JPS6248416B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multipath interference detection circuit for detecting the level of multipath interference in an FM receiver.

(Prior Art) A conventional multipath interference detection circuit in an FM receiver, for example, as shown in FIG. The output signal of the gain control amplifier 1 is subjected to AM detection by an amplitude detector (hereinafter referred to as AM detector) 2, and the AM detection output is supplied to a bandpass filter 3 that passes the 19kHz component in the AM detection output. The output of the filter 3 is configured to be supplied to the indicator 4, and the AM modulation component of the main carrier wave caused by the interference of the incoming reflected RF signal, that is, the envelope of the intermediate frequency signal, is detected.
The level of the pilot signal frequency component in this envelope was displayed.

In addition, 6 indicates the FM antenna, 7 indicates the front end, 8 indicates the intermediate frequency amplification stage, 9 indicates the FM demodulation stage, and 10 indicates the stereo demodulation stage.
It constitutes an FM stereo tuner.

However, when using the conventional multipath interference detection circuit as described above, the interference is caused by the frequency shift caused by the main signal and the sub signal, and by the phase difference between the desired RF signal and the reflected RF signal at the time of incidence on the antenna 6. However, the level of the pilot signal frequency component or the level of the frequency component that is an integral multiple thereof in the envelope of the carrier changes as described later, and there is a drawback that multipath interference cannot be detected correctly. Another disadvantage is that the input signal of the indicator 4 cannot be used as a signal for canceling the reflected RF signal.

Now, assuming that the right channel audio signal and the left channel audio signal are equal, a change in the pilot signal level in the carrier envelope due to the multipath signal will be explained.

A _D ...Desired RF signal, A _R ...Reflected RF signal, ω
_c ...Angular frequency of main carrier when not modulated (=
2π _c ), ω _n ... Angular frequency of main signal (L+R) (=2π _n ), ω _p ... Angular frequency of pilot signal (=2π _p ), _n ... Frequency of main signal, _p ... Pilot signal frequency, _Dn ...Frequency deviation due to the main signal, _Dp ...Frequency deviation due to the pilot signal, r...Relative level of the main carry amplitude of the reflected RF signal to the desired RF signal (1>r>0), t ₀ ... Desired RF of reflected RF signal
When the delay time for the signal is taken as A _D = sin [ω _c t + D _n / _n sinω _n t + D _p / _p sinω _p t] A _R = sin [ω _c (t-t ₀ ) + D _n / _n sinω _n ( t − t ₀ ) + D _p / _p sinω _p (t − t ₀ )] The composite RF signal incident on the antenna 6 is A _D + A _R = R(t) sin [ω _c t + D _n / _n sinω _n t + D _p / _p sinω _p t-α(t)], where R(t)=√1 ² + ² +2 α(t)=tan ^-1 rsinθ/1+rcosθ θ=2D _n / _n sinω _n /2t ₀ cosω _n (t-1/2t ₀
) +2D _p / _p sinω _n /2t ₀ cosω _p (t-1/2t ₀
)+ω _c t ₀ .

Therefore, the ratio between the desired RF signal and the reflected RF signal is
10dB, 7.5k frequency deviation due to pilot signal
Hz, the delay time is 10 μs + 1 period of the carrier, the main signal frequency is 1000 Hz, and the frequency deviation due to the main signal is 0 kHz. Pilot signal frequency component in the carrier envelope for the phase difference between the desired RF signal and the reflected RF signal. The spectrum level of the pilot signal and the spectrum level of the frequency component twice that of the pilot signal are respectively the second
As shown in Figure a and Figure 2b. Figure 2a
As is clear from and b, the spectrum level changes depending on the phase difference.

Also, the ratio of the desired RF signal to the reflected RF signal is
10dB, the frequency deviation of the pilot signal is 7.5kHz,
Delay time is 10μs, main signal frequency is 1000Hz
The spectral levels of the pilot signal frequency components in the carrier envelope with respect to the frequency deviation caused by the main signal are as shown in FIGS. 3a and 3b. In addition,
FIG. 3a shows the case where the phase difference between the desired RF signal and the reflected RF signal is 0 degrees, and FIG. 3b shows the case where the phase difference is 90 degrees.

In addition, when the frequency of the main signal is 9500Hz for Figure 3 a and b, Figure 3 a
3c as shown in FIG. 3c, and FIG. 3b as shown in FIG. 3d.

Also, the ratio of the desired RF signal to the reflected RF signal is
10dB, 7.5k frequency deviation due to pilot signal
Hz, delay time 10μs, main signal frequency 1000
The spectrum level of the component twice the pilot signal frequency in the envelope of the carrier with respect to the frequency of the main signal, when expressed as Hz, is as shown in FIGS. 3e and 3f.
Note that Figure 3e shows the case where the phase difference between the desired RF signal and the reflected RF signal is 0 degrees, and Figure 3f shows the case where the phase difference is also 0 degrees.
The case of 90 degrees is shown.

In addition, when the frequency of the main signal is 9500Hz for Fig. 3 e and f, Fig. 3 e
, as shown in Figure 3g, and Figure 3f
The result is as shown in FIG. 3h.

