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~75kHz Interference

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Simon

Sat Oct 13 2007, 11:13PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

As a part of a university prac, I've been using a scintillator to detect muons and measuring the timing of pulses to estimate halflives.

We've got nice data but there's a huge spike in the ~13us time period, which corresponds to ~75kHz. I'm curious as to what this interference could be. It seems too high fres to be a CRT.

Any thoughts?

This is a lab environment with lots of equipment (nuclear sources and the usual electronic lab gear), so I guess it could be anything.

Amateur-Scientist

Sun Oct 14 2007, 03:39AM

Registered Member #1015 Joined: Fri Sept 21 2007, 06:43PM
Location:
Posts: 14

It could be a nosiy switching power supply.

Dave Marshall

Sun Oct 14 2007, 05:00AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I can hear noise from one of my CRT monitors well into the HF spectrum, above 10Mhz. If theres one thats arcing slightly to the core of the flyback, or that has a dodgy capacitor, it could easily generate a signal in that range. It isn't likely to be quite that narrow though. I'd wager its something using a cheap oscillator or a dying appliance nearby.

Dave

Carbon_Rod

Sun Oct 14 2007, 05:38AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

IIRC CRT fly backs are usually only 12-25KHz

Best way to track the problem is to selectively shutdown (unplug) each piece of equipment to see if it changes.

Some noise sources that can be detected in sensor front ends:

Calculators
Wrist watches
Cellphones or iPods
Florescent fixtures (in laptops too)
Broadcast antenna
Old equipment (oscilloscope or spectrum analyzer too.)
Security System (and smoke detectors)
WiFi or network points (often slip by the FCC)
Exhaust Fans
Corrosion monitoring probes

Rule number 1: Never mix analog and digital areas unless absolutely necessary.
Rule number 2: The Sun's EM spectrum is quieter just before dawn.

Not knowing the design, I can only blindly recommend testing the design with batteries (helps for debugging jfet amps or National's weirdness.)

Cheers

Dr. Dark Current

Sun Oct 14 2007, 06:51AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Simon wrote ...

As a part of a university prac, I've been using a scintillator to detect muons and measuring the timing of pulses to estimate halflives.

We've got nice data but there's a huge spike in the ~13us time period, which corresponds to ~75kHz. I'm curious as to what this interference could be. It seems too high fres to be a CRT.

Any thoughts?

Yes, some CRT monitors can have line frequency up to 100khz, and 75 is fairly common for a computer monitor

Simon

Sun Oct 14 2007, 08:32AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

wrote ...

Best way to track the problem is to selectively shutdown (unplug) each piece of equipment to see if it changes.

Not when "each piece of equipment" includes other people's experiments. :)

The interference is very narrow. Here's a cheap plot of the raw data for curiousity's sake. The bins count the number of double pulse events separated by various time periods. The spike is at ~13us. The full scale represents 80us.

Edit: oops, that's not quite the raw data, and I've been mixing up the scale. Full scale is 8us and the spike is at 1.3us. The interference is ~750kHz.

Bjørn

Sun Oct 14 2007, 03:25PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have a program that I use for such mysteries and it says:
750000 is a multiple of 24 by MIDI datarate (31250 Hz)
750000 is a multiple of 48 by PAL B,G,H,I,N / SECAM horizontal sync frequency (15625 Hz)

Chris Russell

Sun Oct 14 2007, 06:10PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

The 48th harmonic of the PAL sync frequency looks like a good candidate, but I would expect to see other harmonics in your data. Any TV sets running nearby that can be shut down?

This one is a bit further off the mark, but you might want to consider AM broadcast interference as well. I'm not sure where your university is, but it may be in close proximity to a broadcast transmitter at or near that frequency.

Bored Chemist

Mon Oct 15 2007, 06:00AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Can you tune a radio to it? Any modulation might help to ID the source and you can walk about + see where the signal's strong.

Hazmatt_(The Underdog)

Mon Oct 15 2007, 09:37PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

What's the Frequency of your college AM Radio Station?

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