JPH07327056A - Frequency modulator - Google Patents

Abstract

PURPOSE:To obtain a means improving an information transmission bit rate per a carrier frequency band with a simple circuit suitable for LSI by applying band compression to a pulse density modulation (PDM) signal without conversion to a pulse code modulation (PCM) signal at once and providing the compressed signal to a modulation input of a frequency modulator so as to compress the band of the information. CONSTITUTION:A PDM signal (bit stream) outputted from a DELTASIGMA A/D converter 1 is not converted into a PCM signal but its band is compressed and the resulting signal is given to a frequency modulator 2. The output pulse width of the DELTASIGMA A/D converter 1 is limited by one period (1T) of a sampling period and the pulse width of the PDM signal before limit is nT and when the pulse width is limited to 1T, a differential encoding is attained simply by using a trigger flip-flop or the like. The band of a differential encode output signal is limited to 1/4 of the sampling frequency. Then a 1-bit digital signal subjected to differential encoding (bit stream) is band-limited by a low pass filter 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is used in wireless communication. INDUSTRIAL APPLICABILITY The present invention is used for communication performed by converting a single or multiplexed analog signal into a digital signal or communication performed by converting an input digital signal into a higher speed digital signal. INDUSTRIAL APPLICABILITY The present invention is used for simplification, downsizing and power reduction of a device. The present invention is suitable for use in devices that use integrated circuits.

[0002]

2. Description of the Related Art An analog frequency modulation system is mainly used in transmission circuits of mobile phones, which are currently the mainstream. In this method, an analog circuit such as a voice to be transmitted is directly filtered by an analog circuit, a carrier wave is modulated, and the modulated signal is transmitted. FIG. 19 shows this conventional device as a first conventional device.

On the other hand, there are transmission circuits based on digital technology. Digital mobile phones use linear PCM audio
(Pulse Code Modulation) After analog-to-digital conversion (hereinafter referred to as AD conversion) to a signal, the bit rate is compressed by digital processing to perform phase modulation and then transmitted.
FIG. 20 shows this conventional device as a second conventional device. Although there are various methods for AD conversion, a ΔΣ AD converter (converter for converting an input signal into a pulse density modulation signal) is widely known as a method suitable for LSI implementation. The quantization is performed with a low resolution of 1 bit, but the sampling frequency is 100 times or more higher than the normal sampling frequency (oversampling), and the integrator of the quantization loop is used. By shaving the quantization noise to a frequency higher than the signal band by the frequency characteristic of, a high SN ratio of 60 dB or more in the signal band is realized. The output of the ΔΣ AD converter is a pulse density modulation (PDM) signal and is 1
It is a pulse train (bit stream) in bit units.

In addition, as another digital system, in a digital cordless telephone, voice is converted to ADPCM (Adaptive Decode).
32kb with AD converter of lta Pulse Code Modulation method
After converting into a ps digital PCM signal, the carrier is modulated by the QPSK system and transmitted. This is shown in FIG. 21 as this third conventional example device.
The third conventional example device has a simpler circuit and smaller LSI scale than the second conventional example device, and is advantageous in terms of power and cost.

[0005]

The first prior art device is
Already matured analog wireless technology is used, but this analog technology is not suitable for making circuits into LSI. This circuit often requires a large capacity capacitor or coil. The circuit cannot be realized by a digital signal processor (hereinafter referred to as DSP) and software. That is, it is not suitable for downsizing, low power consumption, and economy of the device by the LSI technology, which has made remarkable progress in recent years. Further, since it is difficult to secure stability and accuracy of an analog circuit, it is necessary to provide a margin for mass production design, and there is a limit to improving transmission efficiency and noise resistance. In addition, since it is difficult to realize complicated processing, there is a limit to increasing confidentiality.

Since the second conventional example device contains a lot of high frequency noise, it is necessary to remove it by a digital filter and at the same time convert it into a normal PCM signal. This digital filter requires a large-scale logic circuit. Further, it is known that this PCM signal is compressed with a DSP to a bit rate from 64 kbps to 8 kbps and modulated on a carrier by the QPSK method, and then transmitted. However, band compression by digital processing is performed in a frequency domain and a time domain. It requires an advanced algorithm that also uses the processing of. D with very high processing power and large memory to do this
SP is required. Although it can be realized with the latest VLSI technology, it is not sufficient in terms of power consumption and cost.

Since the third conventional example device has a wide band of the modulated signal, the spread of the spectrum of the modulated transmission signal is large, and the communication using the band of the radio wave cannot be efficiently performed.

