We are having a noise problem on our Varian Inova 500.
When acquiring a one-scan proton spectrum, several (5-6) noise spikes are
observed. These spikes are at 700 Hz intervals. They are not phase
coherent, amplitude varies which each spectrum acquired. It can only be
observed if the gain is set to approximately 25 or greater and is fully
present with the pulse width set to zero.
What I have found so far:
The noise is being injected into the probe through the VT cable, the noise
disappears when the cable is disconnected from the probe.
It is being transmitted into the VT cable via through air transmission, as
the signal is still fully present when the VT cable is disconnected from the
controller.
The signal appears to be coming from the RF side of the console, as
determined by placement of the loose end of the VT cable.
The noise is still present with the following turned off:
The AMT amplifiers
The pulse field gradient unit
The low-band PTS
Shim power supply
Switching the decouple channel from carbon to proton makes no difference.
Output of the PTS units appears normal.
All voltages on the main power supply are in range.
All in all, the noise generation appears to be external to the instrument
path, but is emanating from the console.
Any suggestions?
Thank you,
Bob Shanks
NMR Instrumentation Technologist
Chemistry Instrument Center (608) 262-0563
University of Wisconsin FAX: (608) 262-3160
1101 University Ave Room 2237A
Madison, WI 53706 rbshanks_at_wisc.edu
Received on Wed Apr 14 2004 - 09:11:47 MST