Re: [AMMRL] Signal Blanking on High Band

From: Spin-Doc <bert.heise_at_spin-doc.net>
Date: Wed, 11 Mar 2026 19:59:33 +0100
Dear Shawn,

Do you have a 400MR with VFS synthesizer (19“ rack unit) or synthesizer board
(on the right of the reference generator board)?
Anyway, in both cases you can try to swap the synthesizer channels 1 & 2 by swapping
both RJ45 controller and BNC cables, followed by „su“. An oscilloscope could also
be handy for troubleshooting (expecting to see a cw sine wave at desired frequency plus 20 MHz IF
(so 420 MHz on high-band).
Other things to check would be 
- swap rf1 vs. rf2 controller boards (blanking signal generated there)
- check main power supply voltages, especially 3.3, 5 and 11 V (behind right side panel, white
molex plugs at the bottom). Watch out for browned pins (tighten contact & clean both contacts,
don’t use automotive contact spray!)…

Best regards 
Bert

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Spin-Doc 
Magnetic resonance equipment, service, training & more
Dr. rer. nat. Bert Heise
Owner,applications scientist
Kiefernweg 13
58239 Schwerte
Germany
Mail: bert.heise_at_spin-doc.net
Phone: +49-171-5135315

Representing MestreLab software, Stelar relaxometers & Barthel HF amplifiers


Am 11.03.2026 um 18:24 schrieb Shawn Wagner via groups.io <shawn.wagner=usc.edu_at_groups.io>:

Hi AMMRL Community,
Before I go hunting for the problem, I wanted to check if anyone has seen this before. I have an unusual issue on a Varian 400MR (DirectDrive).
The signal on the high-band channel (channel 1, 1H/19F) blanks for ~0.125 s every ~1.6 s. During the blanked period the receiver output goes completely flat. This does not occur on channel 2.
When tuning the high-band channel you can see the same behavior. If the blanking occurs during the tune sweep, the signal disappears and only noise is visible.
Things I have checked so far:
One additional observation: if I turn the transmitter pulse off and set the receiver gain to 0, the blanked portion of the FID (~0.125 s) actually shows more noise than when it is acquiring normally.
Has anyone seen this type of periodic receiver blanking on a 400MR? If some one recognizes the symptom and can save me from rooting through the console internals, I would greatly appreciate it.
Thanks,
Shawn

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Received on Wed Mar 11 2026 - 12:06:43 MST

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