JPS6219010Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement of a low-pass circuit in a digital tuning circuit such as a frequency synthesizer.

Figure 1 shows part of a digital tuning circuit such as a frequency synthesizer using a conventional electronically tuned tuner. In the figure, 1 indicates a received signal (not shown).
and a reference signal (not shown); 2 is an output terminal of this phase comparator 1;
6 is a smoothing circuit consisting of a smoothing resistor 7 and a smoothing capacitor 8 connected to the phase comparator 1; 10 is a Darlington connection transistor as an impedance conversion circuit connected to the smoothing circuit 26 via a base resistor 9; 15 is the collector resistance of the Darlington-connected transistor 10, 12 and 13 are the first and second voltage dividing resistors connected to the emitters of the downstream transistors of the Darlington connection, and 14 is the base to which the divided voltage of both resistors 12 and 13 is applied. Amplification transistors 16 and 17 are the amplification transistors 14
The above circuit elements 12 to 17 form an amplifier circuit 27. 28 is a negative feedback circuit consisting of the first and second capacitors 18, 19 and a resistor 20, 4 is a power supply terminal, and 3 is a low pass circuit (hereinafter referred to as LPF) consisting of these circuit elements 26, 10, 27, 28. ). Reference numeral 5 denotes an electronic tuning tuner to which the tuning voltage output of the LPF 3 is applied via a tuning voltage application terminal 6.

Next, the operation will be explained.

The output voltage of the phase comparator 1 is smoothed by a smoothing circuit 26. This smoothing circuit 26 includes a Darlington connected transistor 10 which serves as an emitter follower.
Impedance matching is achieved by The output voltage is determined by the first and second voltage dividing resistors 12 and 13.
After the voltage is divided by the amplifier transistor 14, a part of the amplified output is sent to the negative feedback circuit 2.
8 to control the amplification degree of the amplifier circuit 27, and the output of the amplifier circuit 27 is outputted to the electronic tuning tuner 5 as a tuning voltage output of the LPF 3.

By the way, Δf/ΔV of the electronic tuning tuner 5,
That is, it is known that the change Δf in the frequency f with respect to the minute change Δv in the tuning voltage V is not uniform, and the higher the tuning voltage v becomes, the smaller Δf/Δv becomes. On the other hand, since the gain of the LPF 3 is constant, ,
When looking at the digital tuning circuit as a closed loop,
The loop gain changes depending on the tuning frequency or tuning voltage. That is, the higher the tuning voltage becomes, the smaller the loop gain becomes, and this change in loop gain causes undesirable phenomena, especially during channel selection transients. For example,
LPF3 to achieve appropriate gain with low tuning voltage
When a high tuning voltage was applied, the loop gain of the digital tuning circuit was insufficient, and an oscillation mode occurred during the tuning transition. Moreover, when the loop gain is increased as a countermeasure, there is a problem in that an oscillation phenomenon occurs.

The present invention was devised to eliminate the problems of the conventional ones as described above, by providing a negative feedback amount reduction circuit that reduces the amount of negative feedback of the negative feedback circuit of the LPF using a tuning voltage. Therefore, it is an object of the present invention to provide an LPF for a digital tuning circuit that can eliminate undesirable vibration modes and oscillation phenomena during tuning transients.

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

FIG. 2 shows an embodiment of the present invention, in which the same reference numerals as in FIG. 1 indicate the same parts.

22 and 23 are the third and fourth resistors for base bias connected between the output terminal of LPF3 and the ground;
5 is a transistor to which the divided voltage of both resistors 22 and 23 is applied to its base; 24 is the transistor 25;
This is a capacitor connected between the collector of the negative feedback circuit 28 and the connection point of the second capacitor 19 and the resistor 20 of the negative feedback circuit 28, and when the output voltage of the LPF 3 exceeds a predetermined voltage due to the above circuit elements 22 to 25. A negative feedback amount reducing circuit 21 that reduces the amount of negative feedback of the negative feedback circuit 28 is formed.

Next, the operation will be explained.

When the tuning voltage that is the output of the LPF 3 is less than a predetermined voltage, the voltage obtained by dividing it with the third and fourth resistors 22 and 23 is the voltage that is obtained by dividing the voltage between the base and the transistor 25.
Since the emitter-to-emitter voltage is not exceeded, the transistor 25 remains off, thus operating as before.

Conversely, when the tuning voltage exceeds the predetermined voltage, the transistor 25 is turned on. Then
A portion of the feedback amount fed back by the negative feedback circuit 28 of the LPF 3 is bypassed through the transistor 25 by an amount determined by the capacitance of the second capacitor 19 of the negative feedback circuit 28 and the capacitor 24 of the negative feedback amount reduction circuit 21. , falls to the ground. As a result, the amount of negative feedback of the negative feedback circuit 28 decreases, and the apparent gain of the LPF 3 increases.

As described above, according to the LPF of the digital tuning circuit according to the present invention, a negative feedback amount reduction circuit is newly added to the conventional LPF, and when the tuning voltage exceeds a predetermined voltage, the negative feedback is reduced, and the LPF Since the apparent gain is increased, it is possible to eliminate vibration modes or oscillation phenomena during the tuning transition of the digital tuning circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an LPF in a conventional digital tuning circuit, and FIG. 2 is a circuit diagram of an LPF in a digital tuning circuit according to an embodiment of the present invention. 10... Darlington connection transistor (impedance conversion circuit), 21... Negative feedback amount reduction circuit, 26... Smoothing circuit, 27... Amplification circuit, 28
. . . Negative feedback circuit. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of utility model registration request] This is a low-pass circuit of a digital tuning circuit that receives the output of the phase comparator as input and outputs the tuning voltage of the electronically tuned tuner, and includes a smoothing circuit that smooths the input voltage and an impedance that converts the impedance of the output of this smoothing circuit. A conversion circuit, an amplifier circuit that amplifies the output of this impedance conversion circuit and outputs the tuning voltage, a negative feedback circuit that provides negative feedback of the output voltage of this amplifier circuit, and the negative feedback amount of this negative feedback circuit as described above. 1. A low-pass circuit for a digital tuning circuit, comprising: a negative feedback amount reduction circuit that reduces an output voltage of an amplifier circuit when it exceeds a predetermined voltage.