OPERATING CONSIDERATIONS (Cont’d)
By increasing the sampling clock frequency, it may be possible to “push” the spectral lines (noise
peaks) out of the 0...1 kHz bandwidth of BPM response.
Note: When the external clock signal is applied to the BPM module thru the DB9 male connector (pin 8) at the rear
panel of the BPM-RFC chassis, it overrides the clock for all the modules in the chassis.
If the spectral lines cannot be pushed out of the 0...1 kHz bandwidth, or if it is not desible to
change the sampling clock frequency, then the local oscillator (LO) frequency can be changed.
This will work only if LO intermodulation is at the origin of the spectral line. To test this
hypothesis, install the BPM module out of the chassis on a card extender (see ”Procedure”, page 8,
this manual). Put your finger on the quartz crystal oscillator (The quartz is located next to the
"Clock adjust" potentiometer). The LO frequency will change by a few hundreds of Hertz.
Observe the output spectrum: if the spectral lines (noise peaks) move up or down in frequency, it
may be possible to “push” those lines out of the 0...1 kHz bandwidth of BPM response.
More than 2 kHz LO frequency change can be obtained by installing a few-pf capacitor (C94) on
the BPM board.
Aliasing was observed on ALS at LBL
3
while using the LBL-built BPM prototype. Jim Hinkson
suggests a diagnostic of aliasing and a cure: "We know the frequencies for synchrotron and
betatron oscillations in our machine. These frequencies change with machine settings. It would be
a simple matter to program a clock frequency based on tune measurements. This would allow us to
avoid aliasing under all conditions. It's easy to see if aliasing is a problem. Using a low frequency
spectrum analyser I can clearly see coherent "beam" motion in the sub-micron range. If aliasing is
suspected, all one needs to do is change the clock frequency and see if the spectral line moves. If it
does, it's aliasing."
3
Jim Hinkson, private communication, June 1995
BERGOZ Instrumentation
Multiplexed
01630 Saint Genis Pouilly, France
Beam Position Monitor
Tel. +33-450.426.642
Fax +33-450.426.643
Page 27
User’s manual
BERGOZ Instrumentation - 01630 Saint Genis Pouilly, France - Tel.: +33-450.426.642 - Fax: +33-450.426.643
TVA-VAT-IVA-USt. Nº FR88414997130 - Sàrl. capital 152K - Siren 414 997 130 R.C.S. Bourg - APE 332B