In these conventional devices, the modulation signal input is assumed to be a voice input, and a clock signal which is considerably high speed in order to be a multimedia signal in which a digital signal, an image signal, a voice signal, etc. are composite-multiplexed. The frequency or carrier frequency must be set.

In these conventional devices, in order to form a wireless communication network, it is necessary to obtain a frequency allocation corresponding to it.
In recent years, spread spectrum communication has been studied, and it has become possible to realize a communication system that does not require frequency allocation by distributing the spectrum of a transmission signal to an extremely wide frequency and making the transmission output a weak radio wave.

The present invention has been made against the background described above, and includes a mobile phone and a wireless LAN (local area network).
It is an object of the present invention to provide a frequency modulation device for wireless communication suitable for multimedia signals such as networks.
It is an object of the present invention to provide a frequency modulation device that band-compresses information to effectively use radio waves. The present invention is realized by a circuit suitable for LSI, and is miniaturized and made economical.
It is an object of the present invention to provide a frequency modulation device that enables low power consumption. It is an object of the present invention to provide a frequency modulation device that can realize a circuit with a digital signal processor (DSP) and software. A further object of the present invention is to provide a frequency modulator suitable for spread spectrum communication that does not require radio frequency allocation.

[0011]

The present invention is characterized in that the PDM signal is subjected to band compression as it is without being converted into a PCM signal and is applied to the modulation input of the frequency modulator. That is, the present invention includes a PDM modulator that converts an input signal to be transmitted into a pulse density modulation signal, and an encoder that generates a binary signal whose logical value changes every pulse of the pulse density modulation signal. A frequency modulation device characterized in that a value signal is input to a frequency modulator.

The PDM modulator preferably has means for limiting its output pulse width to one clock period.

A low-pass filter is preferably inserted in the path through which the output binary signal of the encoder is supplied to the frequency modulator.

The low pass filter is preferably a first-order integration circuit.

The frequency modulator is preferably a frequency hopping type spread spectrum communication modulator.

It is desirable that the low-pass filter has a limited bandwidth such that the hopped frequencies do not interfere with each other.

It is preferable that the frequency modulator has a modulation index high enough to generate a transmission signal for spread spectrum communication by the binary signal.

[0018]

The input signal to be transmitted is an analog signal or a digital signal. This input signal is converted into a pulse density modulation signal. This pulse density modulation signal is used as a frequency modulation input. At this time, 2 is generated by an encoder that generates a binary signal whose logical value changes every pulse of the pulse density modulation signal.
Divide. This is the frequency modulation input. This allows
A frequency-modulated transmission signal with low redundancy is obtained.

By shaping the pulse width of the pulse density modulated signal to a constant value, the encoder is simplified.

By passing the pulse density modulated signal through a low pass filter, the extra band of the frequency modulated transmitted signal can be limited. Since the higher the frequency component is, the smaller the amplitude is, the high frequency noise is suppressed and the SN ratio is secured. Further, the band of the modulated signal can be limited.
By using this low-pass filter as a primary integration circuit, the most ideal waveform can be obtained as a frequency modulation input.

In the circuit of the present invention, the signal passing through the circuit is a signal having a limited waveform reciprocating with a constant amplitude even if it is a digital signal or an analog signal. Therefore,
The circuit can be made into an LSI by a simple circuit, and the circuit can be downsized, economical, and power can be reduced.

In the circuit of the present invention, by increasing the modulation index of the frequency modulator, that is, by performing deep modulation,
The frequency modulation signal for spread spectrum communication can be generated as it is. A frequency modulator for spread spectrum communication that does not require a pseudo noise (PN) generating circuit is obtained.

Further, since the frequency band of the modulated signal is limited in the circuit of the present invention, mutual interference between hopping frequencies can be reduced even if frequency hopping is performed.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention.

The present invention generates a ΔΣ AD converter 1 as a PDM modulator for converting an input signal to be transmitted into a pulse density modulation signal, and a binary signal whose logical value is converted for each pulse of the pulse density modulation signal. And an encoder 4 for
The frequency modulator is characterized in that this binary signal is input to the frequency modulator 2.

The ΔΣ AD converter 1 has means for shaping the pulse width of the output constant.

A low-pass filter 5 is inserted in the path through which the binary signal output from the encoder 4 is supplied to the frequency modulator 2.

Next, the operation of the embodiment of the present invention will be described. The operational characteristic of the device of the present invention is that the PDM signal (bit stream) output from the ΔΣ AD converter 1 is not converted into a PCM signal but is band-compressed as it is and input to the frequency modulator 2.

