UHF Receiver with decreased distortion due to multipath reception

In a UHF receiver in which volume control is carried out as a function of received field strength, distortion due to multipath reception is prevented by use of a band-pass filter connected to the output terminal of the IF section which furnishes a signal indicative of received field strength. The low frequency noise signals appearing at this terminal in the presence of multipath reception are passed by the band-pass filter and subsequently rectified to create a DC control signal which closes a switch. This in turn increases the time constant of the connecting circuit between the field strength output terminal and the control input of the volume control circuit. The increase in time constant suppresses the above-mentioned low frequency oscillations which occur only during multipath reception, so that the output of the volume control circuit is not affected by such multipath reception.

The present invention relates to UHF receivers and, particularly, to FM 
receivers used in car radios. 
BACKGROUND AND PRIOR ART 
In known UHF receivers, a volume control is provided which varies the 
volume in dependence upon the field strength of the received signal. In 
the absence of a signal received at the antenna or in the presence of a 
very weak signal, the volume is substantially decreased so that the noise 
which would otherwise be generated is suppressed. For car radios, it is 
particularly desirable that the volume control be effected very rapidly 
following a change in field strength. The line connecting the output 
terminal of the IF section which furnishes the field strength signal to 
the control input of the volume control circuit thus has a very small time 
constant. The field strength signal at the corresponding output terminal 
of the IF section is first applied to an amplifier. An RC circuit is 
connected to the output of the amplifier, the time constant of the RC 
circuit determining the time constant of the connecting line. The control 
input of the volume control circuit is connected to the common point of 
the resistor and the capacitor of the RC circuit. 
If now two high frequency carrier oscillations are received at the antenna 
of the UHF receiver which originate from the same point but which, due to 
different transit times, have different modulation content, a so-called 
multipath reception results. This multipath reception causes, among other 
things, an amplitude modulation of the received signals which means that 
the signal is both amplitude and frequency modulated. As a result, 
distortion of a greater or lesser degree will occur. 
These low frequency amplitude modulations also affect the field strength 
signal. This in turn causes a modulation of the volume control signal 
which again causes distortion. 
THE INVENTION 
It is an object of the present invention to furnish a multipath signal 
indicative of multipath reception, in response to which the distortions 
resulting from multipath reception will be eliminated. 
It is a further object of the present invention to furnish a multipath 
signal generating circuit which operates precisely and reliably but is 
simple and inexpensive to construct. 
In accordance with the invention, a multipath signal furnishing apparatus 
comprises band-pass filter means connected to the output of the IF section 
furnishing the field strength signal, and rectifier means connected to the 
output of the band-pass filter means. The band-pass filter is tuned to 
frequencies which are particularly indicative of multipath reception. 
Preferably, the lower frequency limit is selected so that the low 
frequency field strength changes which result from movement of the car do 
not result in an output from the rectifier. The upper frequency limit is 
so chosen that interference from neighboring transmitters will not pass. 
In particularly preferred embodiments, the frequency of the band-pass 
filter is limited to 19 kHz or/and 57 kHz. These frequencies represent 
auxiliary carrier frequencies of stereo or ARI transmitters which tend to 
predominate the amplitude modulation during multipath reception. 
The so-generated multipath signal may be used to control a circuit which 
increases the time constant of the above-mentioned connecting line between 
the field strength output terminal of the IF section and the control input 
of the volume control circuit. Specifically, if an amplifier is connected 
to the field strength output terminal of the IF section and an RC circuit 
is connected to the output of this amplifier, then the time constant of 
the RC circuit is changed during multipath reception so that the RC 
circuit constitutes a low pass filter which suppresses the noise 
frequencies which are superimposed on the field strength signal during 
such multipath reception. 
The time constant of the RC circuit may be changed in a simple manner by 
activating an electronic switch which connects a second capacitor in 
parallel with the capacitor of the RC circuit.

