JPH0342008B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an AFC circuit for a receiver, and more particularly, to an AFC circuit for a receiver, which converts an input high frequency signal into an intermediate frequency signal using at least one local oscillator and a frequency conversion circuit. The present invention relates to an AFC circuit that performs automatic frequency control (AFC) on the output local oscillation frequency of a local oscillator so that a predetermined intermediate frequency can be obtained from a frequency conversion circuit with high accuracy when outputting a local oscillator.

Prior Art The AFC circuit of a conventional receiver supplies the output AFC control voltage of an AFC control voltage generation circuit to which an intermediate frequency signal is supplied to a voltage-controlled local oscillator, and converts the output local oscillation frequency from a frequency conversion circuit. The intermediate frequency signal of
It was determined by the frequency accuracy and temperature characteristics of the AFC control voltage characteristics of the control voltage generation circuit. However, since the resonant circuit built into the AFC control voltage generation circuit is composed of inductance, capacitance, or circuit elements equivalent to these,
Its temperature characteristics are poor, and it is extremely difficult to stably obtain the standard IF frequency over a wide temperature range.Also, even if the temperature fluctuation is extremely small, the resonant frequency of the above-mentioned resonant circuit can be set to the standard IF frequency. Adjustment is required to match with high precision, but such adjustment is nearly impossible and usually results in a large error of about 1000 ppm (ppm = 10 ^-6 ) with respect to the reference IF frequency. It was hot.

Therefore, the present applicant previously proposed in Japanese Patent Application No. 59-235494 an AFC circuit for a receiver that eliminates the above-mentioned drawbacks. FIG. 4 shows a block system diagram of an example of the AFC circuit of the receiver proposed by the present applicant.
In the figure, a high-frequency signal received by the receiver enters input terminal 1, and is supplied to frequency conversion circuit 2, where it is output from voltage-controlled oscillator (VCO) 3, which constitutes a local oscillator. It is converted to the frequency of the difference or sum with the local oscillation frequency and is specified as
It becomes an intermediate frequency signal of the IF frequency. This intermediate frequency signal is applied to the terminal 4a of the switch circuit 4, while being outputted to the output terminal 5. The switch circuit 4 connects the reference IF from the reference frequency generation circuit 6 to the terminal 4b.
frequency is supplied and the switching pulse generation circuit 7
The input intermediate frequency signal at the terminal 4a and the input reference IF frequency at the terminal 4b are alternately selected and outputted by the switching pulses and supplied to the AFC control voltage generation circuit 8.

The AFC control voltage generation circuit 8 has a zero point control voltage input terminal 9, and also has a high level control voltage input terminal 9 for an input signal frequency lower than the inflection point frequency _L.
Generates and outputs the AFC control voltage, and sets it to low level for input signal frequencies higher than the inflection point frequency _H.
The characteristics are selected such that the AFC control voltage is generated and output, and the reference IF frequency is located between the inflection point frequencies _L and _H. The output AFC control voltage of this AFC control voltage generation circuit 8 is applied to the common terminal 10c of the switch circuit 10. The switch circuit 10 is operated by the switching pulse from the switching pulse generating circuit 7.
Terminals 10a and 10b are connected to the switch circuit 4.
are switched and connected alternately. That is, the switch circuit 10 selectively outputs the AFC control voltage to the hold circuit 11 via the terminal 10a during the period in which the switch circuit 4 selectively outputs the input intermediate frequency signal of the terminal 4a, and the switch circuit 4 selects and outputs the AFC control voltage to the hold circuit 11 through the terminal 10a. During the period in which the IF frequency is selectively output, the AFC control voltage is selectively output to the hold circuit 12 via the terminal 10b.

The hold circuits 11 and 12 are smooth control voltage generation circuits, and are provided to solve the problem that the AFC control voltage is intermittently obtained by the switch circuit 10. The output signal of the hold circuit 11 is
It is applied to the VCO 3 as a control voltage, and the output signal of the hold circuit 12 is applied to the zero point control voltage input terminal 9 of the AFC control voltage generation circuit 8. here,
During the period when switch circuits 4 and 10 are connected to 4a and 10a, respectively, frequency conversion circuit 2→switch circuit 4→AFC control voltage generation circuit 8→switch circuit 1
An AFC control loop is formed consisting of 0 → hold circuit 11 → VCO 3 → frequency conversion circuit 2, and is selectively output from the terminal 10a of the switch circuit 10.
An operation is performed to match the IF frequency with the zero point frequency of the AFC control voltage generation circuit 8 using the AFC control voltage. On the other hand, switch circuits 4 and 10 are connected to 4b and 1, respectively.
During the period when connected to 0b, AFC control voltage generation circuit 8 → switch circuit 10 → hold circuit 12 →
A zero point correction loop consisting of the AFC control voltage generation circuit 8 is formed, and the zero point frequency of the AFC control voltage generation circuit 8 is set as a reference from the reference frequency generation circuit 6.
Performs an operation to match the IF frequency.