The output pulse width of the ΔΣ AD converter 1 is limited to one sampling period (1T: T = 1 / fs: fs is full scale), and the pulse width of the PDM signal before the limitation is nT (n is an integer). However, if the pulse width is limited to 1T, differential encoding can be easily performed by a trigger flip-flop or the like.

FIG. 2 shows the waveform of each part. FIG. 3 shows an outline of the frequency spectrum of each part. FIG. 2 is a diagram showing the waveform of each part. FIG. 3 is a diagram showing an outline of the frequency spectrum of each part. As shown in FIG. 2A, the ΔΣ AD converter 1 generates a pulse density modulation signal limited to the sampling period 1T. The band of the differential encode output signal shown in FIG. 2B is 1/4 (fs) of the sampling frequency.
/ 4). By this processing, high frequency noise can be suppressed without losing signal information. Figure 2
As shown in (c), the low-pass filter 5 band-limits the differentially encoded 1-bit digital signal (bit stream).

As shown in FIG. 3, the low-pass filter 5 has a smaller amplitude for a component having a higher frequency, so that high-frequency noise is further suppressed. This makes it possible to secure the SN ratio of the signal and limit the band of the modulated signal. According to the experiment, the band is 1/8 of the sampling frequency.
The quality of the transmission can be sufficiently ensured even if only to a certain degree.

The differentially encoded bit stream signal is input as it is (without being converted into a PCM signal) to the frequency modulator 2 through the low-pass filter 5, and the modulated output is transmitted by the transmitter 3. The power spectrum of the FM modulated wave is shown in FIG. FIG. 4 is a diagram showing the power spectrum of the FM modulated wave. As shown in FIG. 4A, when FM modulation is performed with a normal square wave pulse, a side rope is generated outside the main rope corresponding to the fundamental wave band in the spectrum of the modulated FM signal. Therefore, a wide transmission band is required.

In the present invention, as shown in FIG. 4 (b), the amplitude of the high frequency component of the harmonic is reduced by band limiting the pulse to be transmitted by the low pass filter 5, and the frequency shift in the frequency modulation is reduced. To reduce. As a result, the side lobe falls within the main spectrum of low frequency, so that an FM modulated wave without side lobe can be obtained. this is,
It is equivalent to GMSK (Gaussian filtered Minimum Shift keying) and TFM (Tamed FM). Therefore, it is possible to eliminate interference and crosstalk with the signal of the adjacent channel. In addition, the occupied bandwidth of radio waves per channel (or channel interval) can be reduced.

In particular, although there is a method of hopping to frequencies of a plurality of channels in the spread spectrum method, applying this method enables effective channel arrangement. Since the band of the frequency modulation wave is limited to fs / 4 or less, the hopping frequency in the spread spectrum modulation can efficiently set the channel.
If the frequency modulated wave band is 32 kHz, the radio wave band is 1M
In the case of Hz, 15 frequency channels can be arranged at intervals of 64 kHz.

In the present invention, the random noise component of the PDM signal obtained by the ΔΣ AD converter 1 is used to make F
The spectrum of the M-modulated signal can be spread. In the normal spread spectrum method, a circuit is increased because a dedicated circuit is provided to generate pseudo random noise (PN) and this is superposed on a signal to broaden the spectrum, but in the present invention, a spectrum equivalent to this is obtained. The diffusion process can be easily realized without adding a special circuit. As for the degree of spread, the spectrum becomes wider as the FM modulation index is increased.

Next, FIG. 5 shows a structural example of a circuit for receiving a transmission signal of the apparatus of the present invention. FIG. 5 is a block configuration diagram of a configuration example of the receiving circuit. First, the frequency modulated signal is FM
The signal is detected by the wave detector 50, is equalized by the inverse filter 51 having the frequency characteristic opposite to that of the low-pass filter 5, and then the pulse is regenerated by the comparator 52 and differentially decoded by the differential decoder 53, thereby transmitting. The reproduced PDM signal can be reproduced. This PDM signal can be converted into an analog signal by a simple low pass filter 55.

Next, the ΔΣ AD converter 1 including the sampling circuit 6 will be described with reference to FIGS. 6 and 7. FIG. 6 is a block diagram of the ΔΣ AD converter 1. Figure 7 is ΔΣ
It is a figure which shows the characteristic of AD converter 1. The input signal is passed through a capacitor to remove the DC component, and the offset voltage (-
0.5 V) is set to use the lower half of the integrator dynamic range. First, the integrator 10
Has a very small time constant (that is, no integral action)
Considering that, the integrator 11 outputs a signal to the output when the output integrated voltage reaches the comparison value of the comparator 12 by integrating the input signal.

