CN104796110A - Analog power-frequency comb notch filter and adjusting method thereof - Google Patents

Abstract

The invention relates to an analog power-frequency comb notch filter and an adjusting method thereof. The analog power-frequency comb notch filter is characterized in that an input signal is connected to one input end of a multiplier, a power-frequency noise synthesizer is connected to the other input end of the multiplier, the multiplier is connected with an inverse phase input end of an amplifier, the inverse phase input end of the operational amplifier is connected with the output end of the operational amplifier through an RC parallel feedback network; the output and input signals of the operational amplifier are connected to two input ends of an adder, respectively. The analog power-frequency comb notch filter has the advantages that the interference of power frequency and its harmonics of a weak signal detection circuit can be effectively suppressed, power frequency noise is filtered, and the action of attenuating power frequency harmonics is great; the circuit structure is simple, the attenuation frequency is not influenced by device precision and temperature and is set by programming, attenuation quantity and its attenuation bandwidth are both adjustable, and optimal noise suppression is achieved.

Description

Analog power frequency comb notch filter and its adjustment method

technical field

The invention relates to an analog industrial frequency notch filter of a signal detection circuit, in particular to an analog industrial frequency comb notch filter with multiple attenuation frequency bands and adjustable attenuation characteristics and an adjustment method thereof.

Background technique

In the field of signal detection, the power frequency (my country's power frequency is 50Hz) and its harmonic electromagnetic field generate electromagnetic interference in the signal detection circuit through electromagnetic induction, which seriously affects the efficiency and accuracy of signal detection, and is the main source of interference in the signal detection circuit. . Adding a power frequency notch filter to the detection circuit can filter out power frequency noise, which is beneficial to improve the signal-to-noise ratio and the anti-interference ability of the circuit, and is widely used in the fields of communication, radar, medical equipment and geophysical exploration.

Power frequency notch filters can be divided into digital and analog. The digital power frequency notch filter has the advantages of stable center frequency, easy realization of multiple stop bands, and easy adjustment. However, the digital power frequency notch filter can only be applied after the input signal is converted into a digital quantity, and it is difficult to achieve real-time performance. Moreover, in weak signal detection, the useful signal is submerged in the power frequency and its harmonic noise. When the power frequency interference is large, the circuit cannot be effectively amplified, which is not conducive to measuring weak signals. Therefore, the analog power frequency notch filter plays an irreplaceable role.

The conventional method of simulating a power frequency notch filter mainly includes: 1) using a low-pass filter and a high-pass filter in series, and the cut-off frequency of the high-pass filter is higher than the power frequency, and the cut-off frequency of the low-pass filter is lower than the power frequency; 2 ) use a band-pass filter and a subtractor in series, the center frequency of the band-pass filter is the power frequency, and the subtractor is used to subtract the output of the band-pass filter from the original signal, and the output of the subtractor is the power frequency notch filter Output; 3) Double T power frequency notch filter, using double T network to resonate at power frequency, so that power frequency noise cannot pass through. The conventional analog power frequency notch filter cannot realize multi-stop band attenuation, and it needs to filter the power frequency noise of multiple frequencies through multi-stage series connection, the circuit structure is complex and the cost increases; the attenuation frequency and attenuation of the conventional analog power frequency notch filter are affected The accuracy of capacitance and resistance is affected, debugging is difficult, and the attenuation frequency drifts with temperature changes, which leads to deterioration of attenuation.

CN100562765C discloses a power frequency harmonic suppression circuit in a useful signal frequency band in a weak signal detector, which uses frequency, phase and amplitude to fully track the power frequency harmonics in the useful signal frequency band, and then uses DDS to generate a power frequency harmonic The sine wave with the same noise frequency, phase and amplitude, and finally subtract the sine wave generated by DDS from the useful signal to suppress the power frequency harmonic noise in the useful signal frequency band. This invention can only suppress the power frequency noise of one frequency in the useful signal frequency band, and cannot suppress the power frequency noise of multiple frequencies at the same time. When the frequency, amplitude or phase measurement of the power frequency harmonic is inaccurate, the filtering effect is poor , and even affect the useful signal.

