JPS60192426A - Balance form mixing circuit - Google Patents

Abstract

PURPOSE:To decrease the leakage of a local oscillation output from an output terminal by adding a variable resistor to a ring circuit to adjust the circuit balance. CONSTITUTION:If there is a difference in the impedance in the mixing circuit because of variation of impedances of an impedance converting transformer 1a and mixer diodes 2, 3, 4, 5, the leakage of the output of a local oscillator 6 is produced in the IF output by the difference. Then a mixer diode operating point adjusting variable resistor 10 is provided and the upper/lower impedance is corrected so as to attain an ideal circuit balance by the said adjustment. Then the leakage of local oscillation is attenuated and the conversion loss at the mixing circuit is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a mixing circuit, and particularly to a balanced mixing circuit suitable for use as a first mixing circuit of a double conversion tuner.

[Background of the invention]

Generally, the main operations of a double convergence tuner are as follows. First, in the first mixing circuit, the oscillation output generated from the first local oscillation circuit and the radio frequency (hereinafter referred to as R
Let it be F. ) output. Next, in the second mixing circuit, the oscillation output generated by the second local oscillation circuit and the IP output output from the first mixing circuit are mixed and output.

FIG. 1 is a circuit diagram showing a conventional balanced type mixing circuit used in a double conversion tuner.

In FIG. 1, 1α and 1h are impedance conversion transformers, 2, 5, and 4.5 are mixer diodes, and 6 is a local oscillator.

The large RF input input to the input terminal of the impedance conversion transformer 1α is mixed with the oscillation output of the local oscillator 6 through a ring circuit composed of mixer diodes 2.3° and 4.5, and then output from the output terminal of the impedance conversion transformer 1b. Output as IF output.

In such a balanced type mixing circuit, the oscillation output from the local oscillator circuit rarely leaks to the output terminal of the mixing circuit (hereinafter referred to as one local oscillation leakage), compared to when using a single-ended type mixing circuit. The local oscillation leakage can be attenuated by 20 to 30 dB.

However, when such a balanced mixing circuit is used as the first mixing circuit of a double conversion tuner, it is necessary to add a bandpass filter to the first stage.

This is because in a double conversion tuner,
When local oscillation leakage occurs in the output of the first mixing circuit, the output enters the second mixing circuit.

Interference beats between the local oscillators and the oscillation output of the second local oscillator circuit occur, resulting in internal interference that does not depend on the external signal field strength. This is because the input must be attenuated by at least 100 dB or more.

Therefore, in the conventional art, when a circuit as shown in FIG. 1 is used, a bandpass filter is provided at the next stage for the remaining 70 to BO dB to attenuate the local oscillation leakage.

However, in this conventional technique, the local oscillation leakage outside the band is attenuated by increasing the number of stages of the bandpass filter, but the loss within the passband increases instead.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a balanced type mixing circuit that eliminates the drawbacks of the prior art described above, provides more perfect circuit balance for one local oscillation output, and reduces leakage of the local oscillation output from the output end. It is in.

[Summary of the invention]

According to the present invention, the above object is to add a variable resistor that enables circuit balance adjustment to the ring circuit, and to adjust the circuit balance using the variable resistor so that one local oscillation output does not leak out. achieved by.

[Embodiments of the invention]

FIG. 2 is a circuit diagram showing one embodiment of the present invention.

In FIG. 2, 7α and 7h are respectively intermediate frequency tuning capacitors %8α and 8A are intermediate frequency tuning coils, 9 is a DC cut coil, 10 is a variable resistor for adjusting the mixer diode operating point, and 11α.

11h is a balanced/unbalanced conversion transformer, and 12 is a self-bias applying resistor.

In the mixing circuit, the oscillation output of the local oscillator 6 is divided into upper and lower halves by an impedance conversion transformer 1α.
If the circuit is perfectly balanced with respect to the local oscillation output, both are canceled by the balanced/unbalanced conversion transformer 11h.

Local oscillation leakage is zero from the IF output.

However, if there is a difference in the upper and lower impedances in the mixing circuit due to variations in impedance of the impedance conversion transformer 1α and the mixer diodes 2, 3, and 4.5, local oscillation leaks to the IF output by that difference. will arise.

Therefore, in this embodiment, a variable resistor 10 for adjusting the mixer diode operation is provided, and the upper and lower impedances are corrected so that an ideal circuit balance can be achieved by variable adjustment of the variable resistor. This attenuates local oscillation leakage and also reduces conversion loss in the mixing circuit.

〔Effect of the invention〕

According to the present invention, leakage of local oscillation output from the output end can be reduced, and compared to a conventional balanced type mixing circuit,
Since attenuation of ~20 dB is possible, when used as the first mixing circuit of a double conversion tuner, interference beats can be reduced without increasing the loss caused by the next-stage bandpass filter.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional balanced type mixing circuit used in a double conversion tuner.
The figure is a circuit diagram showing one embodiment of the present invention. Code explanation 1α, 1h... Impedance conversion transformer. 2.3,4,5...mixer diode. 6...Local oscillator. 7α, 7h...Intermediate frequency tuning capacitor, 8α, s
b... Intermediate frequency tuning coil, 9... DC cut capacitor. 10... Variable resistor for adjusting mixer diode operating point,
. 11α, 11b...Balanced unbalanced conversion transformer. 12...Resistor for self-bias. Representative Patent Attorney Akio Takahashi7 ・

Claims (1)

[Claims] 1) A ring circuit is configured by mixer diodes, and RF
A large input local oscillation output is input to the ring circuit and mixed,
And in a balanced type mixing circuit in which one local oscillation output is prevented from leaking by configuring the circuit so that the circuit is balanced with respect to the input local oscillation output. A balanced type mixing circuit characterized in that a variable resistor that enables adjustment of the circuit balance is added to the ring circuit, and the balance adjustment is completed by variable adjustment of the resistor.