Rigol 1102E waveform down at the limit of pickup

[ColH]

Hi,
I have switched off the whole lab except for a Rigol 1102E  it sits there with 1 probe attached to Ch1. Set to x 1. Bandwidth limit OFF. SinX/X is off. Ch2 is off.
I have put the probe in an aluminium box and hooked the GND clip to it. Inside the aluminium box the retractable hook is lodged in a piece of wood (see the bitmap below).

The signal is cleaned up except for the mystery I can't explain. No matter what I do I can't account for the waveform (ii)/(iii) within the following bitmap:


A) The underlying 'sine' is 100MHz.
B) It is modulated by a 20MHz signal
AND
C) on a longer timescale in the 2.5MHz-5MHz range.

I think
A) is generated by the RIGOL sampling system (1GSa/s x 10 samples give the 100MHz claimed bandwidth) Why it should present like this on the input I do not know.

B) and C) are mysterious. I cannot tell if these are created by the scope or something measured in ambient EM that makes it through the aluminium. Remember: everything in the lab is off at the mains except the RIGOL, which is being powered by a fully charged UPS about 4 meters away (the aluminium box kills the rubbish it emits when charging).

Heads up: 1 am a few km away from 4 high power digital TV towers on huge masts on a hill. I am 1km from the nearest cell tower. WiFi is everywhere.

I know it's right at the bottom of the Rigol's vertical scale (2mV/div). I am chasing down signals and I don't like not knowing where things come from.
I am relatively new to the RIGOL. It's generally coping OK but I don't like mysteries.

I'd just like you to have a look and see if anyone here can offer any insight into what I am looking at.

All advice greatly appreciated.

Cheers
Colin

[tautech]

The 100 and 20 MHz might be coming from clocks on the PCB. Download a service manual and the clock frequencies should be listed in the troubleshooting section.


Oh, and welcome to the forum.

[ColH]

Hi Tautech et. al.

To get to the bottom of this I added three more measurement sites. This means I have 4 measurement sites at different distances from the TV towers:

0) ~35km Parkville (University)
1) ~5km Lab Boronia
2) ~4km The Basin
3) <1km Glasgow Rd
 

With the same setup as original, with the probe inside the little faraday cage, with a timebase of 20ns the attached results were measured.



If this was an internal RIGOL noise source, then the signal would have been the same or similar at each site. It clearly decreases with distance from the TV towers (there are 4 or 5). So the most plausible verdict is that the local TV towers (each 50kW in the range 180-226 MHz) are swamping the input of the RIGOL and causing the strange modulation shown in the bitmaps. If I had to guess what is happening I would say that the the common-mode performance of the input circuitry is leading to the artifact.

At least I think I might have solved the mystery!

Anyone got any suggestions as to what kind of Faraday cage would exclude this frequency range?

regards
Colin