[HCARC] Computer Interferance of 2 meter Rig/ Trivia Question

[Kerry Sandstrom]

Gale,

While your computer processor has a speed of 2.3 GHz, There are a lot of 
other things in the computer that are going at much slower speeds.  If you 
look around, you will find the clock frequency (and XTAL) are much slower 
than 2.3 GHz.  The way the processor clock works is it excepts a much lower 
frequency clock from the clock generator on the motherboard. An oscilator in 
the processor chip generates the 2.3 GHz clock that is phase locked to the 
clock from the motherboard.  I don't know what frequencies are currently 
being used but I wouldn't be surprised if they were in the 50-150 MHz range.

These aren't the only clocks in a computer.  Memory runs at a much slower 
rate.  Typical memory clock speeds have been in the 150 MHz to 800 MHz 
range.  That is why the processor designers have put a small amount of 
memory on the processor IC.  It operates at the processor clock speed.  This 
clock is also derived in most cases from the motherboard clock.  Any 
synchronous communication ports you have will have an associated clock often 
very slow for low data rates.  Sometimes these are generated by a separate 
XTAL oscillator and other times they are derived by frequency dividing the 
motherboard clock.  There are also clocks associated with the display, the 
sound card (or portion of the motherboard) disc drives, etc.  the list of 
different frequencies a computer uses is endless.

The lowest power and most difficult to hear is the processor clock because 
it is very low power and contained entirely within the processor chip.

Several years ago, some of the computer manufacturers started using spread 
spectrum clocks.  The reason was to avoid generating a narrowband clock 
signal that would exceed the FCC emission limits.  By randomly spreading the 
clock over a ramnge of frequencies, the enrgy in the specified FCC 
measurement bandwidth was reduced but the total energy radiated remained the 
same.  Computer manufacturers aren't the only ones who switched to spread 
spectrum clocks, the switching power supply manufacturers did also.

Of course the clock is generated using digital techniques so whatever the 
logic levels used in the computer are the clock levels and of course it is a 
nearly square mwave.  I once built a VHF XTAL calibrator using Schottky TTL 
devices.  You could 'see' harmonics of the 3 MHz XTAL and its 600, 300, 60, 
and 30 kHz devided frequencies out to over 1 GHz.

The only really practical solution to this problem is to separate your 
laptop from your 2 meter antenna.
The radiation is probably not coming from the computer but from the cables 
you have connected to the computer including any comm lines and power lines. 
Be sure they are as far from your antenna as possible.

Yes, I've spent a lot of times looking at unintentional emissions from 
computers and other things for a variety of reasons.  There is a lot of VHF 
RF noise radiated by a computer.  Distance between antennas and other 
electronics is the best solution.  Shielding to prevent VHF and UHF emissins 
is not a task for the light hearted!

If you have any other questions, I'd be more than happy to try to answer 
them.

Kerry