IMD is not a single number, even at a fixed supply voltage and power output.
Recent measurements that I have performed on my K3 show a significant variation
in IMD vs. frequency. For example, at 30W (V+ 14.5V), the IMD of my K3, varied
from -58 dB (relative to PEP*) at 3.8 MHz to -40 dB at 14 MHz. Both of these
numbers are astoundingly good, but get worse at 100W, as expected, and get much
worse at 10W, which is not. And you don't even want to think about it at 12W.
I have it on excellent authority that no one understands this IMD variation with
respect to frequency.
As to a rule of thumb for IMD dB per volt, IMHO, there isn't one. A 1 volt
change from 11 to 12 might be fairly significant; a change from 14 to 15, not so
much.
* Some words about the numbers. I believe, and a number of professional
(Collins, EIMAC) and military specifications agree with me, that the IMD
products should be compared to the level of one of the two fundamental spectral
components. ARRL, Elecraft and every other ham equipment manufacturer compare
the IMD products to the PEP of the two fundamental spectral components. So when
they measure -24 dBc on a spectrum analyzer they add -6 dB and call the IMD -30
dBc. Your receiver is an unswept spectrum analyzer; it isn't fooled by the
slight-of-hand.
On 10/29/2015 8:09 AM, frank wrote:
This subject keeps cropping up. Is this a serious issue ?
I have heard that IMD improves by 10db with a one volt increase in supply
voltage. Is this really the case or just a measurement error? 10db is a large
difference. One volt is a small variation.
What is the minimum acceptable IMD? How linear does the amp need to be? If
36db is better than 26db is 100db even better or just overkill?
Are other solid state amplifier products similarly effected?
What is the IMD performance of the K3S at 10 watts over a supply voltage
variation from 11 volts to 14.7 volts? I ask because I believe those ten watts
are generated by Dmos devices as well. Do these devices only misbehave with
varying supply voltage at higher (100w) power levels and lower supply voltages?
Should I wait to purchase a K3S until this issue is resolved? (Assuming this
is a valid issue)
Back in the good old days we used to solve problems. Today we don't have
problems, just issues.
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