R5, C10, D1 and D2 are used to accomplish transmit/receive switching. ** R5 ** R5 limits the amount of current wasted when the KnightSMiTe is keyed. Grounding the cathode of D1 reduces the power supply of U1 to one diode drop above ground. Without R5, the power supply of the KnightSMiTe would be reduced to 0.7 volts and either rapidly deplete the battery or burn out the diode D1. If D1 is removed, R5 can be replaced with a jumper to improve receive dynamic range slightly. The improvement would only be slight however while its presence along with C10 provide a good degree of power supply noise suppression in the audio amplifier. 1 k-ohm is an optimum value for R5 and I wouldn't recommend changing it's value. Making it larger improves noise immunity but reduces the voltage supplied to the audio amplifier. A reduction in dynamic range results which will lead to distortion. Making R5 smaller increases its dissipation during transmission and increases the power wasted by virtue of shorting the cathode of D1 to ground. ** C10 ** R5 and C10 provides form a low frequency bypass of the power supplying the audio amplifier (U1) minimizing the effects of noise on the power supply. ** D1 ** D1 isolates U1 from drawing current from the emitter bias of Q2 during receive. Its function is to provide a short circuit path to ground for U1's power supply during transmit thereby muting the audio amplifier. Eliminate D1 for a slight improvement in power supply efficiency if listening to low level white noise while transmitting isn't objectionable or U1 if is needed to amplify a sidetone from a Tick keyer or other external tone source. ** D2 ** D2 introduces a capacitance change between transmit and receive to offset the oscillator frequency and enable detection of signals that zero your transmit frequency. D2 is reverse biased in receive (minimum capacitance) and zero biased during transmit (maximum capacitance). D2 can be eliminated at the risk of not hearing stations that respond on your transmit calling frequency or if an alternate means of achieving transmit/receive offset control is introduced. Any diode will work here, but feel free to experiment with different diodes types to find one that produces the greatest transmit offset. One that exhibits a high change in capacitance between zero and reverse bias will produce the best result.
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