As described above, the switch circuits 4 and 10 are alternately switched by the switching pulses from the switching pulse generation circuit 7, so this AFC circuit performs the AFC control operation while correcting the zero point of the AFC control voltage generation circuit 8. Therefore, the IF frequency _IF is determined by the above-mentioned AFC control loop and zero point correction loop.
When the loop gains of the two control loops are sufficiently large, the output reference IF of the reference frequency generation circuit 6
The frequency matches _IFO in principle, and extremely high stability matching that of _IFO can be obtained.

Problems to be Solved by the Invention However, in general, the loop gains of the above two control loops are both finite, so the accuracy Δ _IF of the output IF frequency _IF of the frequency conversion circuit 2 is calculated as follows: It is expressed as the sum of the control loop pull-in accuracy _ΔAFCON and the correction accuracy (error) _ΔZON of the zero point correction loop.

Δ _IF = Δ _AFCON + Δ _ZON (1) The pull-in accuracy of the above AFC control loop Δ _AFCON is
Open AFC control loop (i.e. uncontrolled state)
The frequency deviation _{Δ AFCOFF} with respect to _the reference IF frequency _IFO when For _ZOFF , each is expressed as follows.

Δ _AFCON = Δ _AFCOFF /G _AFC (2) Δ _ZON = Δ _ZOFF /G _Z (3) However, in equations (2) and (3), G _AFC is the loop gain of the AFC control loop, and G _Z is the zero point correction. is the loop gain of the loop.

The above frequency deviation _ΔAFCOFF is generally the frequency setting error of VCO3 when the AFC control loop is open,
It is the sum of the frequency error coming into input terminal 1,
On the other hand, the above-mentioned frequency deviation _ΔZOFF is generally a variation of the zero point frequency when the zero point correction loop is open due to temperature or the like.

Now, as an example, Δ _AFCOFF = 30MHz, Δ _ZOFF
=500kHz, G _AFC =2×10 ⁴ , G _Z =200,
The accuracy of IF frequency Δ _IF is calculated from equations (1) to (3) as follows: Δ _IF = Δ _AFCON + Δ _ZON = (Δ _AFCOFF / G _AFC ) + (Δ _ZOFF / G _Z ) = 4.0 (kHz), and the zero point correction loop Correction accuracy (error)
Δ _ZON is quite large at 2.5kHz, and Δ _IF
This was a factor in increasing the

Therefore, the present invention aims to improve the reference level setting voltage and the AFC
It is an object of the present invention to provide an AFC circuit for a receiver that solves the above problems by generating a control voltage proportional to the difference from the control voltage and applying it to a local oscillator.

Means for Solving the Problems The AFC circuit of the receiver according to the present invention includes a local oscillator, a frequency conversion circuit, a frequency generation circuit, first and second switch means, an AFC control voltage generation circuit,
It consists of first and second voltage generation circuits and a differential amplifier. The first switching means is supplied with an intermediate frequency signal obtained by frequency conversion between the input signal and the output local oscillation frequency of the local oscillator by the frequency conversion circuit, and a reference intermediate frequency from the frequency generation circuit. These signals are selected and output alternately. The AFC control voltage generation circuit generates an AFC control voltage at a level corresponding to the deviation of the output signal frequency of the first switch means from the zero point frequency and supplies it to the second switch means. The second switch means inputs the intermediate frequency signal during the period when the first switch means selectively outputs the intermediate frequency signal.
The AFC control voltage is selectively output to the first voltage generating circuit, and during the period when the first switching means is selectively outputting the reference intermediate frequency, the input AFC control voltage is switched and output to the second voltage generating circuit.

The differential amplifier generates a voltage proportional to the difference between the smoothed control voltages extracted from the first and second voltage generating circuits, supplies it to the local oscillator, and variably controls its output local oscillation frequency.