This output signal is a D flip-flop 13
When input to, the pulse "1" is transmitted in synchronization with the sampling clock signal. This pulse is fed back to the adder and the input of the integrator 11 is cut off. As a result, the output of the integrator 11 disappears. That is, this operation means that the comparator 12 has sent one pulse to the output which continues from the timing of the previous clock signal to the timing of this clock signal. That is, the duration of one pulse is equal to the period of the clock signal.

When the level of the input signal is high, this operation is repeated frequently. This operation is intermittent when the level of the input signal is low. That is, a pulse density modulation signal having a constant pulse width is sent to the output.

Therefore, when the time constant of the integrator 10 is large, a two-stage integrating circuit is formed together with the integrator 11, and the characteristic of 12 dB / oct becomes as the frequency characteristic becomes higher. This configuration speeds up the reaction to the input signal.

FIG. 7 illustrates this characteristic.
When the level of the input signal is high, pulses are frequently emitted, that is, a pulse density modulation signal with a maximum duty of 50% is transmitted. When the input signal level is low, the circuit constant is set so that the pulse density modulation signal with the minimum duty of 10% is transmitted. In the middle, a pulse density modulation signal with a middle duty is transmitted.

FIG. 8 is a specific block diagram of the circuit described above. The output of the D-type flip-flop 13 is connected to the gate inputs of a pair of transistor switches included in the circuit 20 surrounded by a broken line. One of the pair of transistor switches is P-type and the other is N-type. Therefore, when one is closed, the other is open. A negative potential is applied to one drain and a positive potential is applied to the other drain. Moreover, the resistors RN and RP connected to the respective source electrodes have unequal resistance values. Therefore, the voltage corresponding to the above-mentioned bias voltage is always applied, and the negative feedback pulse from the D flip-flop is applied to the input of the integrator 11. FIG. 9 is another concrete configuration diagram of the circuit described above. This circuit is shown in Figure 8.
The resistor part of the circuit described in 1. is realized by a switched capacitor. That is, it is configured such that a current equivalent to the current limited and supplied by the resistor is provided by controlling the time when the capacitor is connected. When the capacitors CP and CN are set to be unbalanced here, there is an effect equivalent to unbalanced resistors RP and RN in the example of FIG. 8 with a common switch timing. Since the circuit of FIG. 9 can be realized by a switch circuit, that is, a field effect transistor and a small-capacity capacitor,
It is very suitable to be composed of an integrated circuit. Since there is no resistor, heat is not generated and the degree of integration can be increased.

Next, the encoder 4 will be described with reference to FIGS.
Will be described with reference to. FIG. 10 is a block diagram of the encoder 4 and its input / output signal waveform. FIG. 11 is a diagram showing signal band compression by the encoder 4. Figure 1
When a PDM signal having a pulse width of 1T is input to the encoder 4 shown in 0 (a), an output synchronized with the falling edge of the input signal is obtained as shown in FIG. 10 (b). here,
Although it is synchronized with the falling edge, it can be synchronized with the rising edge.

As shown in FIG. 11, when the PDM signal duty is 50%, the band is fs / 4 and the duty is 25%.
The signal band can be compressed by controlling so that the band is fs / 8 in the case of and the band is further decreased in the case of the smaller duty.

Next, the low-pass filter 5 will be described with reference to FIGS. 12 and 13. FIG. 12 is a block diagram of the primary integrator. FIG. 13 is a diagram showing the frequency characteristics of the low pass filter 5 and the spectrum of the FM modulated wave. The low-pass filter 5 is shown in FIG.
It is effective to use an analog first-order integrator as shown in FIG. FIG. 12A shows an RC type primary integrator, and FIG. 12B shows an operational amplifier type primary integrator. FIG.
Shows the relationship between the frequency characteristics of the integrator and the spectrum of the modulated wave. By removing the high frequency components, a spectrum without such side lobes can be obtained.

