[Pafgbt] Purple Plague

[John Ford]

I should note that we experienced several instances of counterfeit parts
on our builds of the ALMA bias modules.  So it's not out of the question
for packaged parts to be bad in some way or another.

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