Effect The smoothed control voltage taken out from the second voltage generation circuit corresponds to the reference intermediate frequency generated according to the AFC control characteristics of the AFC control voltage generation circuit.
It is proportional to the AFC control voltage and is supplied to the differential amplifier as a reference level setting voltage. The differential amplifier variably controls the output local oscillation frequency of the local oscillator using the voltage obtained by differentially amplifying this reference level setting voltage and the AFC control voltage taken out from the first voltage generation circuit. As a result, the AFC circuit operates so that the AFC control voltage supplied to one input terminal of the differential amplifier is equal to the reference level setting voltage, and the local oscillator converts the output intermediate frequency signal of the frequency conversion circuit to the above reference intermediate frequency. A local oscillation frequency is extracted that matches the .
1 to 3 for each embodiment of the present invention.
This will be explained with figures.

Embodiment FIG. 1 shows a block diagram of a first embodiment of the circuit of the present invention. In the figure, the same components as those in FIG. 4 are denoted by the same reference numerals, and the explanation thereof will be omitted as appropriate. 1st
In the figure, a reference frequency generation circuit 6 is constructed using the output intermediate frequency signal of the frequency conversion circuit 2 and a circuit with extremely high frequency stability such as a crystal oscillation circuit.
The reference IF frequencies _IFO are alternately switched by the switch circuit 4 and supplied to the AFC control voltage generation circuit 13. This AFC control voltage generation circuit 13 does not have a zero point control voltage input terminal, and therefore its zero point frequency _Z is fixed. The AFC control voltage generation circuit 13 has an input signal frequency (IF
If the horizontal axis is the frequency) and the vertical axis is the output AFC control voltage, the AFC control characteristics are as shown by the solid line a in Figure 2, and the input signal frequency is lower than the first inflection point frequency _L. High level AFC against
Outputs a control voltage, and outputs a low level for input signal frequencies higher than the second inflection point frequency _H.
It outputs an AFC control voltage, and further generates an AFC control voltage at a level that gradually decreases from the high level to the low level as the frequency increases for input signal frequencies from the inflection point frequency _L to _H. The zero point frequency _Z is at each of the inflection point frequencies _L and _H.
It is defined as the frequency that generates the voltage V ₀ at the midpoint of the AFC control voltage.

The AFC control voltage extracted from the AFC control voltage generation circuit 13 having such AFC control characteristic a is
The switch circuit 10, in conjunction with the switch circuit 4, alternately supplies the signal to the first and second hold circuits 11 and 12 via the first and second terminals 10a and 10b, where it is subjected to peak hold, etc. It is converted into a smooth control voltage proportional to that level. Here, in the hold circuit 11, the switch circuit 4 is connected to the terminal 4.
The period during which the input intermediate frequency signal of a is selectively output
Hold the AFC control voltage and change the voltage to the first
The AFC control voltage V _AFC1 is applied to the non-inverting input terminal of the differential amplifier 14. On the other hand, the hold circuit 12
holds the AFC control voltage during the period when the switch circuit 4 selects and outputs the input reference IF frequency _IFO of the terminal 4b, and sets that voltage to the reference level setting voltage V _REF
It is applied to the inverting input terminal of the differential amplifier 14 as a signal. This reference level setting voltage V _REF is generated by the AFC control voltage generation circuit 13 according to the AFC control characteristic a when the reference IF frequency _IFO is input.
When the zero point frequency _Z , which corresponds to the control voltage and provides the control midpoint voltage V ₀ , changes, it changes accordingly.

The differential amplifier 14 receives the above-mentioned first AFC control voltage.
Generates a second AFC control voltage V _AFC2 proportional to the difference between V _AFC1 and the reference level setting voltage V _REF , and
It is supplied to VCO3 to variably control its output local oscillation frequency. As a result, the frequency conversion circuit 2
→ Switch circuit 4 → AFC control voltage generation circuit 13
The AFC control loop consisting of → switch circuit 10 → hold circuit 11 → differential amplifier 14 → VCO 3 → frequency conversion circuit 2 has its loop gain
When G _AFC is sufficiently large, the first AFC control voltage V _AFC1 taken out from the hold circuit 11 operates to become equal to the reference level setting voltage V _REF by the hold circuit 12, and becomes stable when it becomes equal.

