Noise Detective

[Eric Foster]

Just spent two days tracking down some noise on the receiver output of the Goldscan 5

 

[Joe Gwinn]

Eric,

Is this fax machine legal to sell in Europe? I though Europe had pretty stringent EMI limiting standards. I'm wondering if something is wrong with the power supply in that fax machine.

Joe Gwinn

 

[Anonymous]

Monday I was helping another engineer test a 12-bit ADC, running FFT's and looking at spurs and noise floor. He was baffled by huge spur at around 75MHz, that was clearly non-harmonic. I looked him over, and told him to leave the room. The spur vanished. It was the cell phone on his belt.

- Carl

 

[bbsailor]

Eric,

It is amazing how much RF noise we normally live with. I thought my Tektronix 475A scope was defective as I was getting a blurred image at the high bandwidth (250 MHZ) setting. I was assured by Tektronix experts on a Tektronix forum that it was just the scope picking up RF. Once I lowered the scope bandwidth to 20 Mhz, the signals cleaned up.

This means that as we attempt to make PI machines more sensitive, shielding becomes more critical. We may even need to start carrying around a pocket spectrum analyzer to select a PI operating frequency that has less competing signals. It's a good thing that some PI machines have a variable frequency to adjust out the interfering beat frequencies.

bbsailor

 

[Eric Foster]

Hi bbsailor,

One project I had required a spectrum analyser and it was quite entertaining to watch signals coming and going, amongst the fixed ones from broadcast and other transmitters. Good job our brains seem to have good rf immunity.

Eric.

 

[Eric Foster]

Hi Carl,

One good thing that came out of my investigations was that I have found a way to eliminate the 70kHz inverter. This did not cause any noticeable noise problem, except at one point on the TX frequency control, where there was a beat. Now I generate the 5V rail off the TX driver, so no spikes at all in the receive period.

I've noticed a couple of times, when detecting while having a mobile phone on standby, the the detector has a short burst of wobblies just prior to the phone ringing.

Eric.

 

[bbsailor]

Eric,

The jury is still out on the long-term conclusive effects of cell phone RF close proximity (near field) on our brain tissue. The political pressure to find no ill effects must be tremendous given the popularity and profits from cell phones. It is good that PI coils are operated far enough away from the head, with very short pulses, that we don't expect PI coils to fry our brains.

bbsailor