[Amps] IMD and spurious measurements

Jim Brown jim at audiosystemsgroup.com
Mon Oct 26 17:35:43 EDT 2015

On Tue,8/11/2015 8:49 AM, Manfred Mornhinweg wrote:
> I can't conveniently run my computer from batteries to break any 
> ground loops, because it's a desktop computer. 

Ground loops are a fiction, a wrong concept to explain noise coupled in 
unbalanced connections between equipment powered from different sources. 
The REAL cause of hum, buzz, and other noise coupled in this manner is 
leakage currents in the Green wire (safety ground) in the AC mains.

See http://k9yc.com/GroundingAndAudio.pdf

The proper solution is BONDING, not isolation.

I've been measuring IMD of various rigs and amplifiers using an Elecraft 
P3 attached to a K3, sampling at a capacitive tap with the amp/rig 
feeding a big 500W dummy load, and the rig transmitting dits in the 
range of 30 wpm.  Because keyed CW is 100% square wave AM of a carrier, 
the harmonics that make up the keying waveform excite IMD in the DUT.

The P3 with the SVGA board is capable of resolution on the order of 2 Hz 
with 80 dB amplitude display. This is 500 times better than the 
frequency resolution of my HP and Rigol spectrum analyzers.They are good 
for looking at harmonics, but useless for looking at CW bandwidth.

Here's an in-progress report on my work. I've used Power Point to show 
it, but this is not a Power Point for a presentation, simply a 
convenient way to display the data and document the test method.


I measured three power amps using a second K3 to drive them -- a Ten Tec 
Titan 425, an Elecraft KPA500, and an ACOM 1010.

73, Jim K9YC

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