The BiDirectional (BiDI) audio stage is based on a single package of TL974 op-amps (4 in a package) and two of the four FST3257 SPDT switches in the same single packageof switches used to switch the B-CAS BiDi IF amplifier. In the drawing, switches S1C and S1D are shown in the Rx state. Asserting the Tx/Rx line high changes the state of the switches.
In the Rx state, the output of the balanced modulator/demodulator stage drives the input of audio stage U1B. This stage has 40 dB of linear voltage gain until the output level begins to approach 1Vpp. At this point, diodes D3 through D6 begin to conduct on signal peaks, providing a ‘soft limiting” of the received signal to suppress noise transients that are too short to be detected by the AGC system. The output of U1B is DC coupled into voltage-follower stage U1C. This stage is augmented by Q1 which serves to improve U1’s current sourcing capability in Tx mode. This will be described later. The output of U1C drives the AGC peak detector formed by Q2, R15, R9 and C4. The DC voltage on C4, which is inversely proportional to the received signal level, drives DC voltage amplifier U1D. U1D’s output begins to drop rapidly when the voltage on C4 drops below the AGC threshold voltage set by R13 and R12. As the AGC voltage drops, the gain of the B-CAS IF amplifier is lowered, providing closed loop gain control. The limiting diodes D3 through D6 serve an auxiliary function of linearizing the AGC control voltage vs insignal level, providing a point to derive an S-Meter function. The overall range of the AGC voltage transfer function is around 90 dB.
In the Tx state, MIC amplifier stage U1A amplfies the microphone voltage by a factor of 10, also providing soft limiting on audio peaks. This server as a sort of “audio compression” function and also helps to prevent overdriving the balanced modulator and following IF/RF stages. The output of U1A drives the U1B & U1C stages through switch S1C. The output of U1C drives the balanced modulator stage to generate a DSB signal. An unfortunate characteristic of TL974 op-amp is that it can only source about 1ma of output current, even though it can sink over 70 ma. The balanced modulator requires 15-20 ma of drive current on both audio peaks in order to develop full output. This problem is fixed by the addition of Q1, which supplies the positive drive current that U1 is incapable of supplying. In the Tx mode, R15 drives the AGC detector high, running the BiDi IF amplifier at full gain in this state.
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