JPS6338887B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved FM demodulator.

A conventional FM demodulator has a configuration as shown in FIG. That is, a signal in a frequency band including a high frequency to be demodulated (hereinafter referred to as an IF band signal) is captured by an antenna (not shown), appropriately high-frequency amplified, and converted to an intermediate frequency, and is sent to the input terminal 2 of the amplitude limiter 1. Apply. The amplitude limiter 1 limits the amplitude of the applied input signal and applies its output to the frequency discriminator 3. The frequency discriminator 3 extracts a frequency modulated signal from the applied signal and applies its output to the low pass filter 4. The low-pass filter 4 passes the components of the applied signal that are below the upper frequency limit of the expected FM modulation component, and outputs them from the output terminal 5.
The FM modulation component is extracted as demodulated output.

This type of FM demodulator not only has the noise suppression effect of the FM method, but also suppresses the desired signal when the IF band signal contains interfering waves that are not intended to be received in addition to the desired signal that is to be received. If the ratio of the signal level to the interference wave signal level (hereinafter referred to as D/U) is greater than 1, the interference wave can be eliminated and the desired wave can be clearly received. Now, the IF band signal E is E = A _D cos (W _D t + θ _D ) + A _U cos (W _U t + θ _U ) ... (1) However, A _D ... the amplitude of the desired wave, W _D ... the carrier frequency of the desired wave, θ _D ...Phase term of the desired wave (the FM modulation component is obtained by differentiating θ _D with respect to time), A _U ...Amplitude of the interfering wave, W _U ...Carrier frequency of the interfering wave, θ _U ...
Assuming that the phase term of the interference wave is A _D = A _U = F when D/U = 1, Equation (1) becomes E = 2F cos (W _D + W _U ) t + θ _D + θ _U /2 ×cos (W _D −W _U )t+θ _D −θ _U /2 ...(2) When the amplitude change due to the cosine component of (W _D −W _U )t is removed by amplitude limiting, the output L becomes L=2Fcos(W _D −W _U )t+θ _D +θ _U /2 ……(3), which is The time differential output D is determined as D=dL/dt=-F( _WD + _WU )+ _θ'D + _θ'U ×sin( _WD + _WU )t/2+ _θD + _θU /2...(4).

However, θ' _D = d/dtθ _D ; FM modulation component of the desired wave θ' _U = d/dtθ _U ; FM modulation component of the interfering wave. Therefore, the rectified and smoothed output of the output D, that is, the output as a frequency discriminator. S is S=A {(W _D +W _U )+θ' _D +θ' _U }...(5) However, -F=A.

Also, when D/U<1, the beat frequency
Using W _B = W _U − W _D , equation (1) becomes E=A _D cos {(W _U − W _B )t+θ _D } +A _U cos(W _U t+θ _U ) ...(6), which can be Summarizing the cosine components of W _U t, we get However, M=cos(W _B t-θ _D ), N=sin(W _B t-θ _D ), P=cosθ _U , Q=sinθ _U φ(t)=tan ^-1 A _D N/A _U − Q/A _D M/A _U +P...(8). Therefore, if the amplitude change of the signal E is removed by amplitude limiting and it is passed through a frequency discriminator (ideal), only the phase change in equation (7) will be extracted, so the discriminator output S
is S=d/dt{W _U t−φ(t)} =W _U −d/dtφ(t) (9).

Here, finding d/dtφ(t) is complicated, but if we approximate it under the conditions of A _D /A _U <<1, (A _D /A _U ) ² <<1, we get d/dtφ (t)=−θ′ _U +(W _B −θ′ _D +θ′ _U ) _∞ 〓 〓 ^n-1 (−1) ^n-1 ×(A _D /A _U ) ⁿ cos{n(W _B t+θ _U −θ _D
)}...(10) becomes. Therefore, the discriminator output S is S = (W _U + θ' _U ) - (W _B - θ' _D + θ' _U ) _∞ 〓 〓 ^n-1 (-1) ^n-1 × (A _D /A _U ) ⁿ cos {n(W _B t+θ _U −θ _D
)}...(11) becomes.

Also, when D/U > 1, under the conditions that A _U /A _D ≪1, (A _U /A _D ) ² ≪1, basically replacing the desired wave and the interference wave yields equation (11). Similarly, the discriminator output S is S = (W _D + θ' _D ) + W _B - θ' _D + θ' _U ) _∞ 〓 〓 ^n-1 (-1) ^n-1 × (A _U /A _D ) ⁿ cos {n(W _B t+θ _U −θ _D
)}...(12) is found.

That is, in equation (12) when D/U>1, the first term on the right side is the FM modulation component of the desired wave and the DC voltage term proportional to the carrier frequency, and the second term on the right side is the desired wave and the interference. These are the beat signal and its harmonics due to wave interference. The influence of interference waves is eliminated by extracting only the first term of this spectrum by a low-pass filter 4 whose passband extends from DC to the upper limit frequency of the FM modulation signal.