Next, the experimental results will be described with reference to FIGS. In FIG. 14, a sampling frequency of 128 kHz is used for the ΔΣ AD converter 1, and RC2 is used for the low-pass filter 5.
It is a figure which shows the concrete experimental data at the time of using the circuit of a stage. FIG. 15 is a diagram showing the measured values of the spectrum of the output of the low-pass filter 5. FIG. 16 is a diagram showing a spectrum of an FM-modulated signal in which the FM modulation index is appropriately set and the spectrum is spread. 14A shows an output waveform of the ΔΣ AD converter 1, FIG. 14B shows an output waveform of the encoder 4, and FIG.
(C) is the measured value of the output waveform of the low-pass filter 5. Figure 1
As shown in FIG. 4 (a), it can be seen that the output waveform of the ΔΣ AD converter 1 is T = 1 / fs. FIG. 14 (b)
As shown in, the output waveform of the encoder 4 is output in synchronization with the falling edge of the output waveform of the ΔΣ AD converter 1. As shown in FIG. 15, it can be seen that the main spectrum is limited to 32 kHz (fs / 4). It can be seen from FIG. 16 that the spectrum is broadened to about 400 kHz. These data demonstrate the operational characteristics of the present invention and its practicality.

[0047]

As described above, according to the present invention, it is possible to realize a frequency modulation device for wireless communication suitable for multimedia signals of mobile phones, wireless LANs (local area networks) and the like. According to the present invention, it is possible to realize a frequency modulation device that effectively compresses a band of information to effectively use a radio wave. According to the present invention, it is possible to realize a frequency modulation device which is realized by a circuit suitable for an LSI and can be downsized, economical, and low in power consumption. According to the invention, a digital signal
It is possible to realize a frequency modulator capable of realizing a circuit by a processor (DSP) and software. Further, according to the present invention, it is possible to realize a frequency modulator suitable for spread spectrum communication that does not require radio frequency allocation.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a waveform of each part.

FIG. 3 is a diagram showing an outline of a frequency spectrum of each part.

FIG. 4 is a diagram showing a power spectrum of an FM modulated wave.

FIG. 5 is a block configuration diagram of a configuration example of a receiving circuit.

FIG. 6 is a block configuration diagram of a ΔΣ AD converter 1 having a pulse width limiting function.

FIG. 7: Analog signal input and output PD of ΔΣ AD converter
The figure which shows the waveform of M signal.

FIG. 8 is a block configuration diagram showing another configuration example of a ΔΣ AD converter having a pulse width limiting function.

FIG. 9 is a block configuration diagram showing a configuration example of a ΔΣ AD converter having another pulse width limiting function.

FIG. 10 is a block configuration diagram of an encoder and a diagram showing input / output signal waveforms thereof.

FIG. 11 is a diagram showing signal band compression by an encoder.

FIG. 12 is a block diagram of a primary integrator.

FIG. 13 is a diagram showing frequency characteristics of a low pass filter and a spectrum of an FM modulated wave.

FIG. 14 is a diagram showing specific experimental data.

FIG. 15 is a diagram showing an actual measurement value of a spectrum of an output of a low pass filter.

FIG. 16 is a diagram showing a spectrum of an FM-modulated signal that has been spread spectrum by appropriately setting an FM modulation index.

FIG. 17 is a block configuration diagram of a first conventional example device.

FIG. 18 is a block configuration diagram of a second conventional example device.

FIG. 19 is a block configuration diagram of a third conventional example device.

[Explanation of symbols]

 1 ΔΣ AD Converter 2 Frequency Modulator 3 Transmitter 4 Encoder 5, 55 Low-pass Filter 10, 11 Integrator 12, 52 Comparator 20 Circuit 21 Switched Capacitor Integrator 50 FM Detector 51 Inverse Filter 53 Differential Decoder

Claims (7)

[Claims] 1. A PDM modulator for converting an input signal to be transmitted into a pulse density modulation signal, and an encoder for generating a binary signal whose logical value changes every pulse of the pulse density modulation signal. A frequency modulation device characterized in that a value signal is input to a frequency modulator. 2. The frequency modulator according to claim 1, wherein the PDM modulator has means for limiting an output pulse width of the PDM modulator to one clock period. 3. The frequency modulator according to claim 1, wherein a low-pass filter is inserted in a path through which the binary signal output from the encoder is supplied to the frequency modulator. 4. The frequency modulator according to claim 3, wherein the low-pass filter is a first-order integration circuit. 5. The frequency modulator according to claim 1, wherein the frequency modulator is a modulator for frequency hopping type spread spectrum communication. 6. The frequency modulator according to claim 5, wherein the low-pass filter has a limited bandwidth such that the hopped frequencies do not interfere with each other. 7. The frequency modulator according to claim 1, wherein the frequency modulator has a modulation index high enough to generate a transmission signal for spread spectrum communication by the binary signal.