JPS59158111A - Demodulation circuit - Google Patents

Abstract

PURPOSE:To perform AM and FM demodulation without switching time constants nor using and changeover switch by converting the frequencies of an AM and an FM signal to the same intermediate frequency, and installing a phase-locked loop which is locked at the intermediate frequency. CONSTITUTION:The AM and FM signals are converted to the intermediate frequency f0 and then amplified by an amplification part 6 and supplied to the common phase-locked loop 14 constituting a demodulation circuit. An FM demodulation output appears at the output side of a filter 18 and is supplied to a frequency mixer circuit 12 for the FM signal. On the other hand, it is passed through a filter 24 to obtain a sensor output. The output of a VCO20 is delayed by pi/2 through a phase circuit 26 and inputted to a synchronous AM detecting circuit 28 together with the output of the amplification part 6, obtaining an AM demodulation output S0. The output S0 is supplied to a mixer circuit 4 as an automatic gain adjustment signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a demodulation circuit that demodulates an AM signal and an FM signal, and particularly relates to a demodulation circuit that makes the intermediate frequencies of the AM signal and the FM signal the same and uses a common phase-locked loop.

Conventionally, when demodulating an AM signal or FM signal,
In a demodulation circuit composed of a phase-locked loop (PLL),
Install a demodulation circuit for each AM signal and FM signal, or
A common demodulation circuit is installed that enables demodulation of M and FM signals. If demodulation circuits dedicated to AM- and FM are configured separately as in the former case, there is a drawback that the configuration becomes complicated and the price of the device increases. Further, when a common demodulation circuit is installed as in the latter case, it is necessary to change the time constants of the filter and the voltage controlled oscillator using a switch. Such switching of the time constant causes errors due to stray capacitance of the switch, and has drawbacks such as poor reliability.

This invention changes the frequencies of AM and FM signals to the same intermediate frequency, installs a phase-locked loop that synchronizes with this intermediate frequency, and eliminates the need for time constant switching and changeover switches as in the past. An object of the present invention is to provide a demodulation circuit capable of performing FM demodulation.

The present invention includes a phase-locked loop that demodulates an FM signal set to the same intermediate frequency as the intermediate frequency of the AM signal, and a synchronous AM demodulation loop that includes this phase-locked loop in the loop and demodulates the AM signal. It is characterized by what it did.

Embodiments of the present invention will be described in detail below with reference to the drawings.

The figure shows an embodiment of the demodulation circuit of the present invention.

The AM signal received by the antenna 2 is converted to an intermediate frequency fo via a frequency mixer circuit 4 serving as a frequency conversion means, and then amplified by an intermediate frequency amplification section (IF) 6. Further, the FM signal received by the antenna 8 is input to the intermediate frequency amplification section 6 via a frequency mixer circuit 10.12 as a frequency conversion means. In this embodiment, the resistance to the intermediate frequency of the FM signal is constituted by two frequency mixer circuits 10.12, or the frequency mixer circuit 12.
may be changed to the intermediate frequency fo using a frequency dividing circuit.

Then, the AM signal or FM signal amplified by the intermediate frequency amplification section 6 is transmitted to a common phase-locked loop 1 constituting a demodulation circuit.
given to 4. That is, this phase locked loop 14 is composed of a phase comparator 16, a filter 18, and a voltage controlled oscillator 20, and the output of the voltage controlled oscillator 20 is fed back to the phase comparator 16. The FM demodulation output is taken out to the output side of the filter 18 of this phase-locked loop 14, and this FM9
The fi-tuned output is given as a frequency control signal to the extreme oscillation part of the frequency mixer circuit 12 for the FM signal via the resistor output 2. Further, this demodulated output becomes one sensor output for controlling the position of the frequency fo via the filter circuit 24.

Further, a phase shift circuit 26 is installed on the output side of the voltage controlled oscillator 20 to delay the phase shift of the output signal by π/2, and the output signal of this phase shift circuit 26 is transmitted to the intermediate frequency amplification section 6.
is input to the synchronous AM detection circuit 28 together with the intermediate frequency signal of , and the AM demodulation output S
You can get it outside.

That is, the phase-locked loop 14, the phase shift circuit 26, and the synchronous AM detection circuit 28 function as an AMM modulation circuit.
An MM tone loop 30 is configured.

The Alvi demodulated output So is given to the frequency mixer circuit 4 as an automatic gain adjustment signal via a filter 32. Further, this AM demodulated output So is passed through a filter 34 to an AND circuit 3 together with the output of the filter circuit 24.
6, and the output generated at the output terminal 38 of this AND circuit 36 is used as a tuning control signal for the autotuning circuit. The auto-tuning circuit includes a system in which the variable capacitor of the tuning circuit is rotated by servo control,
There is a method of tuning the varistor voltage to a desired frequency by varying the voltage of the varistor, and the output of the AND circuit 36 can be used as a control signal for these methods.

According to the configuration described above, the frequencies of the AM signal and the FM signal are changed to the same intermediate frequency, and the phase-locked loop 14 constituting the basic circuit of the demodulation circuit is shared.
When receiving a signal, the output of the phase-locked loop 14 is used to confirm the synchronous frequency f, and the output of the synchronous AM detection circuit 28 is used as an AMM modulation output. Further, when receiving an FM signal, the phase-locked loop 14 is used as an FM demodulation circuit, and the loop of the synchronous AM detection circuit 28 is used as a signal strength detection circuit.

In this way, by making the intermediate frequencies the same for AM signal and FM signal demodulation, the basic loop of the AM/FM demodulation circuit is shared, so each AM-FM demodulation circuit can be configured as a common block, and the configuration can be simplified. In particular, for demodulating AM and FM signals, there is no need to change the time constant by switching the filter or capacitor connected to the voltage controlled oscillator, as in the case of conventional methods, eliminating the need for a changeover switch and reducing errors caused by floating switches. This can be prevented and the reliability of demodulation can be improved. Further, with such a configuration, the auto-tuning circuit has a high ability to follow the input signal strength of the stop signal, which is noticeable during demodulation of an AM signal.

As explained above, according to the present invention, a common phase-locked loop is used for demodulating AM signals and demodulating FM signals. Switching becomes unnecessary, and the reliability of demodulation can be improved.

[Brief explanation of drawings]

The figure is a block diagram showing an embodiment of the demodulation circuit of the present invention. 6... Intermediate frequency amplification section, 14... Phase locked loop,
26... Phase shift circuit, 30... Synchronous AM (Xiagen Shu rule)
F.

Claims (1)

[Claims] A phase-locked loop that demodulates an FM signal set to the same intermediate frequency as the intermediate frequency of the AM signal, and a synchronous AM that includes this phase-locked loop in the loop and demodulates the AM signal.
A demodulation circuit characterized by comprising a demodulation loop.