However, when D/U<1, the desired wave FM modulation component θ' _D disappears from the first term on the right side of equation (11).
For example, the number of users of transceivers using citizen bands has increased recently, and the situation has become such that interference cannot be avoided, and there is a good possibility that D/U will reverse. Therefore, with the conventional demodulator that suppresses the strong components of radio waves and the weak components as described above, there is a possibility that the desired communication may not be possible. In order to eliminate this drawback, there is a method as shown in FIG. 2 (a method related to Japanese Patent Application No. 53-4955 filed by the present applicant).

In FIG. 2, the input terminal 7 of the amplitude limiter 6
Apply IF band signal. Amplitude limiter 6 limits the amplitude of the applied signal and applies its output to frequency discriminator 8 . When D/U is greater than 1, the output of the frequency discriminator 8 is passed through a low-pass filter 9 having a pass band similar to the low-pass filter 4 in FIG. Extract the modulation component. The process up to this point is the same as in FIG.

However, as shown in equation (11) above, D/U is 1
When the FM modulation component θ′ _D of the desired wave is smaller than (11)
Focusing on the point that appears as amplitude modulation in the beat signal in the second term on the right side of the equation, part or all of the beat signal is extracted by the bandpass filter 11, and the envelope detector 12 extracts part or all of the beat signal.
The output of the bandpass filter 11 is detected and the signal containing the FM modulation component θ' _D of the desired wave is sent to the output terminal 1.
3 output signal. Therefore, although the signal from the output terminal 13 is disturbed by the FM modulation component θ' _U of the interfering wave, it also contains the FM modulation component θ' _D of the desired wave, so that the signal from the FM demodulator in FIG. The disappearance of such desired signals can be avoided.

However, the FM demodulator shown in FIG. 2 has the following drawbacks. That is, if the demodulator shown in FIG. 2 is attempted to be implemented in its original form, the improvement effect may not be partially achieved. The upper and lower cutoff frequencies of the band filter 11 are determined as follows. For the purpose of stabilizing the operation of the envelope detector 12, the cutoff frequency on the low frequency side (lower side) is set at the cutoff frequency of the desired wave and the interference wave.
Set the frequency higher than the highest frequency of the FM modulation components θ' _D and θ' _U (eg, audio frequency). On the other hand, the cutoff frequency on the high frequency side (upper side) has the same value as the bandwidth of the IF band (reception output frequency band where desired waves and interference waves exist). This is the maximum frequency of the beat signal obtained when the desired wave exists at one end of the IF band and the interference wave exists at the other end of the IF band. Therefore,
The bandpass filter 11 has a passband extending from a predetermined frequency higher than the highest frequency of the FM modulation components θ' _D and θ' _U to the maximum beat frequency (fundamental wave) of the desired wave and the interference wave. In order to set such a cutoff frequency, when the frequency interval between the desired wave and the interference wave becomes less than half the bandwidth of the IF band, the band filter 11 passes harmonics of the beat signal. It is predicted from the second term on the right side of equation (11) that the level of the desired wave FM demodulated signal included in the signal obtained from the envelope detector 12 becomes unstable due to the presence of harmonics of this beat signal. This can also be observed experimentally, with this level shifting with the D/U and desired-jamming frequency spacing, and even disappearing in extreme cases. Therefore, the emergence of a practical means for stably extracting a signal containing a desired wave FM modulation component is desired.

An object of the present invention is to eliminate the above-mentioned drawbacks and to obtain a practical FM demodulator that can stably extract a signal containing a desired wave FM modulation component.

The FM demodulator shown in Figure 2 above extracts the beat signal of the interfering wave and the desired wave, which contains desired wave FM modulation signal information, when there is interference in the IF band, but the beat frequency is The value is between zero and the frequency value equivalent to the bandwidth of the IF band. In the present invention, paying attention to this point, an unmodulated signal (hereinafter referred to as a local signal) with a frequency sufficiently far away from the IF band where the desired wave and the interfering wave exist is applied as an active interfering wave, and the local signal and the IF band signal are It uses the beat signal between. An embodiment of the present invention will be described below with reference to FIGS. 3, 4, and 5.

FIG. 3 is a block diagram of one embodiment of the present invention. In FIG. 3, the IF band signal applied to the input terminal 14 becomes one input signal of the adder 15. On the other hand, the local signal generator 16
A local signal with a frequency sufficiently distant from the IF band is output and applied to the adder 15 as the other input.
The output of the adder 15 is applied to the amplitude limiter 17,
The output of the amplitude limiter 17 is supplied to a frequency discriminator 18, and the phase change of the signal, that is, the frequency modulation signal is extracted. The output of the frequency discriminator 18 is then applied to a low pass filter 19 and a bandpass filter 20. The low-pass filter 19 is the same filter as the low-pass filters 4 and 9 in FIGS. 1 and 2, and when the desired signal level is higher than the interference signal level, the desired signal FM modulation component θ′ _D is extracted. . The band filter 20 has a band to be described later, and passes only the fundamental wave of the beat signal of the IF band signal and the local signal. The output of the bandpass filter 20 is sent to the envelope detector 21
The signal is detected without losing the desired wave FM modulation component θ' _D information.