CN101666834A discloses a signal sampling method and system for resisting power frequency interference. First, the power frequency signal is extracted from the input mains AC signal, and the power frequency interference superimposed on the detection signal is synchronously tracked through the power frequency signal, and a A power frequency synchronization signal; then, according to the power frequency synchronization signal, the zero-crossing phase of the power frequency interference signal is locked, and the input detection signal is synchronously sampled at the zero-crossing phase. This invention can only suppress the power frequency noise of one frequency, and cannot suppress the power frequency noise of multiple frequencies at the same time. Accuracy of signal detection.

CN2586170Y discloses a device for eliminating 50Hz power frequency interference of a superconducting quantum interferometer. The device includes an inductance coil that couples a measured signal to obtain a reference signal. The inductance coil is connected to an amplifier, and the amplifier is connected to a 50Hz bandpass filter. The pass filter is connected with a phase adjuster that can invert the phase of the reference signal, the phase adjuster is connected with one input of the adder, and the other input of the adder is connected with the feedback loop of the superconducting quantum interferometer, and the adder The output terminal of is connected with the feedback resistor of the feedback loop. This invention can cancel the unnecessary interference signal at the measurement point, but it can only suppress the noise of 50Hz, and cannot attenuate the harmonic of the power frequency, and in order to obtain the reference signal, an inductance coil is added, and the inductance coil itself also generates noise. affect the measurement results.

CN203502602U discloses a nuclear magnetic resonance detection device for eliminating power frequency harmonic interference. It adopts the addition of two groups of nuclear magnetic resonance signals with a difference of half a power frequency harmonic cycle at the same starting point to cancel close to power frequency harmonic noise. The device consists of a receiving coil It is connected with gain-adjustable amplifier, filter and secondary amplifier, one end of the secondary amplifier is connected with strength indicator, the other end of the secondary amplifier is connected with hysteresis comparator, one end of hysteresis comparator is connected with adder through trigger, and the other end of hysteresis comparator is The delayer is connected with the adder, and the adder is connected with the computer to constitute. The device can only filter out power frequency harmonics of one frequency, and the filtering effect is affected by the delay accuracy of the delay device. When the delay does not meet the half-cycle condition or delay drift occurs, the power frequency harmonic suppression effect is poor. To sum up, although the existing technology has solved to a certain extent that the attenuation characteristics of the power frequency notch filter are affected by the precision and temperature drift of the resistors and capacitors, it still cannot achieve multi-stop band attenuation, and needs to be filtered out by multi-stage series connection. Power frequency noise of multiple frequencies, complicated circuit structure and increased cost.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the existing analog power frequency notch filter, and provide an analog power frequency comb trap, which can attenuate power frequency and its harmonics, and its attenuation characteristics can be adjusted. Another object of the present invention is to provide a method for adjusting the attenuation frequency, attenuation amount and attenuation bandwidth of an analog power frequency comb trap filter.

The purpose of the present invention is achieved in the following manner:

A kind of analog power frequency comb-shaped notch filter is connected with the adder 8 through the negative pole of the resistor 6 and the operational amplifier 5 by the power frequency noise synthesizer 1 through the multiplier 2 and the resistor 3 respectively, the adder 8 and the multiplier The device 2 is connected, and the anode of the operational amplifier 5 is connected to the resistor 4 to form a connection.

The power frequency noise synthesizer 1 is connected with the digital-to-analog converter 12 through the counter 11, the frequency divider 13 and the crystal oscillator 10 by the button b16, the button c17 is connected with the button a15 through the programmable memory 9, and the counter 11 is connected through the programmable memory 9 And digital-to-analog converter 12 is connected with passive low-pass filter 14 to form.

The method for adjusting the attenuation frequency, attenuation amount and attenuation bandwidth of the simulated power frequency comb notch filter comprises the following steps:

a. Set the power frequency and its harmonic frequency f ₁ , f ₂ , f ₃ ,..., f _m to be filtered out;

b. In the case of sampling rate f _s =4f _m , synthesized power frequency noise signal n(k)=cos(2πkf ₁ /f _s )+cos(2πkf ₂ /f _s )+cos(2πkf ₃ /f _s ) +…+cos(2πkf _m /f _s ), normalize the magnitude of n(k);

c. Turn off the button c17, and turn on the button a15, so that the programmable memory 9 is in the writing state;

d, disconnect button b16, make the output port of counter 11 be in high-impedance state, the power frequency noise signal n(k) of one cycle is written in the programmable memory 9 through memory address line 18 and memory data line 19 ;

e, disconnect the key a15, and connect the key c17, so that the programmable memory 9 enters the read state;

f, the frequency divider 13 is set, the input clock of the counter 11 is set to f _s ;

