On 01/15/2011 09:15 AM, Moeller wrote:
On 15.01.2011 13:45, Patrick S. wrote:
For flexibility, being able to bypass stages or feed signals e.g. at the
ADC would be cheap. Preparing for different transport systems would makeI wonder how much noise will be introduced with a switch at the ADC.
At least I would use 2 different input connectors, because switching
in front of the LNA would surely introduce too much noise (except
some special shielded RF relays).
The HMC221AE that I’ve shown in the latest diagram has an insertion loss
of
0.4dB (or less at lower frequencies). Further, my assumption is that
the
“direct” inputs would likely have their own gain/filter chain ahead of
them anyway.
I built a science instrument with the BASIC_RX, and had about 40dB of
gain+filtering
in front of the BASIC_RX. Certainly if you wanted those external
inputs to be
“off air” capable, you’d need a gain stage or two.
But one of the significant uses for a BASIC_RX-type of interface to the
outside world
is for doing things like pulling in the IF of an existing receiver,
where there’s already
plenty of signal available, so the insertion loss of the switch is
negligible.
If those external inputs are used for general signal probing, a
slightly-higher noise
figure in front of the ADC isn’t going to matter much. Actually,
looked at another way,
the effective noise figure of the ADC is quite high anyway, so a small
amount of additional
noise added by the RF switch just isn’t going to be an issue.
–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium