JPS5920284B2 - Receiver AFC reference voltage correction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a receiver with a built-in AFC circuit, and the present invention relates to an AFC reference voltage of the receiver that can correct changes in characteristics of an IF filter over time and position the short reception point at the highest point. This invention relates to a correction circuit.

When the AFC circuit is tuned to the receiving frequency, the receiving point becomes the best point of the detector where the least amount of distortion occurs.
It detects the deviation of the reception point from the optimal reference voltage of the wave detector, controls a local oscillator (VCO), and draws the reception frequency to the best point of the wave detector.

In this AFC circuit, it is known that if the field pack loop from the detector has a large gain, the receiving point will be completely drawn to the optimal point of the detector.

(This configuration is called a servo lock circuit.

) This servo detection circuit has a relatively inexpensive configuration compared to the method of fixing the local oscillator with a crystal, etc., and can completely draw the receiving frequency to the best point of the detector, and the System control.

Even with this servo lock circuit, if the characteristics of the IF filter and the optimal point of the detector change over time, the receiving point drawn by AFC will not necessarily be the point where the least amount of distortion occurs. There is.

However, although the deterioration of the characteristics of the detector can be suppressed by temperature compensation and widening the band, it has not been possible to correct the fluctuations in response to the changes in the characteristics of the 1F filter.

In particular, as improvements in detectors in recent years have led to the emergence of products with superior characteristics over a wide band, changes in characteristics due to aging of the IF filter will affect performance when looking at the characteristics of the entire receiver. This is considered problematic.

In view of the above-mentioned points, the present invention provides a circuit for correcting aging of the IF filter in the AFC circuit, and adjusts the oscillation frequency of the local oscillator to the best point of the IF filter.
The present invention provides an AFC reference voltage correction circuit for a receiver, which allows the receiver to always operate at the best reception point in response to aging of the IF filter.

An embodiment of the present invention will be described below with reference to the drawings.

Figure 1 shows the change in distortion rate due to characteristic fluctuations of the detector and IF filter.
Figure 8 shows the case where the same amount of change occurs over time in the filter and detector, resulting in a deviation from the best point.A shows the case where a detector with a narrow linear region is used, and b shows the case where a detector with a wide linear region is used. This is an example.

In the figure, the solid line indicates the optimum setting state, the chain line indicates the case where the IF filter has drifted, and the broken line indicates the case where the detector has drifted.

As can be seen from both graphs, if the detector has a narrow band, the distortion characteristics will be greatly affected by changes in the detector's characteristics.
If the detector has a wide band, it will be greatly affected by changes in the characteristics of the IF filter.

FIG. 2 is a block diagram of this embodiment, showing the configuration of a conventionally known receiver.

The antenna 1 is connected to an RF circuit 3 via a switch 2 as an input release mechanism, and a MIX circuit 4 is connected to the RF circuit 3.

The output of the MIX circuit 4 is connected to the IP circuit 5, and the IF
A detector 6 is connected to the circuit 5, and the output of the detector 6 is an audio signal 7.

A local oscillator 8 is connected to the MIX circuit 4, and a low-pass filter 9 is connected to the detector 6.

The inverting input terminal of a differential amplifier 100 is connected to the low-pass filter 9, and the output of the differential amplifier 10 is connected to the local oscillator 8.

Further, the low-pass filter 9 is connected to a reference voltage generator 12 via a switch 11.
The output of is connected to the non-inverting input terminal of the differential amplifier 10.

Next, the operation of this embodiment will be explained.

When the receiver is being used normally, switch 2 is closed and switch 11 is open.

The radio waves received by antenna 1 are amplified by RF circuit 3, and
It is mixed with the oscillation signal from the local oscillator 8 in the IX circuit 4 to become an intermediate frequency and input to the IF circuit 5.

The selection S and the amplified output in the IF circuit 5 are input to the next detector 6, where the detector 6 demodulates the audio signal 7 and outputs it.

At this time, the output of the differential amplifier 10 is applied to the local oscillator 8 to control the oscillation frequency and draw the receiving frequency to the optimum point of the detector 6.

When the characteristics of the IF filter change over time, the switch 2 is opened and the switch 11 is closed to cancel the input signal entering the receiver.

Due to the noise generated from the first stage (transistor) of the RF circuit 3, a voltage ΔV corresponding to the center frequency of the IF filter is generated at the output of the detector 6.

Here, the noise generated from the first stage (transistor) of the RF circuit 3 is a white noise component having constant and continuous characteristics.

(Refer to ΔV+ in FIG. 3) This voltage ΔV is extracted through the low-pass filter 9, inputted into the reference voltage generator 12 through the switch 11, and this voltage Δ■ is applied to the reference voltage generator 1.
2.

The reference voltage generator 12 stores the voltage ΔV and at the same time outputs the voltage ΔV to the non-inverting input terminal of the differential amplifier 10 to use this voltage value as a reference voltage for operating the differential amplifier 10.

After this, the receiver operates normally by closing switch 2 and opening switch 11, and the servo lock circuit always pulls the oscillation frequency of local oscillator 8 to the point in Figure 3 in response to the received radio waves. .

Point A in this figure 3 corresponds to point A in figure 1, and the IP
It corresponds to the drift amount of the filter, and the voltage ΔV
By performing f correction, the characteristics of the IP filter and the S curve characteristics of the detector 6 are made to match, and the local oscillator 8 is caused to oscillate at the best point where distortion is least generated.

2. In this embodiment, the switch 2 is provided between the antenna 1 and the RF circuit 30 to constitute the input release mechanism, but even if the switch is provided between the MIX circuit 4 and the local oscillator 80, the input release mechanism is not substantially disabled. function can be achieved.

Furthermore, although the switch 2.11 is operated manually, it may be operated automatically within a certain period of time after the switch is turned on in conjunction with the power switch of the receiver.

Since the present invention is configured as described above, even if the characteristics of the IP filter change over time, the AFC reference voltage can be corrected to compensate for the change, and the local oscillator always oscillates at the minimum position where distortion occurs. It is possible to control the frequency and use the characteristics of the entire receiver in its best condition at all times.

[Brief explanation of drawings]

FIG. 1 is a graph showing changes in distortion characteristics due to changes in characteristics of the IF filter and detector, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is a graph showing the operation of the same. 2...Switch as input release mechanism, 4...
...MIX circuit, 5...IP circuit, 6...
-... Detector, 8... Cal oscillator, 1
2...Reference voltage generator.

Claims (1)

[Claims] The AFC circuit consists of an I MIX circuit, an IF circuit, a detector, and a local oscillator, and uses the output from the detector to control the local oscillator to draw the received frequency to the best point of the detector. Then, a reference voltage generator connected to the output of the wave detector is provided, the input release mechanism is used to put the receiver in a no-signal state, and the output voltage of the wave detector in this state is stored in the reference voltage generator. An AFC reference voltage correction circuit for a receiver, characterized in that the voltage is used as a reference for a control voltage for operating a local oscillator, and is capable of correcting frequency deviations due to characteristic fluctuations of an IF filter.