[Amps] IMD and spurious measurements
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.
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
More information about the Amps