Next, the differences between the FM demodulator shown in FIG. 2 and the embodiment of the present invention are as follows. That is, the local signal obtained by the local signal generator 16 in FIG. The passband is
This is the range of the frequency difference between the IF band signal and the local signal. Since a desired wave and an interference wave exist in the IF band signal, that is, the reception output frequency band, by applying a local signal, in addition to the beat signal of the desired wave and interference wave, the beat signal of the local signal and the IF band signal, i.e. A beat signal of the local signal and the desired wave and a beat signal of the local signal and the interference wave are generated. The bandpass filter 20 has a local signal and
Since the fundamental wave of the beat signal with the IF band signal is passed, from the output terminal 22 of the envelope detector 21,
Although it contains the beat components of the interference wave and the local signal, it is possible to extract a signal that contains the FM modulation component of the desired wave, which is superimposed on the beat signal of the desired wave and local signal in the form of amplitude modulation. Furthermore, it is considered convenient in practice to incorporate the adder 15 into a part of the amplitude limiter 17. Note that the terminal 23 is the output terminal of the low-pass filter 19.

4 and 5 are explanatory diagrams of the operation of the FM demodulator of FIG. 3. In Figure 4, local signal 4
1 is placed sufficiently away from the IF band signal 42.
As a result, the frequency spectrum at the output of the frequency discriminator 18 becomes as shown in FIG. Fifth
In the figure, the fundamental wave 511 of the beat signal of the local signal and the IF band input signal is generated at a location sufficiently far from the location where the frequency is zero (that is, direct current). The second harmonic 512 of the beat signal can be completely separated from the fundamental wave 511 of the beat signal. Since the third and higher harmonics of the beat signal have higher frequencies than the second harmonic 512, this frequency arrangement allows the bandpass filter 20 to extract only the fundamental wave 511 of the beat signal. Therefore, although the output of the bandpass filter 20 includes the local signal and the fundamental wave of the beat signal of the interference wave, in this way, the interference of higher harmonics can be completely eliminated. , the desired wave FM included in the signal obtained from the output terminal 22
The level of the modulation component becomes stable without being affected by external factors such as D/U and the frequency difference between the desired wave and the interfering wave.

On the other hand, in FIG. 5, in the frequency discriminator output of the FM demodulator in FIG. harmonic 52
2. Harmonics such as the third harmonic 523 are the fundamental wave 521
may be in the same frequency band as However, the third
The FM demodulator shown in the figure can completely separate the fundamental wave component of the beat signal of the local signal and the IF band input signal from its harmonic component and extract only the fundamental wave component. The instability of the desired wave FM modulation component contained in the wave component is eliminated.

When selecting the frequency of the local signal,
It is desirable to make the beat with the local signal occur at a frequency higher than the third harmonic region of the beat signal between the IF band input signals, but if the beat signal frequency is too high, it may cause problems in other aspects. There may be adverse effects.

As described above, according to the present invention, the FM demodulator can stably extract a signal containing the desired wave FM modulation component, so that even when the interference wave level is higher than the desired wave level, the output is The wave FM modulation component is not lost and can be effectively used to maintain and continue communication.

[Brief explanation of the drawing]

1 and 2 are block diagrams of the conventional example, FIG. 3 is a block diagram of an embodiment of the present invention, FIG. 4 is an explanatory diagram of the operation of the embodiment of the present invention, and FIG. 3 is a block diagram of FIG. 2 and an operation explanatory diagram of an embodiment of the present invention; FIG. 14...Input terminal, 15...Adder, 16...
Local signal generator, 17... Amplitude limiter, 18
...Frequency discriminator, 19...Low pass filter, 20
... Bandwidth filter, 21 ... Envelope detector, 22
...Output terminal, 23...Output terminal.

Claims (1)

[Claims] 1. A receiving means for receiving a signal including a frequency-modulated desired wave, a local signal generator for outputting a local signal of a frequency different from the output signal frequency band of the receiving means, and a local signal from the local signal generator. an adder that adds the signal and the output signal of the receiving means; an amplitude limiter that limits the amplitude of the output signal of the adder; and a frequency discriminator that is supplied with the output signal of the amplitude limiter and extracts a frequency modulated signal. a band filter connected to the frequency discriminator and having a pass band in a frequency range corresponding to the frequency difference between the output signal frequency band and the local signal; and an envelope detection of the output signal of the band filter to detect the desired signal. An FM demodulator comprising a detector that outputs a signal containing a wave frequency modulation component.