> Tomorrow some of us are meeting with Sandy Weinreb and Steve Smith to > talk about a wideband, channelized down-converter. Ultimately we want > to be able to down-convert and digitize 14 - 26Ghz simultaneously. > Sandy's GAVRT design was based on the 2 Gsamp/s CASPER iADCs. Using > that approach, with two polarizations, we would need twelve > down-converter channels (both I and Q sampling). It looks like we could > safely assume that 3 Gsamp/s ADCs can be used with the ROACH boards, > which would reduce the number of channels to eight. Chuck Naudet tells > me that the next step in the VLBI world will go from the current 1 GHz > bandwidth (at one or two bits/sample) to 4 GHz. We need more dynamic > range but even 4 Gsamp/s would be attractive, reducing the number of > downconverter channels to six. So what would you do if you expected to > start building the downconverters in April? in September? in December?
We in Green Bank are also interested in this sort of thing. We have several receivers on the drawing boards with ~8-14 GHz of instantaneous bandwidth, and we are thinking about how to get the astronomical data to the signal processor. I should add that our major heartburn is for large-format (for us!) arrays of about 100 pixels, dual pol. We are considering a 14 channel, 4 to 8 bit dynamic range ADC/transmission system for our normal receivers, so that we can get the data from our new 7 pixel feedhorn array to the signal processors in the lab, and we likely will go down that path you suggest with the 3 GS/s ADC's and roach boards. I don't know if the answer would change much over 6-9 months. Maybe Dan has an idea about that... But that solution clearly does not work for a 200 channel * 8 GHz (16 Gs/s) system for the GBT! We have been looking into some of the photonic ADC's that people have been working on using optical modulators and electric field effects to image lasers onto detectors. These look promising and have been demonstrated, but they are still lab curiosities, to my knowledge. Ideas welcome for this problem, too! John > > In advance, many thanks > > Tom >
