<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css" style="display:none;"> P {margin-top:0;margin-bottom:0;} </style>
</head>
<body dir="ltr">
<div class="elementToProof" style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Hi Paul,</div>
<div class="elementToProof" style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Wouldn't delay fluctuations from the troposphere also look similar?</div>
<div class="elementToProof" style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Claire</div>
<div id="appendonsend"></div>
<div style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display: inline-block; width: 98%;">
<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> evlatests <evlatests-bounces@listmgr.nrao.edu> on behalf of Paul Demorest via evlatests <evlatests@listmgr.nrao.edu><br>
<b>Sent:</b> Monday, October 28, 2024 4:06 PM<br>
<b>To:</b> evlatests <evlatests@nrao.edu><br>
<b>Subject:</b> [evlatests] phase noise on outer antennas</span>
<div> </div>
</div>
<div style="font-size: 11pt;">hi all,<br>
<br>
Now that we're in A-config, recent testing at high time resolution (10ms) has shown excess short-timescale phase noise affecting the outer three antennas (ea21@E72, ea24@W72 and ea18@N72), as well as ea26@W32. A few plots are attached so you can see what I
mean. These were done via 10ms-dump-time observations of a bright source at K-band, and show two of the bad antennas (ea18, ea21) plus a good antenna (ea13) for comparison. I also took data at X and Ka bands, a summary of these results is in the attached
text file. The worst ones have short-term phase RMS of ~20 deg at Ka, which is >10x larger than the good antennas. This is enough to cause decorrelation (sensitivity loss) at the ~5-10% level and may be at least partially responsible for reduced high-freq
performance seen on these antennas in recent stress tests.<br>
<br>
This noise has the following properties which make me suspect something like LO phase jitter is the cause:<br>
- It looks totally random vs time.<br>
- All 4 IFs for a given antenna show exactly the same noise (see zoom-in plot for example).<br>
- The amplitude of the noise scales in proportion to observing frequency, higher freqs are more affected.<br>
<br>
It's also notable that all 3 outer antennas looked fine in B-config, and only started showing this noise once they moved to their A-config locations. I found some 10ms X-band data from last A-config (Aug 2023) and it shows a similar pattern: the outer pads
and W32 are noisier than the rest, even though some different antennas were involved at the time.<br>
<br>
Since this seems to be pad-related rather than antenna-related, my hand-wavy guess is maybe LO optical power is getting a bit too weak over the very long fibers? And as for W32 maybe it just has some problem with its fiber connection causing a similar effect.
Even if that's not the right explanation, I do think this is worth understanding and (if possible) fixing. Let me know if you have any questions/suggestions.<br>
<br>
Cheers,<br>
Paul</div>
<div id="appendonsend"></div>
<div style="font-family: Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display: inline-block; width: 98%;">
<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> evlatests <evlatests-bounces@listmgr.nrao.edu> on behalf of Paul Demorest via evlatests <evlatests@listmgr.nrao.edu><br>
<b>Sent:</b> Monday, October 28, 2024 4:06 PM<br>
<b>To:</b> evlatests <evlatests@nrao.edu><br>
<b>Subject:</b> [evlatests] phase noise on outer antennas</span>
<div> </div>
</div>
<div style="font-size: 11pt;">hi all,<br>
<br>
Now that we're in A-config, recent testing at high time resolution (10ms) has shown excess short-timescale phase noise affecting the outer three antennas (ea21@E72, ea24@W72 and ea18@N72), as well as ea26@W32. A few plots are attached so you can see what I
mean. These were done via 10ms-dump-time observations of a bright source at K-band, and show two of the bad antennas (ea18, ea21) plus a good antenna (ea13) for comparison. I also took data at X and Ka bands, a summary of these results is in the attached
text file. The worst ones have short-term phase RMS of ~20 deg at Ka, which is >10x larger than the good antennas. This is enough to cause decorrelation (sensitivity loss) at the ~5-10% level and may be at least partially responsible for reduced high-freq
performance seen on these antennas in recent stress tests.<br>
<br>
This noise has the following properties which make me suspect something like LO phase jitter is the cause:<br>
- It looks totally random vs time.<br>
- All 4 IFs for a given antenna show exactly the same noise (see zoom-in plot for example).<br>
- The amplitude of the noise scales in proportion to observing frequency, higher freqs are more affected.<br>
<br>
It's also notable that all 3 outer antennas looked fine in B-config, and only started showing this noise once they moved to their A-config locations. I found some 10ms X-band data from last A-config (Aug 2023) and it shows a similar pattern: the outer pads
and W32 are noisier than the rest, even though some different antennas were involved at the time.<br>
<br>
Since this seems to be pad-related rather than antenna-related, my hand-wavy guess is maybe LO optical power is getting a bit too weak over the very long fibers? And as for W32 maybe it just has some problem with its fiber connection causing a similar effect.
Even if that's not the right explanation, I do think this is worth understanding and (if possible) fixing. Let me know if you have any questions/suggestions.<br>
<br>
Cheers,<br>
Paul</div>
</body>
</html>