JPS58131806A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To make a center frequency of an intermediate frequency signal coincident with a demodulation center frequency, by shifting the demodulation center frequency to the center frequency of a shifted intermediate frequency signal if the center frequency of the intermediate frequency signal is shifted from the demodulated center frequency. CONSTITUTION:If the center frequency of the intermediate frequency signal is shifted from the demodulated center frequency, a potential difference produced between the frequencies is detected at a potential detection circuit 6, a correction voltage in response to the potential difference is applied to a varactor diode 5 via a sampling circuit 8 from a voltage amplifier 7, the tuning frequency of a tuning circuit 4 is changed, the demodulated center frequency is shifted to the center frequency of the intermediate frequency signal, and when the circuit 6 detects the zeroed potential difference, the operation of the center frequency correction loop is stopped. Thus, the trouble of the adjustment of the demodulation circuit and the tuning circuit is omitted at the manufacture of products, allowing to reduce the manufacture cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM demodulation circuit that demodulates an intermediate frequency signal while matching the center frequency of the intermediate frequency signal with the demodulation center frequency using a tuning circuit.

Figure 1 shows the conventional F of the so-called quadrature demodulation method.
This figure shows the configuration of the M demodulation circuit. 1 is a demodulation circuit equipped with a phase rotation tuning circuit 2, and the phase rotation tuning circuit 2 converts the input intermediate frequency signal into After the phase is shifted around the center frequency, the internal multiplier is guided to an arithmetic circuit for quadrature demodulation, and a demodulated signal, a reference potential signal and a potential difference signal to be input to an automatic frequency control circuit (not shown) are output. be done.

In the conventional example configured in this way, the center frequency of the intermediate frequency signal is the demodulation center frequency f. When it matches,
The frequency-output voltage characteristic of the demodulated signal becomes as shown in FIG. 2, and the potential difference signal is not output, but the center frequency of the intermediate frequency signal and the demodulated center frequency f. and maintain a consistent state. However, due to deviations in the oscillation frequency of the local oscillation circuit, etc., the center frequency of the intermediate frequency signal changes to the demodulation center frequency f.
. When the voltage becomes lower by Δfa or higher by Δfb, a potential difference of -ΔVa or +Δ■b occurs between them, and the voltage -Δva or +■b is applied to the automatic frequency control circuit, and the oscillation frequency of the local oscillation circuit is changed. changes,
Since the center frequency of the intermediate frequency signal is pulled back to the demodulation center frequency fO, the center frequency of the intermediate frequency signal is changed to the demodulation center frequency f. can be held. By the way, in order to perform such an operation, the demodulation center frequency must be exactly f. When assembling the product, it is necessary to adjust the demodulation circuit 1 and the phase rotation tuning circuit 2, which requires additional equipment and personnel, resulting in an increase in manufacturing costs.

In order to eliminate the drawbacks of the conventional example, the present invention, when the center frequency of an intermediate frequency signal deviates from the demodulation center frequency, shifts the demodulation center frequency to the center frequency of the deviated intermediate frequency signal, and The present invention provides an FM demodulation circuit that matches the center frequency of a signal with the demodulation center frequency. Below, embodiments of the present invention will be described in detail.

FIG. 3 shows the configuration of an embodiment of the present invention, where 3 is a demodulation circuit, 4 is a tuning circuit in which a variable capacitance diode 5 is connected in parallel with a fixed capacitor, and the tuning frequency is made variable; is the difference between the center frequency of the intermediate frequency signal and the demodulation center frequency fc, l! 7 is a voltage amplifier serving as a correction voltage generating means that outputs a correction voltage corresponding to the potential difference detected by the potential difference detection circuit 6; 8 is a sampling circuit; This sampling circuit 8 applies the corrected voltage to the variable capacitance diode 5, and this sampling circuit 8 prevents the voltage amplifier 7 and the variable capacitance diode 5 from being directly connected, thereby reducing the voltage difference between the demodulation circuit 3 and the potential difference detection circuit. 6. Voltage amplifier 7
The relationship between the loop time constant of the center frequency correction loop constituted by the variable capacitance diode 5 and the tuning circuit 4 and the low frequency signal component of the demodulated signal prevents the DC component from not being demodulated.

In this embodiment configured as described above, when the center frequency of the intermediate frequency signal and the demodulation center frequency deviate, the potential difference generated between them is detected by the potential difference detection circuit 6, and a correction voltage is applied to the voltage amplifier according to the potential difference. 7 to the capacitance diode 5 via the sampling circuit 8, the tuning frequency of the tuning circuit 4 changes l~, the demodulation center frequency is shifted by the center frequency 1 of the intermediate frequency signal, and the electric power difference l~ is applied. When the potential difference detection circuit 6 detects that the voltage has become zero, the operation of the center frequency correction loop is stopped.

Note that the present invention can be implemented in any demodulation circuit having a tuning circuit, such as a ratio demodulation circuit, a Foster-Seeley demodulation circuit, a quadrature demodulation circuit, a Travis Smith demodulation circuit, and the like.

As explained above, according to the present invention, even if the center frequency of the intermediate frequency signal and the demodulation center frequency deviate, the tuning frequency of the tuning circuit changes, so that the demodulation center frequency matches the center frequency of the intermediate frequency signal. Therefore, there is no need to adjust the demodulation circuit and the tuning circuit when assembling the product, and the equipment and personnel required for this can be omitted, which has the advantage of reducing manufacturing costs.

5-

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a conventional FM demodulation circuit, Figure 2 is the frequency-output voltage characteristic of the demodulated signal of the conventional FM demodulation circuit, and Figure 3.
The figure is a configuration diagram of an embodiment of the present invention. 1.3 ...... Demodulation circuit, 2,4 ...
...Tuned circuit, 5 ... Variable capacitance diode, 6 ...... Potential difference detection circuit, 7 ...
...Voltage amplifier, 8...Sampling circuit. Patent U1 applicant Matsushita Electric Industrial Co., Ltd.-6=

Claims (1)

[Claims] In an FM demodulation circuit that demodulates the intermediate frequency signal while matching the center frequency of the intermediate frequency signal and the demodulation center frequency using a tuning circuit, when the center frequency of the intermediate frequency signal and the demodulation center frequency deviate from each other, A center frequency comprising a potential difference detection means for detecting a potential difference, a correction voltage generation means for outputting a correction voltage corresponding to the potential difference, and a summing means for applying the sampled correction voltage to a tuning frequency variable element of the tuning circuit. a correction loop 1 to 171, when the center frequency of the intermediate frequency signal and the demodulation center frequency deviate from each other, the center frequency of the demodulation is shifted by the center frequency 1 of the intermediate frequency signal; An FM demodulation circuit characterized in that the center frequency of the FM demodulation circuit matches the center frequency of the demodulation center frequency of the FM demodulation circuit.