As is clear from FIGS. 3a to 3h, the level of the pilot signal frequency component in the carrier envelope and the level of the component twice the pilot signal frequency vary in accordance with the frequency shift caused by the main signal.

(Object of the Invention) The present invention has been made in view of the above, and it is an object of the present invention to provide a multipath interference detection circuit that can eliminate the above-mentioned drawbacks and stably detect multipath interference.

This purpose, according to the present invention, includes an AM detector to which an intermediate frequency signal before amplitude limitation is supplied in the FM receiver, and an AM detector to which the output of the AM detector is supplied and whose center frequency is an integer multiple of the pilot signal frequency. a set bandpass filter, a switch circuit to which the output of the bandpass filter is supplied, a hold capacitor charged by the output of the bandpass filter via the switch circuit, and a hold capacitor charged with the output of the bandpass filter via the switch circuit;
and a comparator that controls the switch circuit to be in the on state when the FM demodulation output of the FM receiver is supplied and the FM demodulation output is within a predetermined range, and the voltage of the hold capacitor is set as the multipath interference detection output. This is achieved by

Hereinafter, the present invention will be explained by examples.

(Structure in one embodiment of the invention) FIG. 4 is a block diagram showing an embodiment of the invention.

In one embodiment of the present invention, an intermediate frequency signal before amplitude limitation in an FM tuner consisting of an antenna 6, a front end 7, an intermediate frequency amplification stage 8, an FM demodulation stage 9, and a stereo demodulation stage 10 is transferred to an automatic gain control amplifier 1. and amplify it. The output signal of the automatic gain control amplifier 1 is supplied to an AM detector 2 for AM detection. The output of the AM detector 2 is supplied to a bandpass filter 3 whose center frequency is set to the pilot frequency to extract the pilot frequency component from the output of the AM detector 2. The output of the band pass filter 3 is supplied to the hold capacitor 12 via the switch circuit 11, and the hold capacitor 12 is charged with the output of the band pass filter 3. The voltage on the hold capacitor 12 supplies the indicator 4. On the other hand, the demodulated output of the FM demodulation stage 9 is supplied to a window comparator (hereinafter simply referred to as a comparator) 13. The comparator 13 has a signal corresponding to a predetermined frequency deviation of the carrier.
DC voltage source 14 equal to the output voltage of the FM demodulation stage 9
₁ and ₁₄₂ voltages are supplied as reference voltages. The output of the comparator 13 is supplied to the switch circuit 11, and when a voltage within the reference voltage range is output from the FM demodulation stage 9, the switch circuit 11 is controlled to be turned on.

(Operation in one embodiment of the invention) In one embodiment of the invention configured as described above, the intermediate frequency signal of the FM tuner before being amplitude limited is amplified by the automatic gain control amplifier 1. Since the intermediate frequency signal before being amplitude limited by the automatic gain control amplifier 1 is amplified, the input level of the antenna 6 reaches almost the same level from a small level to a large level. The intermediate frequency signal amplified to the same level and before amplitude limitation is subjected to AM detection by an AM detector 2, and the detection output of the AM detector 2 is supplied to a bandpass filter 3.

If a reflected RF signal exists together with the desired RF signal, the desired RF signal is AM modulated by the interference of the reflected RF signal. Therefore, the intermediate frequency signal before amplitude limitation is also AM modulated, and the AM detector 2 detects the AM signal, and its envelope output becomes the AM signal.
Only the pilot signal frequency component outputted from the wave detector 2 is outputted from the bandpass filter 3.

On the other hand, when the frequency deviation of the carrier is less than the predetermined frequency deviation, the comparator 13 generates a high potential output, and the switch circuit 11 is restricted to the on state.
Hold capacitor 12 is bandpass filter 3
It is charged by the output of On the other hand, when the frequency deviation of the carrier exceeds the predetermined frequency, the comparator 13 generates a low potential output, and the switch circuit 1
1 is controlled to be in an off state, and the charging voltage of the hold capacitor 12 is held.

Therefore, by setting the predetermined frequency deviation low, even if the output level from the bandpass filter 3 fluctuates due to the frequency deviation caused by the main signal as shown in FIG. Voltage fluctuations are suppressed to a minimum, and the influence of output level changes due to frequency deviation fluctuations is almost eliminated, allowing the indicator 4 to accurately and stably indicate multipath interference.

The same effect can be obtained even if the center frequency of the bandpass filter 3 is set to a frequency that is an integral multiple of 2 or more of the pilot signal frequency.

Next, other embodiments of the present invention will be described.
(Structure in Another Embodiment of the Invention) FIG. 5 is a block diagram showing another embodiment of the invention.

In another embodiment of the present invention, the level of the input signal is further compared between the band pass filter 3 and the switch circuit 11 to select the higher level of the input signal. A large amplitude selection circuit 17 to output, a bandpass filter 15 which receives the output signal of the AM detector 2 and whose center frequency is set to twice the pilot signal frequency, and an amplifier to amplify the output of the bandpass filter 15. 16, and the output of the bandpass filter 2 and the output of the amplifier 16 are connected to a large amplitude selection circuit 17.
and the output of the large amplitude selection circuit 17 is supplied to the switch circuit 11.

