[Pafgbt] Phased Array Coordination meeting
Anish Roshi
anish.roshi at gmail.com
Thu Apr 11 07:46:17 EDT 2013
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> 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> 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> 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); Anish
>> Roshi; Steven White;
>> Robert Simon (bsimon at nrao.edu); Michael
>> Shannon; Matthew Morgan
>> (mmorgan2 at nrao.edu); DAVID SAROFF (RIT
>> Student)
>> (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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