[Pafgbt] Purple Plague

[Rick Fisher]

Great!  Looks like that's the next step.

Rick

On Wed, 10 Apr 2013, Steven White wrote:

> Yes, we can make these measurements with a power meter.  We also have other
> options for down conversion and detection using the absorber and cold sky.
>
> ?  Steve  ?
>
>
> ?-----Original Message-----
> ?From: pafgbt-bounces at nrao.edu [mailto:pafgbt-bounces at nrao.edu] On
> ?Behalf Of Rick Fisher
> ?Sent: Wednesday, April 10, 2013 3:25 PM
> ?To: pafgbt at nrao.edu
> ?Subject: Re: [Pafgbt] Purple Plague
> ?
> ?Thanks, Matt.  That's good to know.
> ?
> ?Looks like the immediate task is to verify the noise and gain performance
> of
> ?the amplifiers currently in the Dewar.  Bob, Steve, is there an easy way
> to do
> ?this without the PAF back-end and fiber link?
> ?
> ?Rick
> ?
> ?On Wed, 10 Apr 2013, Matt Morgan wrote:
> ?
> ?> Hi Rick,
> ?>
> ?> No, the ones with packaged transistors shouldn't have this problem,
> ?> unless they are bonded incorrectly inside the package, which as David
> ?> said is highly unlikely considering it is a commercial part.
> ?>
> ?> So perhaps in this instance the Purple Plague was actually a Red
> Herring.
> ?>
> ?> Unfortunately, then, It still begs the question to me why so many of
> ?> the amps appear to have failed at this time.
> ?>
> ?> Matt
> ?>
> ?>
> ?>
> ?> On 4/10/2013 2:21 PM, Rick Fisher wrote:
> ?>>  Matt, I cannot recalled whether you said that Sandy's packaged
> ?>> amplifiers,  like the ones in the PAF, are subject to purple plague.
> ?>>
> ?>>  Rick
> ?>>
> ?>>  On Wed, 10 Apr 2013, Steven White wrote:
> ?>>
> ?>> >  Figure 2 is the same as the amplifier designated as the spare.
> ?>> > When  Mitch  tested this amplifier, he encounter oscillations in
> ?>> > both and could only  test  one amplifier at a bias of 1.2 V.  This
> ?>> > may explain the high noise  temperature measured ~ 13-14K versus
> ?>> > Figure 3 & 4 measurements.
> ?>> >  Yesterday,
> ?>> >  the amplifier was tested in GB and did not oscillate, but the gain
> ?>> > was  closer to 33 dB than 39 dB of S21 measurement of the document.
> ?>> > *IF* all  the  amplifiers in the dewar are the packaged design, the
> ?>> > situation may not  be as  dire as if the chip version was used.  We
> ?>> > are planning to test the spare  amplifier cold next week for noise
> ?>> > and S parameters.
> ?>> >
> ?>> >  Also, a swap of IF paths at the output of the dewar is suggested
> ?>> > to  verify  the problem, a good channel with a low gain LNA
> ?>> > channel.
> ?>> >
> ?>> >  Steve
> ?>> >  ?
> ?>> >  ?But we definitely need to take some steps to resolve this.
> ?>> > Mainly  because  it is  ?urgent to let Toney Minter know if we can
> ?>> > meet the 2nd week of June  ?schedule, or need to fall back.  ?He
> ?>> > has been alerted to the fact that  we  have  ?this issue.
> ?>> >  ?
> ?>> >  ?Anish said he was confident that the high noise temp channels
> ?>> > were "in  the  ?dewar" and not in the backend.  But it would be
> ?>> > good to do something to  ?prove that irrefutably.  Steve, you
> ?>> > suggested a cable swap test.  Or,  is  there  ?any bias monitoring
> ?>> > on the amplifiers themselves?
> ?>> >  ?
> ?>> >  ?Beyond this further diagnostic, we should decide whether there is
> ?>> > any  more  ?software/hardware to test (it may be useful to resolve
> ?>> > the  ?attenuation/sensitivity issue), while the receiver is in OTF
> ?>> > and cold,  or  ?whether to bring the receiver back to the lab.
> ?>> >  ?
> ?>> >
> ?>> >
> ?>
> ?>
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