Re: QCX+ - QSD some questions and build issues
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Very interesting, Hans. Like you, I don't understand the reason why it should be so. Some very subtle non-linearity somewhere?
?Your last paragraph is spot-on: there are better places to look for potential improvement.
JZ
Hi JZ, Markus
A transceiver such as QCX+ which is all analog, uses a larger value QSD capacitor for lower bandwidth. We only need enough bandwidth to be able to detect 700Hz well. On an SDR like QMX (and mcHF) we need smaller capacitors for larger bandwidth. QMX samples at 48ksps for example.?
The bias point of the QSD actually DOES make a difference to IP3 (strong handling signal capability). 2.5V midrail is NOT necessarily optimum. However:
1. It isn't clear from a theoretical perspective WHY this should be the case (at least I don't think I've ever seen any explanation let alone a convincing explanation). But it is measurable nonetheless.
2. The difference (improvement) is small.
3. The optimum bias voltage also varies by operating frequency.?
4. IP3 is difficult to measure accurately for most people.
5. Different optimum bias voltages are found for different MUX switch part numbers.?
Ideally to squeeze out every last dB of performance one should have a Digital to Analog Converter providing the MUX DC bias voltage, a measurement of IP3, and a lookup table of values per band. It would take an extremely high performance receiver before it would be worth worthing about such things: for anything less, there would be easier ways to improve it. Lower hanging fruit.
73 Hans G0UPL
Markus,
The QSD integration capacitor values will affect the bandwidth of the QSD filtering property, but not affect its sensitivity or signal handling ability.
The precise value of the bias voltage on the mux is unimportant. Anything close to centered between ground and supply works fine. Only small signal swings should be allowed to exist here. Large signals that might bump against the rails would completely overwhelm the high gain amplifiers that follow.
JZ KJ4A?
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