G, the counting upper limit of setting counter 11 is the discrete point number minus one of the industrial frequency noise n (k) of a cycle;

h, the conversion rate of the digital-to-analog converter 12 is set to f _s ;

i, the cut-off frequency of passive low-pass filter 14 is set to 0.5f _s ;

j. Press the button b16 to enable the output of the counter 11, so that the programmable memory 9 can cyclically output data;

k, adjust the resistance value of the resistor 3, adjust the attenuation of the power frequency comb trap;

l. Adjust the values of the resistor 6 and the capacitor 7 to adjust the attenuation bandwidth of the power frequency comb trap.

Beneficial effects: the present invention can effectively suppress the interference of the power frequency and its harmonics in the weak signal detection circuit, which is obtained by using the frequency to be attenuated to synthesize the power frequency noise signal, and multiplying the synthesized noise signal with the original input signal. The modulated signal is demodulated by the RC parallel feedback network of the inverse proportional amplifier, and the noise signal with the same amplitude and opposite phase as the power frequency and its harmonic noise in the original signal is obtained. Finally, these noises are subtracted from the original signal to achieve the suppression work. frequency and its harmonics; the circuit structure of the present invention is simple, the center frequency is not affected by the precision and temperature of components, the attenuation frequency is set by programming, and the attenuation and its attenuation bandwidth can also be adjusted to achieve the best noise suppression Effect.

Description of drawings

Accompanying drawing 1 is the structural block diagram of analog industrial frequency comb trap.

Accompanying drawing 2 is the structural block diagram of power frequency noise synthesizer in accompanying drawing 1.

Accompanying drawing 3 is the transmission characteristic curve diagram of the simulated power frequency comb trap filter in the ECG signal detector.

Accompanying drawing 4 is the transmission characteristic curve diagram of the simulated power frequency comb trap in the nuclear magnetic resonance water finding instrument.

1 power frequency noise synthesizer, 2 multiplier, 3, 4, 6 resistors, 5 operational amplifier, 7 capacitor, 8 adder, 9 programmable memory, 10 crystal oscillator, 11 counter, 12 digital-to-analog converter, 13 Frequency divider, 14 passive low-pass filters, 15, 16, 17 keys, 18 memory address lines, 19 memory data lines.

Detailed ways

Below in conjunction with accompanying drawing and embodiment the present invention is described in further detail:

Accompanying drawing 1 is the functional block diagram of the circuit of the power frequency and its harmonic comb trap. The power frequency noise synthesizer 1 is connected to one input terminal of the multiplier 2, and the input signal s(k) is connected to the other input terminal of the multiplier 2, then the output signal of the multiplier is:

x(k)=s(k)×v(k),k=0,1,2,...,f _s /f ₁ -1

Since v is synthesized by multiple cosine waves, according to Euler's formula, v is expressed as:

$\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; j</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

Using the frequency shift characteristic of F[s(t)exp(jω ₀ t)]=S(ω-ω ₀ ), we get

$\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

According to the properties of Fourier transform, for a real signal s, the real part of its spectrum S(ω) is an even signal, and the imaginary part is an odd signal, and the output spectrum of the multiplier 2 is expressed as:

$\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>$

The output terminal of the multiplier 2 is connected to one end of the resistor 3, and the other end of the resistor 3 is connected to the inverting input terminal of the operational amplifier 5. The resistance of the resistor 3 is R _i , the operational amplifier can be OP27, and the non-inverting input terminal of the operational amplifier 5 The input end is connected to the reference ground through a resistor 4, the resistance of the resistor is about 1kΩ, the inverting input end of the operational amplifier 5 is connected to one end of the resistor 6, and the other end of the resistor 6 is connected to the output end of the operational amplifier 5 , the resistance of the resistor 6 is R _f , the inverting input terminal of the operational amplifier 5 is connected to one terminal of the capacitor 7, the other terminal of the capacitor 7 is connected to the output terminal of the operational amplifier 5, and the capacitance of the capacitor is C, according to the inverting proportional amplifier The amplification factor formula obtains the frequency spectrum U (ω) of the output voltage of operational amplifier 5 as:

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; )</span>}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>$

The output of the operational amplifier 5 is connected to an input terminal of the adder 8, and the input signal s(k) is connected to the other input terminal of the adder 8, so that the frequency spectrum V _o (jω) of the total output signal is:

