JPS6410122B2 - - Google Patents

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.

FIG. 1 shows the configuration of a conventional FM demodulation circuit using the so-called quadrature demodulation method. 1 is a demodulation circuit equipped with a phase rotation tuning circuit 2; This means that the input intermediate frequency signal is phase-shifted around the center frequency of the intermediate frequency signal, and then guided to an internal multiplication circuit for quadrature demodulation.The demodulated signal and automatic frequency control circuit (not shown) The input reference potential signal and potential difference signal are output.

In the conventional example configured in this way, when the center frequency of the intermediate frequency signal matches the demodulation center frequency f ₀ , the frequency-output voltage characteristic of the demodulation signal becomes as shown in Fig. 2, and the potential difference signal is not output, and the state in which the center frequency of the intermediate frequency signal and the demodulation center frequency f ₀ match is maintained. However, if the center frequency of the intermediate frequency signal becomes lower by Δf _a or higher by Δf _b than the demodulation center frequency f ₀ due to a shift in the oscillation frequency of the local oscillator circuit, etc., the center frequency of the intermediate frequency signal becomes lower than the demodulation center frequency f 0 by Δf _a or higher by Δf _b . A potential difference is generated, and the voltage ΔV _a or ΔV _b is applied to the automatic frequency control circuit, and the oscillation frequency of the local oscillation circuit changes, so that the center frequency of the intermediate frequency signal becomes the demodulation center frequency.
Since it is pulled back to f ₀ , the center frequency of the intermediate frequency signal can be maintained at the demodulation center frequency f ₀ .
By the way, in order to perform such an operation, the demodulation circuit 1 and the phase rotation tuning circuit 2 must be connected during product assembly so that the demodulation center frequency becomes exactly f ₀ .
It was necessary to make adjustments, and equipment and people were required for this purpose, which had the disadvantage of increasing manufacturing costs.

In order to eliminate the drawbacks of the conventional example, the present invention provides that when the center frequency of an intermediate frequency signal deviates from the demodulation center frequency, the demodulation center frequency is shifted to the center frequency of the deviated intermediate frequency signal, and the intermediate frequency The present invention provides an FM demodulation circuit that matches the center frequency of a signal with the demodulation center frequency. below,
Examples 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 a potential difference detection circuit that detects the potential difference that occurs between the center frequency of the intermediate frequency signal and the demodulation center frequency when they deviate from each other;
Reference numeral 7 denotes a voltage amplifier as a correction voltage generating means that outputs a correction voltage corresponding to the potential difference detected by the potential difference detection circuit 6. Reference numeral 8 denotes a sampling circuit that applies the sampled correction voltage to the variable capacitance diode 5. The circuit 8 prevents the voltage amplifier 7 and the variable capacitance diode 5 from being directly connected, so that the demodulation circuit 3, the potential difference detection circuit 6, the voltage amplifier 7, and the variable capacitance diode 5
The relationship between the loop time constant of the center frequency correction loop constituted by the tuning circuit 4 and the low frequency signal component of the demodulated signal prevents the DC component from not being demodulated.

Next, the operation of this embodiment configured as described above will be explained.

Note that f _if indicates the center frequency of the FM demodulated signal, f ₀ indicates the demodulation center frequency of the demodulation circuit 3, and Δfa or Δf _b indicates _the deviation frequency from the demodulation center frequency, respectively.

Now, in the circuit shown in FIG. 3, an FM demodulated signal with a frequency deviation of Δf _a or ΔF _b from the demodulation center frequency f ₀ of the demodulation circuit 3 is input to the demodulation circuit 3, and the center frequency of the FM demodulation signal When the demodulation center frequency f ₀ of the demodulation circuit 3 deviates from f _if by Δf _a or Δf _b , an FM demodulated signal with a potential difference ΔV _a or ΔV _b corresponding to the shifted frequency Δf _a or Δf _b is output from the demodulating circuit 3. Output.

Therefore, when the deviation frequency Δf _a or Δf _b is held constant, the potential difference ΔV _a or ΔV _b is also held constant, and the output of the voltage component of the FM demodulated signal becomes DC, so the voltage of the FM modulated signal Even if the components do not include a DC component, the voltage component of the FM demodulated signal will include a DC component.

Since the center frequency f _if of the FM demodulated signal and the demodulated center frequency f ₀ of the demodulation circuit 3 are relative frequency differences, and the deviation frequency Δf _a or Δf _b is the difference between both frequencies, the FM demodulated signal The output of corresponds to the relative frequency difference. A potential difference of ΔV _a or ΔV _b is always added to the FM modulation signal.

That is, if the voltage component of the FM modulation signal includes a DC component, the voltage component of the FM demodulation signal will output a potential different from the voltage component of the FM modulation signal.