Here, the AFC controlled characteristic with the control voltage applied to the VCO 3 on the vertical axis and the IF frequency taken out from the frequency conversion circuit 2 on the horizontal axis is as follows:
This is indicated by the solid line b in FIG. Therefore, the first
When the AFC control voltage V _AFC1 becomes equal to the reference level setting voltage V _REF and is stable, the differential amplifier 14
As can be seen from FIG. 2, the output second AFC control voltage V _AFC2 is a voltage that provides the reference intermediate frequency _IFO of the AFC controlled characteristic b. As a result, the above AFC control loop
The IF frequency _IF will operate to match the reference intermediate frequency _IFO .

In this way, according to this embodiment, the IF frequency _IF
The accuracy of depends only on the pull-in accuracy of the AFC control loop (that is, the loop gain), and depends on the fluctuation of the zero point.
No deterioration of _IF accuracy occurs.

Note that the inflection point frequencies _{L and H} of the AFC control characteristic a are the conditions for guaranteeing the operation of the circuit of the present invention.
It is necessary to always satisfy the relationship _L < _IFO < _H between and the reference IF frequency _IFO . Therefore, when the change in the AFC zero point frequency _Z due to temperature change is large (that is, when the change in _L and _H is large), the slope of the AFC control characteristic a is made gentler.
Make the inflection point frequency _L sufficiently lower than the reference IF frequency _IFO , and set the inflection point frequency _H to the reference IF frequency.
It needs to be high enough for _IFO . As a method for making the slope of the AFC control characteristic a gentler, for example, a method such as reducing the Q of the resonant circuit (tuned circuit) in the AFC control voltage generation circuit 13 can be considered.
It can be easily achieved.

Next, a second embodiment of the circuit of the present invention will be explained with reference to the block system diagram shown in FIG. In the same figure, the first
Components that are the same as those in the figures are given the same reference numerals, and their explanations will be omitted. In FIG. 3, the AFC control voltage generation circuit 8 is the same as the AFC control voltage generation circuit 13 in that it has the AFC control characteristic a shown in FIG. The difference is that by applying V _REF to the zero point control voltage input terminal 9, the zero point frequency _Z is corrected to be equal to _IFO . Such zero point frequency _Z correction means uses a variable capacitance element as a part of the LC tuning circuit that provides the zero point frequency _Z in the AFC control voltage generation circuit 8, and adjusts its value by the input voltage of the zero point control voltage input terminal 9. This can be easily realized by a method such as variably controlling the capacitance value of a variable capacitance element.

According to this embodiment, the AFC control voltage generation circuit 8
→ Switch circuit 10 → Hold circuit 12 → AFC
Since the change in the inflection point frequencies _L and _H can be made small by the zero point correction loop made up of the control voltage generation circuit 8, unlike the first embodiment, the AFC
There is no need to intentionally make the slope of the control characteristic a gentle. Furthermore, since the purpose of the zero point correction loop in this embodiment is to keep the fluctuations of the inflection point frequencies _L and _H below a predetermined value, the AFC circuit proposed earlier by the applicant shown in FIG. Of course, a high loop gain such as the zero point correction loop is not necessary. In addition, the basic operation in this example is the first
As in the embodiment, _{the IF} accuracy depends only on the pull-in accuracy of the AFC control loop, and it goes without saying that the presence of the zero point correction loop does not cause any deterioration in the _IF accuracy.

It should be noted that the present invention can be suitably applied to any signal state of the high frequency signal that enters the input terminal 1. For example, a four-phase
It can also be applied to cases where unmodulated carrier waves and modulated carrier waves are transmitted alternately, such as in PSK (phase shift keying). In this case, the output switching pulse of the switching pulse generation circuit 7 is generated in synchronization with the cycle of the presence/absence of the unmodulated carrier wave,
The incoming period of the unmodulated carrier wave is determined by switch circuits 4 and 1.
0 is connected to the terminals 4a and 10a, and during the incoming period of the modulated carrier wave, it is switched and connected to the terminals 4b and 10b.
As a result, the carrier frequency of the input signal completely matches the reference frequency. Therefore, the present invention is even more effective when applied to stabilizing the IF frequency of a phase-shift keyed high-frequency signal.

Further, in most cases, the VCO 3 has a function of selecting a large number of input high frequency signals using a tuning voltage having a wide frequency variable range. In this case, the VCO 3 may be controlled by using a voltage obtained by adding the channel selection voltage to the AFC control voltage as the control voltage, and it goes without saying that the present invention operates satisfactorily in such an embodiment.

Further, the hold circuits 11 and 12 may output a continuous constant voltage proportional to the magnitude of the voltage taken out from the switch circuit 10; It may also be configured to output the data.