Note that the gain of the amplifier 16 is set to about 15 dB here.

(Operation in other embodiments of the present invention) In one embodiment of the present invention configured as described above, the pilot signal frequency component in the output signal of the AM detector 2 is extracted by the bandpass filter 3, and the pilot signal frequency component is extracted by the bandpass filter 3. A frequency component twice the frequency is extracted by a bandpass filter 15.
In this case, the frequency component twice the pilot signal frequency is approximately higher than the pilot signal component.
Since it is 15 dB lower, the amplifier 16 amplifies it by about 15 dB. The output level of the bandpass filter 15 amplified by the amplifier 16 and the output level of the bandpass filter 3 are compared in the large amplitude selection circuit 17, and the large output level side is selected, and the output level is transferred to the hold capacitor 12 via the switch circuit 11. is supplied to charge the hold capacitor 12. However, as in the case of one embodiment of the present invention, when the frequency deviation of the carrier is less than a predetermined frequency deviation, the switch circuit 11
is turned on, the hold capacitor 12 is charged to the output level of the large amplitude selection circuit 17, and when the predetermined frequency deviation is exceeded, the switch circuit 11 is charged to the output level of the large amplitude selection circuit 17.
is turned off, and the charging voltage of the hold capacitor 12 is held. Therefore, the influence of changes in the output level of the capacitor 12 due to changes in frequency deviation is almost eliminated.

Furthermore, even if the phase difference between the desired RF signal and the reflected RF signal changes as shown in FIG.
Both the pilot signal frequency component and the frequency component twice the pilot signal frequency in the output signal of the wave detector 2 do not decrease, and in another embodiment of the present invention, the large amplitude selection circuit 17 selects the Bandpass filter 15 amplified by 16
Since the high level side of the output of the bandpass filter 3 and the output of the bandpass filter 3 is selected, even if the phase difference between the desired RF signal and the reflected RF signal fluctuates, the influence of the fluctuation is eliminated, and the output voltage of the hold capacitor 12 is stable.

Therefore, according to another embodiment of the invention, the multipath jammer detection signal is
It is also stabilized by fluctuations in the phase difference between the RF signal and the reflected RF signal.

(Effects of the Invention) As explained above, according to the present invention, the multipath interference detection signal is not affected by the frequency deviation caused by the main signal and sub signals, and the multipath interference detection signal is not affected by the frequency deviation caused by the main signal and sub signals, and the desired RF It is not affected by the phase difference between the signal and the reflected RF signal and remains stable. Therefore, the interference detection signal is stable when displayed on an indicator, and can also be used as a signal for canceling reflected waves.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional multipath interference detection circuit. Figures 2 and 3 are characteristic diagrams showing the level change of the carrier amplitude modulation component with respect to the phase difference between the desired RF signal and the reflected RF signal, and the level change of the carrier amplitude modulation component with respect to the frequency shift caused by the main signal. Characteristic diagram. FIG. 4 is a block diagram showing one embodiment of the present invention. FIG. 5 is a block diagram showing another embodiment of the present invention. 1... Automatic gain control circuit, 2... AM detector,
3 and 15...Band pass filter, 4...Indicator, 7...Front end, 8...Intermediate frequency amplification stage, 9...FM demodulator, 11...Switch circuit, 12...Hold capacitor, 13... ...Comparator, 17...Large amplitude selection circuit.

Claims (1)

[Claims] 1. An amplitude detector to which an intermediate frequency signal before amplitude limitation in an FM receiver is supplied; and an amplitude detector to which the output of the amplitude detector is supplied and whose center frequency is set to a frequency that is an integral multiple of the pilot signal frequency. a switch circuit to which the output of the band pass filter is supplied, a hold capacitor charged by the output of the band pass filter via the switch circuit, and an FM demodulator output of the FM receiver. and a comparator that controls the switch circuit to turn on when the FM demodulator output is within a predetermined range, and the voltage of the hold capacitor is used as a multipath interference detection output. Multipath interference detection circuit. 2. an amplitude detector to which an intermediate frequency signal before amplitude limitation in the FM receiver is supplied; a first bandpass filter to which the output of the amplitude detector is supplied and whose center frequency is set to the pilot signal frequency;
a second bandpass filter to which the amplitude detector output is supplied and whose center frequency is set to twice the pilot signal frequency; an amplifier for amplifying the output of the second bandpass filter; an amplitude selection circuit that compares the output level of the first breadth pass filter with the output level of the amplifier and selects the larger level output; a switch circuit to which the output of the amplitude selection circuit is supplied; a hold capacitor charged by the output of the amplitude selection circuit through the FM receiver;
and a comparator that controls the switch circuit to be on when an FM demodulation output is supplied and the FM demodulation output is within a predetermined range, and the voltage of the hold capacitor is used as a multipath interference detection output. Multipath interference detection circuit.