JPS583611B2 - FM receiver - Google Patents

Description

[Detailed description of the invention] The present invention relates to an FM receiving device.

In order to improve the S/N of an FM detector, there are methods to reduce the noise generated from the semiconductor components used in the detector itself, or to improve the S/N by devising the detector itself.

In the former method, there is a limit to the noise reduction of the semiconductor component itself.

As an example of the latter method, attempts have been made to improve the S/N ratio by performing so-called double conversion using a detector such as pulse count detection in which the demodulation efficiency increases as the carrier frequency becomes lower. S/N even with the method
It is difficult to make significant improvements in

An object of the present invention is to provide an FM receiver that can significantly improve S/N.

The FM receiving device of the present invention converts the received signal into an intermediate frequency signal (hereinafter abbreviated as IF signal) of 10.7 MHz, and then
Nth harmonic signal of the IF signal (N is an integer of 2 or more)
is selected and derived by a resonant circuit, and then this Nth-order high-wave signal is again frequency-converted into an IF signal frequency using a heterodyne method.

It is characterized in that this IF signal is subjected to FM detection by a wideband detection circuit in the next stage.

According to the FM receiver of the present invention, only the Nth harmonic signal included in the IF signal is extracted, and this is converted back to the IF signal frequency, and then FM detection is performed. The frequency deviation in the harmonic signal is N times the frequency deviation (75 KHz) of the IF signal, 10.7 MHz. Therefore, the signal with such a large frequency deviation is re-transformed using the heterodyne method. If the frequency is converted to 10.7 MHz, a frequency shift of N times that of the initial IF signal is obtained.

As a result, if the signal is input to a well-known wideband FM detector and detected, the detected output will be multiplied by N, resulting in a significant improvement in the S/N ratio.

The present invention will be described below with reference to embodiments and the accompanying drawings.

The figure is a block diagram showing an embodiment of the present invention, in which the FM signal received by the antenna 1 is high-frequency amplified by the front end 2 and transmitted to the IF signal having a carrier wave of 10.7 MHz.
converted into a signal.

This IF signal is amplified by an intermediate frequency amplification circuit 3 at the next stage, and is input to a limiter circuit 4 to reduce noise.

lagoon be done.

For example, in the IF signal that is the input signal to the resonant circuit 5, a third harmonic signal component, that is, a signal of 32.1 MHz exists at a level of about 173 of the fundamental wave signal. A wave signal is selectively derived by a resonant circuit 5.

By the way, the frequency deviation of this third harmonic is three times that of the fundamental wave.

In other words, if the frequency deviation is 75 KHz at 10.7 MHz, the frequency deviation is 225 KHz at 32.1 MHz.

The signal whose frequency shift has become three times larger in this way is converted to the original IF by the frequency conversion circuit 10 using the heterologin method.
Frequency conversion is performed to the signal frequency (10.7 MHz).

The frequency conversion circuit 10 includes a local oscillation circuit 6, a mixing circuit 7, and a resonance circuit 8.

If the oscillation frequency of the local oscillation circuit 6 is 10.7 MHz x 2 or 10.7 MHz x 4, the output of the mixing circuit 7 has carrier frequencies of 10.7 MHz and 53.5 MHz or 10.7 MHz and 74.9 MHz. A signal is output.

Therefore, only the IF signal having a carrier frequency of 10.7 MHz is selectively derived by the next-stage resonant circuit 8.

The frequency deviation of the IF signal obtained by this resonant circuit 8 is 225 KHz, that is, three times the frequency deviation of 75 KHz at the input of the resonant circuit 50.

Therefore, this IF signal is transmitted to the next stage broadband detection circuit 9.
If M detection is performed, the detection output will be three times that of the conventional one, and as a result, the S/N will be improved by about 10 dB.

In the above embodiments, a method using a third harmonic signal was shown, but in general, an Nth harmonic signal is used.
Frequency conversion circuit 100 local oscillation frequency (N+1)
The next or (N-1)th order harmonic frequency can be used.

Of course, even if extremely high-order harmonics are used, the level will be extremely small in the IF signal and will not contribute much to improving the S/N ratio.

[Brief explanation of the drawing]

The figure is a block diagram showing an embodiment of the present invention. Explanation of symbols of main parts, 3... Intermediate frequency amplification circuit, 5, 8... Resonant circuit, 6... Local oscillation circuit, 7... Mixed circuit, 9...F
M detection circuit, 10... frequency conversion circuit.

Claims (1)

[Claims] A signal conversion circuit that converts an IFM reception signal into an intermediate frequency signal, and an Nth harmonic signal (N is 2) of the intermediate frequency signal.
a resonant circuit for selectively deriving the Nth integer);
An FM receiver comprising: a frequency conversion circuit that converts the frequency of a harmonic signal to the frequency of the intermediate frequency signal; and a detection circuit that performs FM detection on the intermediate frequency signal obtained by the frequency conversion circuit. 2 The frequency conversion circuit converts the intermediate frequency signal to the (N+
1) or a local oscillation circuit that generates a (N-1)th harmonic frequency, and a mixing circuit that mixes the output signal of the local oscillation circuit and the Nth harmonic signal of the resonant circuit;
2. The FM receiving device according to claim 1, further comprising a resonant circuit that selectively derives the intermediate frequency signal from the output signal of the mixing circuit.