Furthermore, in order to perform feedback control such as AFC operation and zero point correction operation well, appropriate low-pass filters are connected to the output terminals of hold circuits 11 and 12 for system stability, and necessary It is sufficient to adopt a configuration that guarantees the loop gain.

Furthermore, the circuit of the invention can also be applied to double superheterodyne receivers. In this case, AFC control is usually performed on the second local oscillation frequency because it is easy to control because the frequency is low and because the final intermediate frequency signal obtained is the reference IF frequency. It is.

Furthermore, the polarity of the input to the differential amplifier 14 is not limited to that of the embodiment, and may vary depending on the polarity of the input/output of the AFC control voltage generation circuits 8, 13, the hold circuit 11, the differential amplifier 14, etc. Then,
Of course, the polarity should be selected so that the AFC control operation converges.

Effects of the Invention As described above, according to the present invention, the voltage proportional to the AFC control voltage when inputting the intermediate frequency signal and the reference IF
Since the local oscillation _frequency is controlled by a voltage proportional to the difference from the reference level setting voltage, which is proportional to the AFC control voltage when inputting the frequency IFO, the AFC
The accuracy of the IF frequency _IF can be made to depend only on the pull-in accuracy of the control loop. Therefore, compared to the AFC circuit previously proposed by the applicant, a more accurate and stable IF frequency can be obtained, and the temperature It has a number of features, such as being able to completely eliminate changes in the zero point frequency due to humidity, etc., changes over time, and changes in the IF frequency due to initial adjustment deviations, etc., and also being able to omit the zero point correction loop.

[Brief explanation of drawings]

1 and 3 are block system diagrams showing first and second embodiments of the circuit of the present invention, respectively; FIG. 2 is a diagram showing AFC control characteristics and AFC controlled characteristics for explaining the operation of the circuit of the present invention; FIG. 4 is a block diagram showing an example of an AFC circuit previously proposed by the applicant. 1... High frequency signal input terminal, 2... Frequency conversion circuit, 3... Voltage controlled oscillator (VCO, 4, 10
... Switch circuit, 5 ... Intermediate frequency signal output terminal, 6 ... Reference frequency generation circuit, 7 ... Switching pulse generation circuit, 8, 13 ... AFC control voltage generation circuit, 9 ... Zero point control voltage input terminal, 11,12...
...Hold circuit, 14...Differential amplifier.

Claims (1)

[Scope of Claims] 1. A local oscillator whose output local oscillation frequency is variably controlled by an external input control signal, and a frequency converter between the input signal and the local oscillation frequency from the local oscillator to obtain an intermediate frequency between a predetermined intermediate frequency. A frequency conversion circuit that outputs a frequency signal, a frequency generation circuit that generates and outputs a reference intermediate frequency, and alternately selects and outputs the intermediate frequency signal from the frequency conversion circuit and the reference intermediate frequency from the frequency generation circuit. a first switch means; an AFC control voltage generation circuit that is supplied with the output signal of the first switch means and generates and outputs an AFC control voltage of a level corresponding to the deviation of the signal frequency from the zero point frequency;
The first switch means outputs the AFC control voltage supplied from the control voltage generation circuit to its first output terminal during the period in which the intermediate frequency signal is selectively outputted, and the first switch means outputs the AFC control voltage supplied from the control voltage generation circuit to the first output terminal thereof. The period for selectively outputting the intermediate frequency is determined by a second switch means for switching and outputting the intermediate frequency to the second output terminal thereof, and a second switch means for outputting the intermediate frequency from the first and second output terminals of the second switch means.
First and second voltage generation circuits that are separately supplied with the AFC control voltage and output smoothed control voltages proportional to the magnitude thereof; and smoothed control voltages taken out from the first and second voltage generation circuits, respectively; An AFC circuit for a receiver, comprising a differential amplifier that generates a voltage proportional to the difference between the supplied voltages and outputs it to the local oscillator as the external input control signal. 2 The AFC control voltage generation circuit has a first and a second
2. The AFC circuit for a receiver according to claim 1, wherein the zero point frequency between the inflection point frequencies is fixed to a frequency that is equal to or very close to the reference intermediate frequency. 3 The AFC control voltage generation circuit has a first and a second
The zero point frequency between the inflection point frequencies is variably controlled by a smoothing control voltage from the second voltage generating circuit so that it becomes equal to or extremely close to the reference intermediate frequency. An AFC circuit for a receiver according to claim 1.