>At 10:13 1997-12-11 -0800, Rich Measures wrote:
>
>>- Two friends built G2DAF amplifiers. They conducted two-tone tests.
>>The linearity was acceptable as long as they kept the grid current low.
>>I measured the IMD of these amplifiers, on the air when SSB
>>voice-modulation was in use. The total IMD products from either
>>amplifier was in the 21 - 23 db range when normal amounts of grid current
>>were flowing. (measurements were referenced against HP-355 step
>>attenuators}
>>.
>
>This makes me curious, as I`m trying to define a different methode to measure
>IMD myself...
>
>Exactly how, did you do that Rich?
I used two**, double-filter receivers whose -60db bandwidth was <3.6kHz.
For a 2.4kHz transmitted bandwidth signal, I compared the signal strength
on frequency to the signal strength 3.6kHz above or below the signal
being measured. For measuring a LSB signal, I measure the signal level
up 3.6KHz. For measuring an USB signal, I measure the signal level down
3.6 kHz. The offset must be such that it separates edge of the transmit
window skirt from the edge of the receive window skirt by roughly 40db.
If the two windows overlap, the relative measurements are not going to be
accurate.
>What point in the the speech envelope did you use as reference?
The detector was the S-meter/AGC circuitry, which normally uses a peak
detector.
>What word/phrase was used?
Ordinary speech, but NO swear words.
>How did you establish a constant reference?
- The measurement was of differential signal level, not absolute signal
level. The reference was the HP-355 1db/step attenuator set that was
used to generate a calibration graph of the S-meter reading/s versus real
decibels.
>What bandwidth did you cover?
garden-variety SSB.
>How did the IMD products decay? i.e. what level at say 5 KHz from f0?
I did not measure it. I was not concerned with distant products. I
wanted to see what the ratio (in db) of the signal level in the
fundamental window was compared to the signal level just outside the
window.
- Every SSB radio and every linear amplifier generates splatter outside
the fundamental transmitting window. A signal report of 'you are not
splattering' is abundant bovine feces. The question is relatively, how
much splatter? A signal report of S9 splatter is good if the
fundamental signal is 40db higher. On the other hand, a signal report of
S6 splatter is not good if the fundamental signal is S9 (+18db).
{**a single receiver can be used to make the measurement by doing the
3.6kHz offset with the RIT.}
cheers, Lars
Rich...
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|