JPS588772B2 - Local oscillator circuit for automatic sweep receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a local oscillation circuit for an automatic sweep receiver using a digital tuner, which has a simple configuration and operates accurately even when the receiving frequency changes from the highest state to the lowest state. This provides an excellent local oscillation circuit.

Generally, a digital tuner uses a phase-locked loop (hereinafter referred to as a PLL circuit) to configure a local oscillation circuit.

When the oscillation frequency of this local oscillation circuit is automatically changed and used as an automatic sweep receiver, the frequency division ratio of a programmable frequency divider connected between the voltage controlled oscillator and the phase comparator circuit constituting the PLL circuit. are automatically changed sequentially.

Incidentally, in a local oscillation circuit for this type of automatic sweep receiver, no provision is normally made to the low-pass filter connected after the phase comparison circuit.

Therefore, in the case of a conventional local oscillation circuit, the division ratio of the programmable frequency divider is automatically changed sequentially so that the oscillation frequency increases one after another, and when the oscillation frequency reaches the maximum, the oscillation frequency returns to the minimum. Even when the division ratio of the programmable frequency divider is reset to its original state, the oscillation frequency may not reach the minimum due to the action of the low-pass filter, resulting in the problem that accurate reception may not be possible.

The present invention eliminates the above-mentioned conventional drawbacks and provides an excellent local oscillation circuit that has a simple configuration and operates accurately.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A local oscillation circuit according to the present invention will be described below with reference to drawings of an embodiment.

In the figure, 1 is a reference frequency oscillator, 2 is a phase comparison circuit that compares the output of the reference frequency oscillator 1 with the output of a programmable frequency divider 5, which will be described later, and outputs an output proportional to the phase difference between the two outputs, and 3 is a phase comparison circuit. A low-pass filter receives the output of the comparator circuit 2 as an input; 4 is a voltage-controlled oscillator that receives the output of the low-pass filter 3 as an input and oscillates at a frequency proportional to the output of the low-pass filter 3; 5 controls the output of the voltage-controlled oscillator 4 This is a programmable frequency divider that divides the frequency at a required frequency division ratio according to a control signal applied to a terminal.

As shown in the figure, the low-pass filter 3 includes three transistors TR1, TR2, TR3 and seven resistors R1 to R.
7, 5 capacitors C1-C5, 1 diode D
It consists of ,.

In the above embodiment, when a control signal that changes sequentially is applied to the control terminal of the programmable frequency divider 5, the frequency division ratio of the frequency divider 5 changes sequentially, and the oscillation frequency of the electrically controlled oscillator 4 changes to the frequency division ratio. Change in response to change.

Therefore, if the oscillation output of the voltage controlled oscillator 40 is used as it is as the output of the local oscillator, the reception frequency will change in accordance with the change in the frequency division ratio.

On the other hand, the control signal applied to the programmable frequency divider 5 generally changes sequentially in the direction larger or smaller than a certain value, and momentarily returns to the original certain value when it reaches the maximum or minimum value. Usually it is a control signal.

Therefore, in terms of reception frequency, the frequency shifts sequentially from the minimum frequency or maximum frequency of the reception band to the maximum frequency or minimum frequency side, and when the maximum frequency or minimum frequency is reached, it instantaneously returns to the minimum frequency or maximum frequency.

By the way, the capacitor C1 in the low-pass filter 3 is charged instantly at the start of the sweep, and the output of the time constant circuit composed of the resistor R1 and the capacitor C1 must rise quickly, but if the diode D1 is not present,
A problem arises in that the potential rises only gradually due to the action of the resistor R1, and the voltage applied to the voltage controlled oscillator 4 decreases during the period until it rises to a predetermined potential, making it impossible to sweep correctly during that time.

However, according to the above embodiment, the diode D1 is connected to the resistor R.
1, so when the receiving frequency instantaneously returns from the upper limit to the lower limit, the loop is unlocked and the output of the phase comparator circuit 2 becomes H level.At that moment, the H level output is connected to the diode. Applied to capacitor C1 via D1.

Therefore, the capacitor C1 is rapidly charged, and the output of the time constant circuit quickly rises.

For this reason, a predetermined voltage is also applied to the voltage controlled oscillator 4 from the beginning of the sweep, thereby reliably preventing sweep errors at the start of the sweep.

Incidentally, if the receiver is of a type in which the reception frequency value instantaneously changes from the lower limit to the upper limit, the diode D1 may be connected with the opposite polarity to that in the above embodiment.

As described above, the present invention provides a programmable frequency divider whose frequency division ratio is changed by a control signal between the output terminal of a voltage controlled oscillator constituting a phase-locked loop and the input terminal of a phase comparator circuit. The structure is configured to automatically change the division ratio of the programmable frequency divider sequentially and change the oscillation frequency of the voltage controlled oscillator according to changes in the control signal, and the output terminal of the phase comparison circuit and the input terminal of the voltage controller A low-pass filter is connected between the input circuit of the above-mentioned low-pass filter, and a diode is connected in parallel with the resistance of the time constant circuit that constitutes the input circuit of the above-mentioned low-pass filter. The output of the phase comparator circuit is applied to the capacitor of the time constant circuit via the diode, and this capacitor is charged instantaneously when By simply adding a diode, it is possible to accurately return the receiving frequency to its original state and perform an accurate sweep from the start of the sweep.

[Brief explanation of the drawing]

The figure is a schematic electrical connection diagram of a local oscillation circuit for an automatic sweep receiver according to the present invention. 1... Reference frequency oscillator, 2... Phase comparator circuit, 3...
Low-pass filter, 4... Voltage controlled oscillator, 5... Programmable frequency divider, TR1 to TR3... Transistor,
R1 to R7...Resistor, C1 to C5...Capacitor, D
1...Diode.

Claims (1)

[Claims] 1 A programmable frequency divider whose frequency division ratio changes according to a control signal is provided between the output terminal of the voltage controlled oscillator constituting the phase-locked loop and the input terminal of the phase comparator circuit, and the programmable frequency divider whose frequency division ratio changes according to the change in the control signal The oscillation frequency of the voltage controlled oscillator is changed by automatically sequentially changing the frequency division ratio of the frequency generator, and a low pass filter is provided between the output terminal of the phase comparator circuit and the input terminal of the voltage controlled oscillator. A diode is connected in parallel with the resistor of the time constant circuit that constitutes the input circuit of the low-pass filter, and when the division ratio of the programmable frequency divider changes instantaneously from the maximum to the minimum or from the minimum to the maximum. A local oscillation circuit for an automatic sweep receiver, characterized in that the output of the phase comparator circuit is applied to a capacitor of the time constant circuit via the diode to instantaneously charge the capacitor.