JPS58154936A - Channel selecting device - Google Patents

Abstract

PURPOSE:To reduce distortion and to obtain sufficiently less gain change, by keeping a resonance frequency of a channel selecting resonance circuit and a local oscillation resonance circuit independently of the change in a gain controlling voltage. CONSTITUTION:In a local oscillation circuit LOC, the local oscillation resonance circuit A2 is formed with a coil L3, a varactor diode D3, a capacitor C3, and a varactor diode D4, a gain controlling voltage V2 is applied to the diode D4 via a resistor R7, an oscillator OSC1 is oscillated in the resonance frequency of the circuit A2, and the oscillation output is frequency-divided at a frequency divider DV. The phase of the frequency-divided output of the DV and of the output of a reference signal generator OSC2 are compared at a phase comparator PC, an output of the PC is applied to the D3 via a low-pass filter LPF, allowing to compensate the change in the local oscillating frequency due to the change in a gain controlling voltage V2, and as a result, the local oscillation frequency is kept at the same frequency even if the gain control voltage V2 is changed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a channel selection device built into a radio receiver or the like.

Conventional channel selection devices use a resonant circuit to select and extract signal frequencies, and as shown in Figure #11, the input signal is sent to a resonant circuit formed by a coil L and a capacitor C. A frequency component corresponding to the resonant frequency of the resonant circuit is input, amplified by a transistor Q through a coupling capacitor C2, and supplied to the next stage circuit from a coupling transformer TIK. In this case, the gain is
The bias voltage between the gate and source of the transistor Q is changed and adjusted by the gain control voltage (2) using the voltage dividing resistor via R2. Note that R3 is a resistor and C3 is a capacitor.

However, in such a configuration, since an unattenuated signal is applied to the gate of transistor Q,
Distortion was likely to occur and sufficient gain changes could not be obtained.

Therefore, an object of the present invention is to provide a channel selection device that can reduce distortion and obtain a sufficient change in gain.

An embodiment of this invention is shown in FIG. That is, this channel selection device changes the gain by adjusting the signal level itself applied to the gate of transistor Q, rather than changing the bias voltage of transistor Q. To explain in more detail, this channel selection device is
The tuning resonant circuit A is composed of a coil L2, a capacitor C4, and voltage variable capacitance diodes D and D2.The signal is input to the resonant circuit A, a predetermined frequency component is extracted, and the signal is transmitted through the capacitor C6. It is amplified by transistor Q2 and supplied to the next stage circuit by coupling transformer T2. In this case, the signal extracted by the channel selection resonant circuit A is divided between the voltage variable capacitance diode D, the capacitor C4, and the voltage variable capacitance diode D2, and is applied to the transistor Q, and is applied to the voltage via the resistor R5. By changing the gain control voltage v2t applied to the variable capacitance diode D2, the capacitance of the voltage variable capacitance diode D2 changes, the division ratio changes, and the signal level applied to the transistor Q2 changes. However, as it is, the resonant frequency of the channel selection resonant circuit A composed of the coil Lg, the capacitor C41, the variable capacitance diode D, and D2 changes. Therefore, there is a variable voltage capacity.

By changing the capacitance of the diode D, the difference in resonance frequency is corrected without changing the capacitance of the capacitor C4, the voltage variable capacitance diode D2, and the metal capacitance of the diode D2. Variantly, PLL type local oscillation circuit Loc
is used. This local oscillation circuit LOCif, coil L
3. Voltage variable capacitance diode) -D3゜Capacitor C5,
The variable capacitance diode D4 constitutes a local oscillation resonant circuit A, and a gain control voltage V is applied to the voltage variable capacitance diode D4 via a resistor blade.
The oscillator 08C1 is oscillated at a resonant frequency of The phase is compared using a PC, and the output of the phase comparator PC is applied to the voltage variable capacitance diode D3 through a low-pass filter LPF, and the change in the local oscillation frequency due to the change in the gain control voltage v2 is corrected, and as a result, the local oscillation frequency is kept at the same frequency even if the gain control voltage v2 changes.

Then, the low-pass filter L of this local oscillation circuit LOG
The output of the PF is connected to the current through the resistors R and t-, and the variable capacitance diode D is also added to the resonant circuit for tuning by tracking the resonant circuit A2 for local oscillation.

The resonant circuit A for tuning on the signal side changes in the same way as the resonant circuit A2 for local oscillation of the local oscillation circuit LOG, and the resonant circuit A for tuning on the signal side changes within the range of normal trunking deviation. The resonant frequency of is kept constant.

When changing the tuning frequency, it is sufficient to change the division ratio of the frequency divider DV. In this case, the resonant frequency of the tuning resonant circuit and the local oscillation resonant circuit A2 changes according to the division ratio. The resonant frequency after the change is kept constant regardless of the change in the gain control voltage.

R8 and R are resistors and C7Vi bypass capacitors.

In this way, this embodiment uses a resonant capacitor (C4°D
, take the output from the resonant circuit A for channel selection in D2), change the output level by changing the capacitance division ratio, and at the same time use the local oscillation circuit LOG to correct the resulting shift in the resonant frequency. By automatically correcting rounding, distortion can be reduced, and a sufficient gain change can be obtained without lowering resonance Q.

As described above, the channel selection device of the present invention can select jll and @
a series circuit of two voltage variable capacitance elements and a first coil] applying a gain control voltage to the second voltage variable capacitance element to generate an output from the midpoint of the first and second voltage variable capacitance elements; a local oscillation resonant circuit that applies the gain control voltage to the fourth voltage variable capacitance element, which is made up of a series circuit of jll3 and a fourth voltage variable capacitance element, and a coil of jll2; An oscillator that oscillates at the resonant frequency of this local oscillation resonant circuit, a divider that divides the output of this oscillator, a reference signal generator, and a phase that compares the phase of the output of this reference signal generator and the output of the divider. a comparator; and a low-pass filter that filters the output of the phase comparator and applies it to the first and JllI3 voltage variable capacitance elements, and the tuning resonant circuit Furthermore, since the resonant frequency of the local oscillation resonant circuit is kept constant, distortion can be reduced and a sufficient gain change can be obtained.

[Brief explanation of the drawing]

#I1 is a circuit diagram of a conventional channel selection device, and FIG. 2 is a circuit diagram of an embodiment of the present invention. A1... Resonance circuit for tuning, A2... Resonance circuit for local oscillation, C4''5... Capacitor, Dl, D2.D3.
D4... Voltage variable capacitance diode, LOC... Local oscillation circuit, O20... Oscillator, 08C3... Reference signal generator, DV... Branch, PC... Phase comparator, LP
F...Low pass filter Figure 1 Figure 2

Claims (1)

[Claims] jll and the second voltage variable capacitor-child series circuit and the first
a resonant circuit for tuning, which applies a gain control voltage to the second voltage variable capacitance element and outputs an output from the midpoint of the first and second voltage variable capacitance elements; a local oscillation resonant circuit that applies the gain control voltage to the fourth voltage variable capacitance element; and a resonant frequency of the local oscillation resonant circuit; an oscillator that oscillates at , a frequency divider that divides the output of this oscillator, a reference signal generator, a phase comparator that compares the phases of the output of this reference signal generator and the output of the frequency divider, and this phase comparator. The output of J is low-pass filtered and
11 and a low-pass filter applied to the third voltage variable capacitance element, and the resonant frequency of the tuning resonant circuit and the local oscillation resonant circuit is kept constant regardless of the change K of the gain control voltage. station equipment.