JPS6144419B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides a phase-locked loop (hereinafter referred to as
The purpose of a synthesizer receiver using a PLL (PLL) circuit is to remove transient noise that occurs immediately after a channel selection operation.

Generally, when changing the receiving frequency in a synthesizer type receiver, the frequency of the local oscillator circuit is changed by changing the division ratio of the programmable counter that makes up the PLL circuit. The PLL repeats vibrations above and below until it reaches the desired value.
Exhibits frequency fluctuations specific to the circuit. During this time, beats and noise corresponding to the above-mentioned frequency fluctuations occur in the audio output of the receiver, degrading the sound quality, so measures such as muting were taken to prevent sound from being output during this period. By the way, in PLL circuits used in synthesizer type receivers, it has been considered a good idea to shorten the lock time of the PLL circuit as much as possible in order to reduce the above-mentioned transient noise. However, in order to shorten the lock time, it is necessary to reduce the time constant of the low-pass filter and raise the cutoff frequency, but this will reduce the attenuation of the reference frequency component, and the output of the low-pass filter will have a higher cutoff frequency. Ingredient leakage increases. Therefore, the carrier-to-noise ratio (C/N ratio) of the local oscillation circuit deteriorates, causing noise and beats in the reproduced sound. and,
In order to remove this noise, the operation of the low frequency amplifier is temporarily stopped during this time. Conventionally, a method of applying so-called muting has been used, but when the channel selection operation is performed continuously, the muting operation is repeated continuously, resulting in frequent interruptions in the audio, making it difficult to hear the received sound clearly. There were times when I couldn't get a grip on the sound, or the sound was even more unpleasant.

The present invention eliminates these drawbacks and provides effective means for realizing good channel selection operation.The present invention will be described below with reference to drawings of embodiments.

FIG. 1 shows an embodiment of the invention. In FIG. 1, a signal radio wave entering an antenna is amplified by a high frequency amplifier 2, and is converted into an intermediate frequency signal by a local oscillation circuit 8 and a frequency mixer 3.
If the input signal is an AM modulated signal, it is amplified by the IF amplifier 4 and detected by the AM detector 5. If the input signal is an SSB modulated wave, the IF has a relatively narrow band.
The signal is amplified by an amplifier 6 and detected by an SSB detector 7. The detected signal thus obtained is passed through a changeover switch 9, amplified by a low frequency amplifier 10, and converted into audio by a speaker 11. On the other hand, a part of the output of the local oscillation circuit 8 is supplied to a programmable counter 16 and is divided by a frequency division ratio preset by a frequency division ratio code generator 15. This output and the output of the reference frequency generator 21 are compared by a phase comparator 22, and the result is passed through a low-pass filter block 23 to control the local oscillation circuit 8 again. In other words, a PLL circuit is configured.

The low-pass filter block 23 includes a plurality of low-pass filters 24, 25, 26 each having a different time constant or voltage gain, and time constant changeover switches 27, 28 for selecting the low-pass filters 24, 25, 26. The operation of the time constant changeover switch 27 is controlled by a monostable multivibrator 18 that generates a pulse with a time width τ ₁ , and the time constant changeover switch 28 is operated by a monostable multivibrator 1 that generates a pulse with a time width τ ₂ .
9 and a monostable multivibrator 20 that generates a pulse with a time width τ ₃ .

and the monostable multivibrator 19,
The 20 pulses are applied to the time constant changeover switch 28 via the adder 30 and also to the muting section included in the low frequency amplifier 10. Here, the monostable multivibrator 1
Time widths of pulses 8, 19, and 20 τ ₁ , τ ₂ , τ
₃ is set to satisfy the relationship τ ₂ <τ ₁ <τ ₃ .
The monostable multivibrators 18 and 19 are driven by applying the output of the tuning pulse generator 14, which sets the code of the frequency division ratio code generator 15, via the mode changeover switch 17. 20 is driven using the output of the memory recall circuit 12 which calls up a division ratio code stored in advance in the memory 13.

In the synthesizer type receiver having such a configuration, first, when receiving an AM modulated wave (AM mode), the mode changeover switch 17 is switched to the AM position. At the same time as setting the code of the frequency division ratio code generator 15 by the channel selection pulse generator 14,
The monostable multivibrator 18 is driven to generate a pulse with a time width τ ₁ . This pulse drives the time constant changeover switch 27 during τ ₁ and switches from the low-pass filter 25 to the low-pass filter 24. Here, the low-pass filter 25 has an optimal time constant in a steady state, and the low-pass filter 24 has a sufficiently larger time constant or has a lower voltage gain. In other words, during τ ₁ immediately after tuning, the low-pass filter 2
The leakage of the reference frequency component from the local oscillation circuit 8 is minimized to prevent deterioration of the C/N ratio of the local oscillation circuit 8. By doing so, noise and beats in the detection output can be reduced as much as possible, making it unnecessary to apply muting as described above. The low-pass filter 24 is a filter designed primarily to remove the reference frequency component. These situations are shown in 1AM mode in Figure 2.

On the other hand, when demodulating SSB waves (SSB mode), immediately after tuning, if the time constant is increased as in the AM mode, the noise and beat components due to the jitter component of the local oscillator circuit 8 will increase rapidly, and the reproduced sound quality will suddenly deteriorate. Getting worse. Therefore, at this time, the output of the channel selection pulse generator 14 is transmitted to the monostable multivibrator 19 via the mode changeover switch 17 (position SSB) to generate a pulse with a time width τ ₂ . This pulse passes through the adder 30 and drives the time constant changeover switch 28 during τ ₂ , and the low-pass filter 2
5 is switched to the low-pass filter 26, and the local oscillation circuit 8 is changed rapidly. This low-pass filter 26 has a time constant that is sufficiently smaller than that of the low-pass filter 25, or has a high voltage gain, so that the lock-up time during transient response can be sufficiently shorter than when the low-pass filter 25 is used. . Note that transient noise generated during this time is removed by applying muting to the low frequency amplifier 10. Muting is performed by applying the output of the monostable multivibrator 19 to the muting section of the low frequency amplifier 10 via the adder 30. When tuning continuously in AM mode, activating muting produces an unpleasant sound, but in SSB mode, the detection method is different, so τ
If ₂ is made small, it will hardly be detected even if muting is applied. The above situation is shown in 2 and 4 SSB modes in FIGS. 2 and 3, respectively.

