From: Mike Stennes To: Al Wootten Subject: Re: Vanes (fwd) Date: Thu, 10 Jan 2002 21:26:41 -0500 (EST) Hi Al, My comments are embedded in your text below: On Thu, 10 Jan 2002, Al Wootten wrote: > Hi Mike > > Matt Carter at IRAM is building such a system. semi-transparent vanes > have been tried at OVRO and at IRAM but not with such good results. I > recall that they needed more engineering, and simpler solutions were > available. I agree that reflections are a problem. Matt has measured > properties of a number of possible materials > Our intention on ALMA is to try them on the European > prototype. There are potential problems both with this method and with > the method being developed at BIMA for implementation on the US prototype > antenna, of hot loads behind the subreflector described in the project book. > > We had a report on Carter's progress in June at the Calibration > PDR for ALMA. My notes are available: > > The final report, which includes the reviewers names, is at: > http://www.cv.nrao.edu/~awootten/mmaimcal/asac/calpdrrpt2.txt > > My notes on the meeting are also available, at: > http://www.cv.nrao.edu/~awootten/mmaimcal/asac/calpdr.htm > the semi-transparent load discussion is the first item. My recollection is > that this calibration system is most important for SIS systems where > a hot load would take the receiver response into a nonlinear regime. > We were under the impression that this would not occur for a HEMT type > receiver but perhaps we were wrong. For the ALMA HEMT receiver bands, > probably at 68-90 and 31.3-45 GHz, we had planned to use just a hot load. > I'd be interested in your thoughts on whether this is appropriate. For an 86 GHz receiver front-end having just one 5- or 6-stage HEMT amplifier, a 300 Kelvin load would not drive the amplifier into a nonlinear regime, for bandwidths of 20 GHz or so. (GkTB is approximately 83 nW for G=30 dB, B=20 GHz, and T=300K). The CDL 86 GHz amps can output up to 1 microwatt without worry of nonlinearity. This corresponds to an input temperature of 3,623 K. In my case, with the GBT 3mm receiver, I need to consider the use of two 86 GHz HEMT amplifiers. Here, the gain would be 60 dB rather than 30 dB. The second amplifier would certainly saturate when the feed is looking at a 300 K load. This is why I'm investigating the use of semi-transparent vanes or cold loads. The alternative would be to to my continuum detection at IF (after the first mixer). Thanks again for your help. Best regards, Mike