Lifespeed, The cryogenic amplifiers were used in very broadband situations (often hundreds of MHz BW) with "signals" that were basically noise. Most radio astronomy lies in the art of measuring very small *changes *in noise level, such as between pointing at an object of interest or pointing away from it at a known quiet spot in the nearby sky. So we are talking about radiometry, where the "Radiometer Equation" rules. In this game, the best results are had by using the widest possible RF/IF BW, then running the noise into a square law detector, then passing the output of the detector through a very low BW filter.
I mention the above because I suspect it means that phase noise in the predetection part of the path has little of no effect (unless, of course, there is a strong signal lurking not far outside the IF passband). To my knowledge, we've never tried to measure phase noise of any of the cryogenic amplifiers. Our LNAs mostly have noise temperatures in the range of 2K to 3K, which is roughly 0.03 to 0.04 dB NF. Dana On Sun, Jan 12, 2020 at 3:33 PM <lifesp...@claybuccellato.com> wrote: > -----Original Message----- > From: time-nuts <time-nuts-boun...@lists.febo.com> On Behalf Of Dana > Whitlow > Sent: Saturday, January 11, 2020 8:28 AM > To: Discussion of precise time and frequency measurement > <time-nuts@lists.febo.com> > Subject: Re: [time-nuts] Low Phase Noise Amplifiers > > FWIW, at the Arecibo Observatory all our cryogenic LNAs had bias stabilized > with active stabilizers based on opamps. Since the opamps do not work at > ~15K, bias connections to the drain and gate of the RF FETs were brought > out > separately from the RF connections, and the opamp circuitry was at room > ambient temp. > This approach would nicely stabilize both drain DC voltage and drain > current > over the whole temperature range from room ambient to 15K, which was handy > for testing and monitoring LNA behavior during cool-down, which takes > several hours (or more). > > Dana > ************** > Opamp stabilization is nice. Do you have any idea of the residual phase > noise or broadband noise floor you were getting with this bias, or was that > not a figure of merit for the cryo LNAs? Even though an opamp circuit can > be designed for low noise, probably large passive filtering components > would > be required to tamp down the broadband noise. > > Lifespeed > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.