$\begin{array}{c}\mathrm{<span\; class="notranslate">\; Vo</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; )</span>}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; *</span>\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>\end{array}$

In the above formula, "*" represents the convolution operation, and finally the transmission characteristic function of the power frequency and its harmonic comb trap circuit is:

$\begin{array}{c}\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; *</span>\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; ]</span>}\end{array}$

Accompanying drawing 2 is a structural block diagram of power frequency noise synthesizer, and power frequency noise synthesizer 1 is to be connected with the clock input end of counter 11 by frequency divider 13 by crystal oscillator 10, the clock of crystal oscillator 10 and digital-to-analog converter 12 The input terminals are connected, the crystal oscillator provides clocks for the counter 11 and the digital-to-analog converter 12 respectively, and the button 16 is connected with the enabling terminal of the counter 11 to control the operation or termination of the counter 11, and the output of the counter 11 communicates with the programmable The memory 9 is connected, and the memory is 16×4096 bytes of Flash memory. The Flash memory is used as a non-volatile programmable memory. The programming data can be saved after power off, avoiding repeated programming under the condition that the attenuation characteristics remain unchanged, and the counter 11 is binary Counter, the output count value is used to read the data in the programmable memory 9, the button 15 is connected with the write enable pin of the programmable memory 9, and the button 17 is connected with the read enable pin of the programmable memory 9, respectively controlling The write operation or read operation of the programmable counter 9, the programmable memory 9 is connected with the digital-to-analog converter 12 through the memory data line 19, and the digital-to-analog converter 12 is controlled to output synthetic power frequency noise, and the digital-to-analog converter 12 is connected with the passive low The pass filter 14 is connected to form, the power frequency synthesized noise of the smooth output, the passive low pass filter can use RC low pass filter.

A method for adjusting the attenuation frequency, attenuation amount and attenuation bandwidth of an analog power frequency comb notch filter, comprising the following steps:

a. Set the power frequency to be filtered out and its harmonic frequency f ₁ , f ₂ , f ₃ , ..., f _m , under normal circumstances, f ₁ is 50Hz, and f _m is the highest power frequency to be filtered out harmonic.

b. In the case of sampling rate f _s =4f _m , synthesized power frequency noise signal n(k)=cos(2πkf ₁ /f _s )+cos(2πkf ₂ /f _s )+cos(2πkf ₃ /f _s ) +…+cos(2πkf _m /f _s ), normalize the magnitude of n(k):

$\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;kf</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0,1,2</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}$

c. Turn off the button 17, and turn on the button 15, so that the storage is in the writing state. Since a cycle of power frequency noise synthesis waveform is composed of f _s /f ₁ point, when a 16×4096 byte memory is selected, the calculation formula of the highest suppressable power frequency harmonic frequency f _m is:

$\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; 4096</span>}$

When f ₁ is 50Hz, the highest frequency that can be filtered out is 51200Hz, which is the 1024th harmonic of the power frequency.

d, disconnect button 16, make the output port of counter 11 be in high-impedance state, the power frequency noise signal n (k) of one cycle is written in the programmable memory 9 by memory address line 19 and memory data line 20 , write the data vector D as:

D(k)＝round(32767×n(k)),k＝0,1,2,...,f _s /f ₁ -1

round indicates rounding to the nearest integer.

e. Turn off the button 15, and turn on the button 17, so that the programmable memory 9 enters the read state.

f. Set the frequency division coefficient of the frequency divider 13 as f _clk /f _s , and obtain the counting frequency of the counter 11 as f _s .

g. The counter 11 is selected as a 12-bit binary counter, and the counting upper limit of the counter 11 is set to be f _s /f ₁ -1, and one is subtracted because the counting starting point of the counter is 0.

h, the digital-to-analog converter 12 selects a 16-bit digital-to-analog converter, the reference voltage is V _ref , and the conversion rate is set to f _s , the output voltage of the digital-to-analog converter 12 is controlled by the data in the programmable memory 9, and the output voltage v (k) is:

$\mathrm{<span\; class="notranslate">\; v</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 32767</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0,1,2</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}$

i. Set the cut-off frequency of the passive low-pass filter 14 to 0.5 f _s to filter out high-frequency noise generated during the discretization process of the digital-to-analog converter.

j. Press the key 16 to enable the output of the counter 11, and make the programmable memory 9 cyclically output the data vector D.