The UHF receiver has a UHF section 10 whose input is connected to an 
antenna 11 and whose output is connected to an intermediate frequency 
section 12. This IF section 12 is an integrated circuit 13 and has a field 
strength output terminal 14. The integrated FM-IF circuit 13 includes a 
limiting amplifier 15, an FM demodulator 16 and an AM demodulator 17. The 
output of the amplifier 15 is connected to the input of the AM demodulator 
17 whose output constitutes the field strength output 14 of the integrated 
circuit 13. The output of FM demodulator 16 is connected to an output 
terminal 18 which, in turn, is connected to the input of a volume control 
circuit 19. Volume control circuit 19 has a control input terminal 20 and 
an output terminal 21. Output terminal 21 is connected to the low 
frequency section of the UHF receiver. 
For volume control as a function of received field strength, the control 
input 20 of volume control circuit 19 is connected to output terminal 14 
of IF section 13 through a transmission or connecting line 22. An 
amplifier 23 is connected in line 22. Connecting line 22 has a small time 
constant which is determined by an RC circuit 24 connected to the output 
of amplifier 23. RC circuit 24 includes a resistor 25 and a capacitor 26 
connected in series to each other, the series connection being connected 
in parallel to the output of amplifier 23. The control input of volume 
control circuit 19 is connected to the common point of resistor 25 and 
capacitor 26. 
The circuit for detecting, and furnishing the multipath signal indicative 
of multipath reception is denoted by reference numeral 27 and has an input 
terminal 28 connected to output terminal 14 of IF section 13. Circuit 27 
has an output terminal 29 at which the multipath signal is furnished 
during multipath reception at the antenna. 
A time constant regulator unit 30 having an input 31 connected to terminal 
29 causes the time constant of RC circuit 24 to be increased in response 
to the multipath signal. Specifically, in a preferred embodiment, circuit 
30 comprises an electronic switch 32 which closes in response to the 
multipath signal and causes a capacitor 33 to be (e.g. C26=47 .mu.F) 
connected in parallel with capacitor 26. (e.g. C33=0.47 .mu.F) 
Circuit 27 includes a band-pass filter 34, and a rectifier 35 connected to 
the output of the band-pass filter. The lower limiting frequency of 
band-pass filter 34 is in the order of several kHz, while its upper 
limiting frequency is less than 100 kHz. These limiting frequencies are so 
chosen that changes in the signal at terminal 14 resulting from field 
strength changes due to the movement of the car in which the receiver is 
located are blocked by band-pass filter 34 and, further, that interference 
which is the result of transmitters located in the vicinity of the car 
will also be blocked. In a preferred embodiment, band-pass filter 34 has 
an extremely small band-pass centered around either 19 kHz and/or 57 kHz. 
These frequencies, which are auxiliary carrier frequencies transmitted by 
stereo or ARI transmitters, tend to predominate in the amplitude 
modulation resulting from multipath reception. 
OPERATION 
As mentioned previously, circuit 19 controls the volume as a function of 
received field strength so that loud noises in the speaker during weak or 
missing antenna signals are suppressed. Especially when the receiver is 
located in an automobile, it is desirable that this volume regulation take 
place very rapidly following a change in field strength. RC circuit 24 
therefore has a relatively low time constant, that is the capacity of 
capacitor 26 is relatively small. (e.g. R25=15 k.OMEGA.; C26=47 .mu.F) The 
field strength signal derived from terminal 14 is amplified in amplifier 
23 and is then derived at the common point of resistor 25 and capacitor 26 
and applied to control input 20 of volume control circuit 19. If two high 
frequency carrier oscillations are received at antenna 11 which originate 
at the same point but, due to different transit times, have different 
modulation content, the so-called multipath reception results. This 
multipath reception leads to an amplitude modulation of the received 
signals. This amplitude modulation is detected by AM demodulator 17 and is 
superimposed as a low frequency oscillation on the signal at terminal 14. 
This low frequency oscillation is applied to input terminal 28 of 
band-pass filter 34. This low frequency oscillation is within the 
band-pass of band-pass filter 34. Thus, filter 34 detects the low 
frequency amplitude modulation of the signal at terminal 14. A signal is 
therefore applied to the input of rectifier 35, causing a DC output signal 
to appear at terminal 29. This DC output signal, the multipath signal, 
causes switch 32 to close, connecting capacitor 33 in parallel with 
capacitor 26. This increase in capacitance causes RC circuit 24 to act as 
a low pass filter which blocks the low frequency oscillations appearing at 
terminal 14 due to multipath reception from control input 20 of volume 
control circuit 19. The distortion in the output signal of volume control 
circuit 19 due to multipath reception is therefore prevented. As soon as 
the multipath reception ceases, no signal appears at terminal 29, switch 
32 opens and capacitor 33 is again disconnected from the circuit. 
Various changes and modifications may be made within the scope of the 
inventive concepts.