[TenTec] 1 kHz IMD with Preamp ON/OFF

[Bill Tippett <btippett@alum.mit.edu>]

Hi Michael,

        I have a few questions which others may be interested
in as well.  In studying Orion's ETR, I noticed that the IMD
performance is actually better with the preamp ON rather than
OFF.  This seems very strange to me since I would have guessed
that an additional preamplifier and gain would have resulted in
degraded IMD performance, which I believe is the usual case in
IMD tests.  In Orion's case, I read the following from the bottom
set of swept IMD graphs on page 20 of the ETR.

http://www.arrl.org/members-only/prodrev/pdf/lab/orion_etrr.pdf

Orion Swept IMDDR3, 1 kHz spacing, 14 MHz:

Preamp OFF - 84 dB (left bottom graph on page 20)
Preamp ON - 91 dB (right bottom graph on page 20)

Does this make sense to you?  Needless to say I am very happy
if it is true, since my normal mode of operation is with preamp ON,
but I am quite surprised that IMD measurements improved instead
of degrading.  Could the data have been transposed between preamp
ON and OFF?

        For comparison, it is very interesting to open the equivalent
K2 graph at the top of page 20 on the K2's ETR here:

http://www.arrl.org/members-only/prodrev/pdf/lab/k2.pdf

When the K2 tests were made in 2000, ARRL only included the
+/- 200 kHz swept graph, so we should compare K2's graph to
Orion's 200 kHz sweeps which are the MIDDLE set of graphs
on page 20 of Orion's ETR.  If you want an interesting comparison,
open two browser windows so you can view both of the above
ETR's simultaneously.

	I am left wondering:

1.  Why does Orion's performance improve with Preamp ON?
2.  Why does K2's performance degrade so much from 5 kHz
(88 dB) to 1 kHz (67 dB)?

Is there a simple explanation or am I possibly misreading the
data?

				73,  Bill  W4ZV

	

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