k. Adjust the resistance value R _i of the resistor 3 to adjust the attenuation of the power frequency comb trap. According to the above formula, if the input signal contains power frequency noise with a frequency ω ₀ of 2πf _r , and the synthesized power frequency noise n(k) has a cos(2πkf _r /f _s ) term, then at frequency ω ₀ , the amplitude-frequency response of the power frequency comb notch filter is:

$\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; mR</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}$

Therefore, the amount of attenuation can be changed by adjusting R _i , and when R _i =R _f V _ref /m, all power frequency noise at the attenuation frequency set in step a can be filtered out. In the present invention, the frequency for suppressing power frequency and its harmonics is set by step a, and the attenuation frequency is obtained by frequency division of a crystal oscillator, and is not affected by the precision of resistance or capacitance. .

l. Adjust the values of the resistor 6 and the capacitor 7 to adjust the attenuation bandwidth of the power frequency comb trap. When the input signal frequency ω ₀ is not equal to the frequency of the power frequency and its harmonics, that is, Δω=ω ₀ -2πf _r , the amplitude-frequency response of the power frequency comb notch filter at frequency ω ₀ is:

$\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}\underset{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{\mathrm{<span\; class="notranslate">\; m</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&Delta;\&omega;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}$

It can be seen from the above formula that when the frequency of the input signal deviates from the notch frequency set in the power frequency noise synthesizer 1, the output amplitude of the circuit increases with the increase of Δω, and the equivalent noise bandwidth at the attenuation frequency f _r is:

$\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{<span\; class="notranslate">\; \&Integral;</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; d\&omega;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; Hz</span>}<span\; class="notranslate">\; )</span>$

Therefore, the larger R _f C is, the narrower the attenuation bandwidth of the power frequency comb notch filter is. Increasing R _f or C can reduce the attenuation bandwidth and reduce the impact on useful signal frequencies.

Example 1

The following uses an electrocardiogram (ECG) detector as an example for illustration. ECG reflects the electrical changes of the heart during the process of excitation generation, conduction and recovery, and is an objective representation of the electrical activity of the heart. However, because the ECG signal is weak and is detected indoors, it is easily affected by power frequency and its low-order harmonics. interference.

Set the power frequency and its harmonic frequency to be filtered as 50Hz, 100Hz, 150Hz, 200Hz, 250Hz. When the sampling rate f _s is 1000Hz, the power frequency noise signal n(k) is synthesized and its amplitude is normalized:

$\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

D(k)=round(32767×n(k)), write 20 data contained in D(k) into the programmable memory, and set the programmable memory to enter the read state after writing. A crystal oscillator with an oscillation frequency f _clk of 1M Hz is selected, and the frequency division coefficient of the frequency divider is set to 1000, so that the counting frequency of the counter 11 is 1000Hz. Choose a 12-bit binary counter, and set the upper limit of the counter to 19. The digital-to-analog converter 12 is a 16-bit digital-to-analog converter, the reference voltage is 5V, and the conversion rate is set to 1000Hz. Set the cut-off frequency of the passive low-pass filter to 500Hz to filter out the high-frequency noise generated during the discretization of the digital-to-analog converter. Enable the counter to start counting, and get the synthesized power frequency noise frequency from the output of the low-pass filter.

Set R _i =R _f =10kΩ, C=100μF, and the attenuation bandwidth is 0.5Hz, which is far lower than the attenuation bandwidth of conventional power frequency notch filters. The transmission characteristic curve of the analog power frequency comb notch filter in the ECG signal detector is shown in Figure 3.

Example 2

The following is an example of an NMR (Magnetic resonance sounding, MRS) water finder. The MRS water finder uses the artificially excited electromagnetic field to make the hydrogen protons in the groundwater form a macroscopic magnetic moment. This macroscopic magnetic moment produces a precession motion in the electromagnetic field. The coil receives the electromagnetic signal generated by the macroscopic magnetic moment precession to detect the existence of groundwater. . However, because the MRS signal is extremely weak, generally at the nV level, it is easily affected by power frequency harmonics in the surrounding power lines when measuring in the field. The frequency of the MRS signal (Larmor frequency) is related to the strength of the local geomagnetic field, generally 1kHz-3kHz. For example, the Larmor frequency in Changchun City, Jilin Province is 2326Hz. Although the detection circuit of the MRS water finder has added A bandpass filter with a bandwidth of 100Hz, but the 46th harmonic 2300Hz and the 47th harmonic 2350Hz of the power frequency still interfere a lot with the MRS signal, which has become one of the main problems affecting MRS detection.