Also, store the division ratio code in memory 13 in advance.
In some cases, the frequency of the local oscillation circuit 8 is set by storing it in memory and calling it by the memory recall circuit 12. In this case, the width of the frequency change is often very large, and there is no need to check the received sound during the change. Therefore, the monostable multivibrator 20 is driven by the output of the memory recall circuit 12, and the time width τ
₃ pulses are generated. This pulse drives the time constant changeover switch 28, and the low pass filter 2
6 and lock rapidly, and at the same time, mutating is applied by applying the pulse to the muting section of the low frequency amplifier 10 via the adder 30. The situation at this time is shown in Figures 2 and 3.
The figures show 3 and 5 memory recall modes respectively. Note that the pulse width _τ3 is set to a time that is sufficiently locked and stabilized even when the receiving frequency is switched from the lowest to the highest frequency of the band, or vice versa.

In addition, in the above explanation, the low pass filter 26 is used in SSB mode and memory recall mode.
However, it is of course possible to replace these with separate low-pass filters and set the optimum values for each. In addition, although τ ₁ , τ ₂ , and τ ₃ are fixed times, it is determined whether or not the PLL circuit is locked by another means, and the time until locking is made to correspond to the above-mentioned τ ₁ , τ ₂ , and τ ₃ . It's okay.

Next, an example of a specific circuit of the low-pass filter block 23 is shown in FIG. In the same figure, resistor 3
1, 33, 36, 39, capacitor 34, 38,
The differential amplifier 35 constitutes the low pass filter 25 in FIG. Also, when selecting a channel in AM mode, the analog switch 27 controls the capacitor 3.
7 is grounded, and the time constant is increased by a resistor 36 and a capacitor 37. This state corresponds to the low-pass filter 24 in FIG. Also, SSB mode,
In the memory recall mode, analog switch 28 is conductive, and resistor 32, which is sufficiently smaller than resistor 31, is inserted to increase the voltage gain of the filter block. This state corresponds to the low pass filter 26 in FIG. Note that the resistor 39 and capacitor 38 are for correcting the phase characteristics of the filter.

As described above, according to the present invention, the time constant or gain of the low-pass filter of the PLL circuit is set to AM mode,
By controlling the SSB mode and the memory recall mode to suit each other, a great effect can be obtained in that a good receiver can be constructed without noise or beat sound when selecting a channel.

In addition, for convenience of explanation, we did not discuss FM wave reception (FM mode) above.
A similar effect can be obtained by replacing "AM" with "FM", and a combination of FM and SSB, or AM, FM,
All combinations of SSB modes can be handled in common.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the synthesizer type receiver of the present invention, Figs. 2 and 3 are explanatory diagrams of the operation of the receiver, and Fig. 4 is a concrete diagram of the low-pass filter block in the receiver. FIG. 2 is a circuit diagram showing an example of a circuit. 8... Local oscillation circuit, 15... Code generator,
16...Programmable counter, 21...Reference frequency generator, 22...Phase comparator, 23...Low pass filter block, 18, 19, 20...
Monostable multivibrator, 27, 28... time constant changeover switch.

Claims (1)

[Scope of Claims] 1. A low-pass filter group comprising a local oscillation circuit controlled by a phase-locked loop circuit and constituting the phase-locked loop circuit includes a plurality of low-pass filters having different time constants and voltage gains; A synthesizer-type receiver comprising a changeover switch for selecting a low-pass filter, and the changeover switch changes the low-pass filter in a steady state and for a certain period of time immediately after channel selection according to a predetermined reception mode. 2 The low-pass filter that is switched and inserted into the phase-locked loop circuit for a certain period of time immediately after tuning is an FM
Or, when receiving AM mode, the time constant is set to be larger or the voltage gain is smaller than that of the low-pass filter in a steady state when receiving that mode. Synthesizer type receiver. 3 The low-pass filter that is switched and inserted into the phase-locked loop circuit for a certain period of time immediately after tuning is an SSB.
When receiving a mode, the time constant is set to be smaller or the voltage gain is larger than that of the low-pass filter in a steady state when receiving that mode, and the audio output is stopped during the above-mentioned certain period of time. A synthesizer type receiver according to claim 1, characterized in that: 4 The low-pass filter that is switched and inserted into the phase-locked loop circuit only for a certain period of time immediately after channel selection has a smaller time constant or a larger voltage gain when receiving memory recall compared to the low-pass filter in the steady state when receiving that mode. 2. The synthesizer type receiver according to claim 1, wherein the receiver is set so as to have the following characteristics, and the audio output is stopped during the certain period of time. 5 The low-pass filter that is switched and inserted into the phase-locked loop circuit for a certain period of time immediately after tuning is
When receiving in AM or FM mode, the time constant is set to be larger or the voltage gain is smaller than that of the low-pass filter in steady state, while when receiving in SSB mode, the time constant is set to be smaller than that of the low-pass filter in steady state. A patent characterized in that the voltage gain is set to be small or the voltage gain is large, and the time is set to be shorter than the fixed time when receiving in AM or FM mode, and audio output is stopped during that time. A synthesizer type receiver according to claim 1.