[time-nuts] Low Phase Noise Amplifiers

[Dana Whitlow]

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 <lifespeed at claybuccellato.com> wrote:

> -----Original Message-----
> From: time-nuts <time-nuts-bounces at 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 at 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 at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>