Set the power frequency and its harmonic frequency to be filtered as 2300Hz and 2350Hz. In the case of a sampling rate _fs of 9.4kHz, the power frequency noise signal n(k) is synthesized and its amplitude is normalized:

$\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2300</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 4700</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2350</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}{\mathrm{<span\; class="notranslate">\; 4700</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

D(k)=round(32767×n(k)), write 188 data contained in D(k) into the programmable memory, and set the programmable memory to enter the read state after writing. Select the crystal oscillator whose oscillation frequency f _clk is 10MHz, set the frequency division coefficient of the frequency divider to 1064, and obtain the counting frequency of the counter 11 as 9398.5Hz. Choose a 12-bit binary counter, and set the upper limit of the counter to 187. The digital-to-analog converter 12 is a 16-bit digital-to-analog converter, the reference voltage is 5V, and the conversion rate is set to 9398.5Hz. Set the cut-off frequency of the passive low-pass filter to 4700Hz to filter out the high-frequency noise generated during the discretization of the digital-to-analog converter. Enable the counter to start counting, and get the synthesized power frequency noise frequency from the output of the low-pass filter.

Set R _i =25 kΩ, R _f =10 kΩ, C=22 μF, and the attenuation bandwidth is 2.3 Hz.

Since the output frequency of the frequency divider is 9398.5Hz, which deviates from 9400Hz, the attenuation frequencies of the power frequency comb notch filter are 2299.6Hz and 2349.6Hz. When the attenuation bandwidth is 2.3Hz, the power frequency 46th and 47th The sub-harmonics are all in the attenuation frequency band, achieving the suppression effect. The transmission characteristic curve of the simulated power frequency comb trap in the nuclear magnetic resonance water finding instrument is shown in Figure 4.

Claims (3)

1. a simulation power frequency comb notch filter, it is characterized in that, be connected with adder (8) with the negative pole of operational amplifier (5) through resistor (6) respectively again with resistor (3) through multiplier (2) by industrial frequency noise synthesizer (1), adder (8) is connected with multiplier (2), and positive pole and the resistor (4) of operational amplifier (5) connect and compose.

2. according to simulation power frequency comb notch filter according to claim 1, it is characterized in that, industrial frequency noise synthesizer 1 is connected with digital to analog converter (12) with crystal oscillator (10) through counter (11), frequency divider (13) by button b (16), button c (17) is connected with button a (15) through programmable storage (9), and counter (11) connects and composes with passive low ventilating filter (14) through programmable storage (9) and digital to analog converter (12).

3., according to the method for adjustment of the frequency of fadings of simulation power frequency comb notch filter according to claim 1, attenuation and attenuation bandwidth thereof, it is characterized in that, comprise the following steps:

A, setting need power frequency and the harmonic frequency f thereof of filtering ₁, f ₂, f ₃..., f _m;

B, in sample rate f _s=4f _mwhen, synthesis industrial frequency noise signal n (k)=cos (2 π kf ₁/ f _s)+cos (2 π kf ₂/ f _s)+cos (2 π kf ₃/ f _s)+... + cos (2 π kf _m/ f _s), the amplitude of n (k) is normalized;

C, disconnection button c (17), and switch-on keying a (15), make holder able to programme (9) be in write state;

D, disconnection button b (16), make the output port of counter (11) be in high-impedance state, industrial frequency noise signal n (k) of one-period is written in programmable storage (9) by holder address wire (18) and holder data wire (19);

E, disconnection button a (15), and switch-on keying c (17), make programmable storage (9) enter reading state;

F, frequency divider (13) is set, the input clock of counter (11) is set to f _s;

G, the count upper-limit arranging counter (11) are that discrete the counting of industrial frequency noise n (k) of one-period subtracts one;

H, the switching rate of digital to analog converter (12) is set to f _s;

I, the cut-off frequency of passive low ventilating filter (14) is set to 0.5f _s;

J, the b that pushes button (16), enable counter (11) exports, and programmable storage (9) is circulated and exports data;

The resistance of k, regulating resistor (3), the attenuation of adjustment power frequency comb notch filter;

The value of l, regulating resistor (6) and capacitor (7), the attenuation bandwidth of adjustment power frequency comb notch filter.