Therefore, in order to perform accurate FM modulation, it is necessary to adjust the demodulation circuit 3 so that f _if =f ₀ [ΔV _a =0] or f _if = f ₀ [ΔV _b =0].

Next, the demodulation circuit 3 equipped with a frequency correction loop controlled by the potential of ΔV _a or ΔV _b obtained by sampling the potential of the FM demodulated signal will be described.

As described above, when the demodulation center frequency f ₀ of the demodulation circuit 3 shifts by Δf _a or Δf _b , the potential of the FM demodulated signal becomes ΔV _a or ΔV _b .

Now, when the potential of the FM demodulated signal is ΔV _a or ΔV _b , if the frequency correction loop continues to operate in a closed state, the potential of the FM demodulated signal ΔV _a or ΔV _b is equal to the loop constant of the frequency correction loop. Even if it is a constant, it operates so that the potential ΔV _a or ΔV _b becomes zero.

By the way, if an FM modulation signal containing an AC component (FM modulation signal) with a value earlier than the loop time constant is input to the demodulation circuit 3 while the frequency correction loop remains closed, the frequency correction loop cannot follow it. Therefore, the FM modulated signal is demodulated and output as is.

Conversely, when an AC component, a DC region, or an FM modulation signal containing a DC component with a value slower than the loop time constant is input to the demodulation circuit 3, the frequency correction loop can follow it, so that the potential ΔV _a or ΔV _b becomes zero. As a result, it cannot be output as an FM demodulated signal.

Therefore, the demodulation circuit 3 with only a frequency correction loop
In principle, it cannot be used as a demodulator for demodulating the DC region or DC components.

Furthermore, a demodulator 3 will be described which includes a circuit in which sampling means is provided in the frequency correction loop to open and close the frequency correction loop.

Now, the potential ΔV _a or _ΔV b generated by f _if ~ f ₀ = Δf _a or f _if ~ f ₀ = Δf _b is sampled and the frequency correction loop is operated sequentially, so f _if = f ₀ becomes. Therefore, if the frequency correction loop is opened and maintained in this state, the center frequency f ₀ of the demodulation circuit 3 will remain fixed even if the FM modulation signal contains the same DC region or DC component as described above. It is possible to demodulate and output the DC region or DC component according to the FM modulation signal.

In this way, when demodulating by the demodulation circuit 3 provided with a frequency correction loop, when it is necessary to demodulate even the DC component, a specific means for sampling within the frequency correction loop is necessary and effective. It is.

That is, when the center frequency of the intermediate frequency signal and the demodulation center frequency deviate, the potential difference that occurs between them is detected by the potential difference detection circuit 6, and a correction voltage corresponding to the potential difference is sent from the voltage amplifier 7 via the sampling circuit 8. is applied to the variable capacitance diode 5, the tuning frequency of the tuning circuit 4 changes, the demodulation center frequency is shifted to the center frequency of the intermediate frequency signal, and the potential difference detection circuit 6 detects that the potential difference has become zero. , the center frequency correction loop stops operating.

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, or 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, and the demodulation center frequency is biased toward the center frequency of the intermediate frequency signal. Since the present invention is moved to another location, 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 lowering manufacturing costs.

Further, the sampling means is a correction voltage generating means,
By connecting between the variable tuning frequency element, it prevents the correction voltage from being directly applied to the variable tuning frequency element and prevents the DC component from being demodulated, providing a wideband FM modulation circuit. It has the advantage of being possible.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional FM demodulation circuit, FIG. 2 is a frequency-output voltage characteristic of a demodulated signal of the conventional FM demodulation circuit, and FIG. 3 is a block diagram of an embodiment of the present invention. 1, 3... Demodulation circuit, 2, 4... Tuning circuit, 5
... Variable capacitance diode, 6 ... Potential difference detection circuit, 7 ... Voltage amplifier, 8 ... Sampling circuit.

Claims (1)

[Claims] 1. When the center frequency of the intermediate frequency signal and the demodulation center frequency deviate, the tuning frequency of the tuning circuit changes, and the demodulation center frequency follows the center frequency of the intermediate frequency signal and becomes polarized. FM to be transferred
The demodulation circuit includes: a potential difference detection means for detecting a potential difference when the center frequency of the intermediate frequency signal and the demodulation center frequency deviate; a correction voltage generation means for outputting a correction voltage corresponding to the potential difference; and a correction voltage generation means for outputting a correction voltage corresponding to the potential difference; An FM demodulation circuit comprising a center frequency correction loop comprising sampling means for applying a correction voltage to a variable tuning frequency element of the tuning circuit.