JPS6013635B2 - How to adjust the mixing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for adjusting a key combination circuit in an RF converter that suppresses cross-modulation components of a video signal and an audio signal.

A mixing circuit that mixes a video signal and an audio signal derives not only the primary component of the mixed output but also secondary or higher-order components due to the nonlinearity of the circuit elements.

That is, when focusing on the color subcarrier (frequency Wc) and audio carrier (frequency Ws) of the video signal, the mixed output has a. cos
wct+b. In addition to the frequency component expressed by coswst, it includes a second-order component, and such a component can be expressed as follows. (
agyoswct+Qcoswst)2=agyos2wc
T+pine 2b gi oswc to oswst 10b vine cos2ws
ta wife (, - exchange water Ct) ten skin, one thing OS or St) earth {C
OS (WC + WS) t + cos (w. - wS) t} Among the above frequency components, (ws - wc) is 92 Hz (
= 4.8M ratio - 3 MHz), and is included in the video signal band, and also causes beat noise to be generated in the video signal.

Therefore, in view of the above points, the present invention proposes a mixing circuit adjustment method that suppresses beat noise by reducing the output level of the (ws-wc) component included in the secondary component of the mixed output. .

The present invention will be explained below according to an illustrative embodiment. As shown in the figure, the mixing circuit of this embodiment is a circuit that inputs a video signal to one of a pair of FET elements constituting a differential amplifier and an audio signal to the other to derive a mixed output.3. Color video signal Sv containing a color signal modulated by the subcarrier of the script z
is regenerated as a direct current by the first capacitor C and diode D and inputted to the gate of the IFETQ, while the FM audio signal S is FM-modulated using a carrier wave with a ratio of 4.9M.
A is input to the gate of ETQ2 via a second capacitor C2 for cutting off the DC component, and a mixed output is derived from the drains of both FETs Q, Q2, and the mixed output is input to the next stage R.
It is input to an F conversion circuit (not shown).

In this embodiment, as in the subordinate differential amplifier circuit, variable resistors VR are arranged at the source terminals of both FETs to complementarily adjust the bias points. When adjusting this example, after assembling the RF converter, a 3 MHz sine wave (color subcarrier) was used instead of the video signal at the video input terminal, and a 4.9 MHz audio carrier wave was installed at the audio input terminal. Enter both gates at the bell,
Display the mixed output on a spectrum analyzer.

Next, while observing the spectrum level equivalent to 920K on the spectrum analyzer, adjust the variable resistor VR so that the level becomes the minimum to determine the bias point of both FETs, and then 3. Porridge MHz spectral level; 4.
A variable resistor (not shown) controls the input signal level applied to each gate in order to set the 9MHz spectrum level to a predetermined level.
By adjusting this, it is possible to minimize beat noise due to cross-modulation distortion without changing the audio signal level and video signal level. The input signal level may be adjusted after the bias point is determined by inputting the video signal and the audio signal to a mixing circuit and monitoring them with a television receiver. As described above, according to this embodiment, beat noise can be suppressed by simply adding a simple adjustment, and the effect is great.

[Brief explanation of the drawing]

The figure shows a mixing circuit illustrating one embodiment of the invention. VR...Variable resistance, Sv...Color video signal, Sa...FM audio signal.

Claims (1)

[Claims] 1 By inputting a color video signal and an FM audio signal to two inputs of a differential amplifier, in order to make the differential amplifier function as a mixing circuit of an RF converter, a color subcarrier and other signals are input to one input of the differential amplifier. A variable resistor for changing the bias point is installed so that the output level of the difference frequency component between the color subcarrier frequency and the audio carrier frequency is minimized among the output levels of the frequency components mixed and derived when the audio carrier waves are respectively applied to the inputs. and further, the levels of the color video signal and FM audio signal applied to both inputs of the differential amplifier circuit are variably adjusted on the input side to mix and derive both input signals at a predetermined level. How to adjust a mixing circuit.