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Reed Relays ? (a bit off topic)
Bob,
you made an easy question and a difficult (for me) question. The easy question. Haven't semiconductor switches, caught up with reed relays for this application by now, or do they "still" provide a lower "on" resistance, etc. ? The problem of using diode-based switches is not in their higher "on" resistance, but in the fact that, even if strongly forward biased, they do not behave linearly, like a pure resistance. So, in presence of strong in-band or out-of-band signals, they can create intermodulation products. Increasing the diode forward biasing improves the situation, but the intermodulation performance of a mechanical contact anyway remains superior. Accordingly, some of the latest state-of-the-art transceivers (e.g. ICOM IC-7700) boast the use of mechanical switches instead of diode-based switches. The difficult question. I am retired now, so haven't kept up withese things, or the state of the art nowadays, but are they still using electromechanical relays for front ends feeding a 16 bit A to D ? I would re-formulate your question in this way: which is the point of avoiding the use of diodes OUTSIDE the A/D converter, if the A/D converter chip anyway includes switching diodes INSIDE it. This question is to be put to somebody who is familiar with the internal electrical design of A/D converter chips, and knows whether their internal structure is such to cause intermodulation products in presence of strong signals. 73 Tony I0JX Rome, Italy |
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