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