JPS594888B2 - AM receiver automatic channel selection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an AM receiver that automatically scans the oscillation frequency of a local oscillator and stops this scanning when a broadcast radio wave is received to perform automatic tuning. This invention relates to an automatic channel selection device.

Auto scan tuning in AM receiver is FM
It has very difficult problems compared to receivers.

This is because the carrier is AM modulated and the broadcast frequency interval is set to 10KH7 (in the near future 9KH7), so there is a need to accurately extract narrow band signals in IKHz steps. .

As a conventional automatic tuning device of this type, one has been proposed in which an AM intermediate frequency signal is applied to a ratio detector, and scanning of the local oscillation frequency is stopped by the ratio detection output.

However, ratio detectors have extremely poor temperature characteristics.
It has the disadvantage of being easily detuned from broadcast radio waves due to temperature fluctuations.

Therefore, in view of the above-mentioned drawbacks of the conventional technology, it is an object of the present invention to provide an accurate and narrow-band The purpose of this invention is to provide a channel selection device that is capable of automatic channel selection with specific characteristics, and will be described below based on illustrated examples.

That is, in the figure, 1 is an antenna, 2 is a high frequency amplification circuit, 3 is a mixer that mixes the signal selected by the high frequency amplification circuit 2 and the signal from the local oscillator 4 to produce an intermediate frequency signal, and 5 is an intermediate frequency signal. An amplifier circuit, 6 an AM detection circuit, 1 a buffer amplifier, and the output end of this buffer amplifier is the AM audio output end.
The blocks indicated by symbols 1 to 7 above represent well-known AM receivers.

A part of the intermediate frequency signal obtained by the mixer 3 is applied to a buffer and limiter circuit 8, the output of which is applied to one input of a mixer 9, and the other input of this mixer 9 is connected to a reference signal oscillator. A reference frequency obtained by dividing the output from 10 by a frequency divider 11 is applied via a buffer amplifier 12.

As the reference signal oscillator 10, a 6.4 MHz quartz oscillator with a custom-made temperature, for example, is used, and 6.
The oscillation frequency of 4 MHz is divided by 14 by the frequency divider and converted into 457 KH2.

On the other hand, the above intermediate frequency signal is selected to be 460 KH2, so when both are mixed by mixer 9, 3KH2 is selected.
A beat signal of H2 is obtained, and the beat signal is applied to one input terminal of the phase comparator 13.

A series of loops consisting of a low-pass filter 14 including this phase comparator 13, a voltage-controlled oscillator 15, and a divider 16 is a well-known PLL. It has a capture range of about .

Therefore, if a signal of about 3KH2±300H2 is applied to the phase comparator 13, the PLL will be locked, and at this time the lock detection circuit 17 will generate a signal.

The lock detection circuit 17 is a well-known multiplication circuit that compares the phases of the input to the phase comparator 13 and the output of the divider 16, and generates an output only when the phases match.

When a detection output is generated from the lock detection circuit 17,
In response to the output, an auto-stop signal is generated from the auto-stop signal generating circuit 18, and the signal is applied to a control circuit 19 including an up-down counter.

This foot circuit 19 has a function of controlling the direct current value applied to the varactor diode D in the local oscillator 4 by amplifying or down-counting using the signal from the reference signal oscillator 10 as a clock, and therefore has the function of controlling the DC value applied to the varactor diode D in the local oscillator 4. When the autosdog signal arrives from the signal generation circuit 18, the amplifier or down-counting is stopped, and the count immediately before the stoppage is held to hold the DC value applied to the varactor diode D.

In the above configuration, when either the up or down instruction button provided on the control circuit 19 is operated, the control circuit 19 sequentially changes the DC value applied to the varactor D by amplifying or down-counting according to the signal from the oscillator 10. let

Therefore, the oscillation frequency of the local oscillator 4 changes sequentially, and when a particular broadcast wave is received, an intermediate frequency signal close to 460 KH2 is generated.

Therefore, a beat signal close to 3KH2 is added to the phase comparator 13 of the PLL, and this beat signal is 3KH, z, ±
When it enters the range of 300H2, the PLL locks, so a detection signal is provided from the lock detection circuit 17. Therefore, an auto-stop signal is given from the automatic stop signal generation circuit 1B to the control circuit 19, and the scanning puff of the local oscillator 4 is stopped. will be done.

The first feature of the automatic tuning device of the present invention is that it generates a beat signal using an intermediate frequency signal and a reference frequency signal relatively close to the intermediate frequency signal, and has extremely good temperature characteristics as a reference frequency signal source. Since an Oates oscillator or the like can be used, it is possible to obtain the effect that reliability with respect to temperature can be extremely improved.

The second feature of this invention is that it uses a PLL that receives the beat signal as input, and uses the narrow lock range of the PLL to perform automatic tuning in only a very narrow frequency range centered on the optimum tuning point. Since the scan can be stopped, it is also possible to always obtain the optimum tuning point with little distortion.

[Brief explanation of the drawing]

The drawing is a block diagram showing an automatic channel selection device of the present invention. 3...Mixer, 4...Local oscillator, 5...Intermediate frequency amplification circuit, 9...
Mixer, 10...Reference signal oscillator, 13...
... Phase comparator, 14 ... Low pass filter, 15 ... Voltage controlled oscillator, 16 ...
Divider 117...Lock detection circuit, 18.
...Ringing stop signal generation circuit, 19...Control circuit.

Claims (1)

[Claims] A beat signal of the intermediate frequency signal of the I AM receiver and the reference frequency signal from the reference signal oscillator is extracted, and the reference input signal of a phase-locked loop phase comparator having a capture range of a predetermined width with the beat signal as the center frequency is obtained. This automatic tuning device for an AM receiver is configured to stop scanning of the local oscillation frequency of the AM receiver by the output of a lock detection circuit which generates a signal when the phase-lock druse locks to a beat signal.