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80W Broadcast FM Stereo Transmitter

80W Broadcast FM Stereo Transmitter80W Broadcast FM Stereo Transmitter Schematic

The two single-ended line level audio signals enter through feedthrough capacitors, and are greeted by a low pass LC filter to get rid of any RF that might be on them. In each channel there is a buffer stage, and then a combination of pre-emphasis and soft-limiting step. The advantage of limiting and pre-emphasis in one step is to avoid heavy overdeviating pitched sounds or loud sounds serious to flatten three times, without the need for a multiband limiter. The gain of the limited portion of the audio signals can be adjusted using potentiometers. Then comes a low pass filter that removes the six-pole signals above 15 kHz.

74HC4060 chip derived signals 38 kHz and 19 kHz, square wave form of a quartz crystal to measure. Two resonant circuits with ferrite cores pot to turn these very clean square waves, waves of low noise breast. Potentiometers to adjust the levels, while the adjustable coil cores allow precise adjustment. Bridges allow you to disable each of these signals for testing and adjustment.

Once out of fashion, but low noise and low distortion analog multiplier chip modulates the LR signal produced by a differential amplifier, operational amplifier, the 38 kHz subcarrier. This circuit has three adjustments to maintain balance. Its production level is also adjustable. The signals that are required only for stereo sound can be switched off to test via a bridge.

The adder combines the signal output of L, R signal, (LR) * 38 kHz signal and the pilot tone. The first two signals are fixed at this stage, while the (LR) * 38 kHz can be adjusted by adjusting the potentiometer itself, and the pilot tone adjustment potentiometer before LC circuit. Then there is a final level adjustment, used to set the transmitter deviation, then a buffer stage with low output impedance, which drives the output through a resistor to avoid the instability of capacitive loads.