# US 4,397,040 A · UHF Receiver with decreased distortion due to multipath reception

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- Number: US 4,397,040 A
- Document ID: US4397040A
- Status: Granted
- Kind: A
- Application number: 06/111,706
- Filed: 1980-01-14
- Granted: 1983-08-02
- Earliest claimed: 1979-01-24
- Assignees: Blaupunkt-Werke GmbH
- Inventors: Peter Jandel; Klaus Vathauer
- Web page: https://patentgrep.com/patent/US4397040A

## CPC

- H04B 1/1081: {Reduction of multipath noise (by equalising H04B7/005)}
- H04B: TRANSMISSION

## Abstract

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.

## Claims

### Claim 1

In a UHF-FM receiver, particularly FM car radio, having
an intermediate frequency (IF) section including
amplitude demodulator means (17) having an output (14) strength signal indicative of the field strength of the signal received by said receiver, said field strength signal having amplitude modulation superimposed thereon only in the presence of multipath reception and automatic volume control means (19) having a signal input connected to said IF section and a control input;
and means (22, 23-26) for connecting said control input of said volume control means to said output (14) furnishing said field strength signal, said connecting means including means (25, 26) for establishing a predetermined time constant;
apparatus (27) for detecting multipath reception and furnishing a multipath signal indicative of said multipath reception connected to said demodulator means output (14), comprising
band-pass filter means (34) connected to said amplitude demodulator means for furnishing a filter output signal only in response to a field strength signal having said amplitude modulation; and
rectifier means (35) connected to said band-pass filter means and having a multipath signal output (29) for furnishing said multipath signal in response to said filter output signal; and
means (32, 33) connected to and controlled by said rectifier means (35) for increasing the time constant of said connecting means (22, 23-26) in response to said multipath signal.

### Claim 2 (depends on 1)

A UHF receiver as set forth in claim 1, wherein said band-pass filter means has a lower frequency limit of several kHz and an upper limiting frequency of less than 100 kHz.

### Claim 3 (depends on 1)

A UHF receiver as set forth in claim 1, wherein said band-pass filter means has a band-pass limited to 19 kHz.

### Claim 4 (depends on 1)

A UHF receiver as set forth in claim 1, wherein said band-pass filter means is limited to a frequency of 57 kHz.

### Claim 5 (depends on 1)

A UHF receiver as set forth in claim 1, wherein said time constant establishing means comprises a first capacitor (26); and
wherein said time constant increasing means comprises a second capacitor (33), and switch means (32) operative in response to said multipath signal for connecting said second capacitor in parallel with said first capacitor.