[Pafgbt] Phased Array Coordination meeting
Karl Warnick
warnick at ee.byu.edu
Fri Apr 19 17:43:42 EDT 2013
Anish,
In your attachment, I couldn't figure out what noise figure you were
calculating for the BYU receiver. Can you tell me what your measured
noise figure for the BYU receiver is?
Karl
On 4/11/2013 5:46 AM, Anish Roshi wrote:
>
> Hi Brian,
>
> The discussion Rick has raised in the mail is based on some
> measurements I made early April. Attached is a summary of these
> measurements. We see a higher NF for the BYU receiver. Is this
> consistent with your earlier measurements ? A possibility Rick raised
> is that the LO power level may be low and so the mixer conversion loss
> may be high. Is it possible ?
>
> Anish
>
>
> On Wed, Apr 10, 2013 at 2:49 PM, Rick Fisher <rfisher at nrao.edu
> <mailto:rfisher at nrao.edu>> wrote:
>
> Hi Anish,
>
> I think it would be good to put this discussion on 'pafgbt' so
> here goes.
>
> You are probably right that there's not much margin for adding
> attenuation at the output of the Dewar. It depends on the noise
> figure of the amplifiers at the input of the fiber link package
> or, more accurately, the effective noise figure of the fiber link
> input. This will be good to know, but for now we can probably
> assume that Roger designed the fiber link with just low enough
> noise figure to work with the Dewar design as he knew it. This
> would be good to verify, however, once the uncertainty of the cold
> LNA integrity is resolved.
>
> Keep in mind that terminating the fiber link input adds 300K of
> noise to its input. Hence, if the noise figure of the fiber link
> input amplifiers is better than 3 dB, you're more than doubling
> its noise power with the termination.
>
>
> Rick
>
> On Tue, 9 Apr 2013, Anish Roshi wrote:
>
>
> Hi Rick, Bob,
>
> It looks like the disagreement is in the measured noise figure
> and estimate
> noise figure of BYU receiver. May be we can repeat the noise
> figure
> measurement of BYU receiver. Bob, would you mind measuring the
> output noise
> spectrum of BYU receiver after terminating its input for
> couple of boards
> with a spectrum analyzer.
>
>
> Regarding putting the attenuator between A and B -- won't it
> reduce the
> noise margin of cold sky signal ? With the current gain, the
> cold sky signal
> is about 20 dB above the noise level when terminated.
>
> -----------------------------------
>
> I checked the noise due to the first stage op-amp LMH6639. The
> voltage noise
> from the spec is 16 nV/sqrt(Hz). The source impedance I have
> taken as 50
> Ohms (ie the mixed output impedance) and so I neglected the
> noise due to the
> current source. The noise power is -143 dBm/Hz and reference
> at the input
> with 23 dB gain ahead of op-amp is -163 dBm/Hz -- comparable
> to the noise
> due to the first stage !! Does this make sense ?
>
> Anish
>
>
> On Tue, Apr 9, 2013 at 4:09 PM, Rick Fisher <rfisher at nrao.edu
> <mailto:rfisher at nrao.edu>> wrote:
> Hi Anish,
>
> >From your short write up, the noise temperature at the
> output
> of the fiber link (point C) with the link input (B)
> terminated
> is about 3e6 Kelvin. This says that either the input noise
> figure of the fiber link is much higher than 300 K or
> the net RF
> gain of the link is nearly 40 dB, or some combination of the
> two. Judging by the noise figure and insertion loss
> specs on
> the input components of the BYU receiver it seems
> unlikely that
> its net input noise temperature is much greater than
> about 1000
> K, unless something has failed or an LO has been set to the
> wrong frequency or power level.
>
> Your noise measurement at the output of the BYU receiver (E)
> with the fiber link output connected to the BYU receiver
> input
> shows a net gain in the receiver of about 66 dB (133.7
> dBm/Hz -
> 67.7 dBm/Hz). This would suggest that the receiver is
> working
> more or less correctly.
>
> For the sake of argument, let's say that the gain in the
> Dewar
> is 40 dB, the net gain if the fiber link is 35 db, and
> the BYU
> receiver gain is 66 dB for a total gain of 141 dB. With
> 300 K
> connected to the Dewar input (array looking at the hot
> load) the
> receiver output power density would be 141 - 174 = -33
> dBm/Hz.
> The BYU receiver output bandwidth is about 600 kHz so its
> output power would be +24.8 dBm or about 3.9 Vrms, which
> is far
> more than the ADC wants or needs and more than the BYU
> receiver
> can deliver. Hence, there's way too much total gain in the
> system.
>
> Where to add attenuation is determined by a balance
> between the
> need to dominate the noise of the system on the
> down-stream side
> of the attenuator when looking at cold sky and the
> requirement
> that all stages of the system be well within their linear
> operating range when looking at the hot load. My guess
> is that
> this attenuation should be divided between the A-B
> junction and
> the C-D junction since the BYU receiver output level is
> reasonably well matched to the ADC level requirements,
> judging
> by previous experience.
>
> The ADC range is +/-1 V with 12-bit resolution so one
> ADC level
> spacing is 2.0 / 4096 = 0.49 mV, and the minimum voltage
> level
> presented to the ADC when looking at cold sky should be
> about 2
> mV rms into a 50 ohm load, and the maximum should be
> about 200
> mV rms when looking at the hot load. Hence, a good level
> for the
> ADC input is between -1 and -41 dBm
>
> Someone should check my figures.
>
> Rick
>
> On Tue, 9 Apr 2013, Anish Roshi wrote:
>
>
> Hi All,
>
> Attached is a short write up on the measurements
> done on the PAF system.
> Anish
>
>
> On Mon, Apr 8, 2013 at 4:59 PM, Bill Shillue
> <bshillue at nrao.edu <mailto:bshillue at nrao.edu>> wrote:
> Meeting tomorrow Tues April 8th
>
> Phased Array Coordination
>
> 9.30--10.30 NTC-200 and GB-137 by video
>
> topics:
>
> 1. Data from last week (Anish). Discussion of
> power levels and
> other issues (see Anish email)
> 2. Testing schedule going forward
> 3. Cornell tests indefinitely postponed
> 4. Dates for testing with BYU backend (2nd
> half of May ?)
> 5. Australia travel: travel is being setup.
> Waiting for NDA.
> Presentation needed.
> 6. Summary of meeting with Tony Beasley
> 7. Summary from David on setting up Roach
> testbed
> 8. Other
>
>
>
>
>
> -----Original Message-----
> From: Bill Shillue
> Sent: Monday, March 25, 2013 4:28 PM
> To: Rick Fisher (rfisher at nrao.edu
> <mailto:rfisher at nrao.edu>); Anish
> Roshi; Steven White;
> Robert Simon (bsimon at nrao.edu
> <mailto:bsimon at nrao.edu>); Michael
> Shannon; Matthew Morgan
> (mmorgan2 at nrao.edu
> <mailto:mmorgan2 at nrao.edu>); DAVID SAROFF (RIT
> Student)
> (dps7802 at rit.edu <mailto:dps7802 at rit.edu>)
> Subject: Phased Array Coordination meeting
>
> Our usual meeting time of 9.30--10.30 is
> impacted tomorrow by a
> Division head meeting starting at 10 am GB137
>
> So we'll cover as much ground as we can before
> 10 am.
>
> We'll start with testing status (Anish) and
> Cornell/BYU tests
>
> Bill
>
>
>
>
>
>
>
>
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--
Karl F. Warnick
Department of Electrical and Computer Engineering
Brigham Young University
459 Clyde Building
Provo, UT 84602
(801) 422-1732
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