> Ed Hare has said that the ARRL labs choose input powers
> down around -100 or so dBm, near the S5 meter reading
> level. Both Ten Tec and Rohde use input powers from
> 80 to 100 dB higher. That is going to make a significant
> difference in the results depending completely on the
> "real" slopes of the IM curves through out the complete
> input power spectrum response of each radio tested.
Just a quick check of an unspecified receiver I just tested in the screen room:
Reference level IP3
-120 dBm 3 dBm
-100 dBm 5 dBm
-80 dBm 2.5 dBm
-75 dBm 3 dBm
-70 dBm 3.5 dBm
-65 dBm 8.5 dBm
-60 dBm 12 dBm
-40 dBm 11 dBm
-20 dBm 8.5 dBm
These tests were done with 3 generators and a spectrum analyzer. One generator
was set
at the reference level and the other two were set to create a third-order IMD
response.
The S meter was pinned for the upper two levels, so it could not be used as an
indicator
of receiver output. The reference tone was set at the reference level and the
two undesired
tones were set until the product was at the same level as the reference tone in
the receiver
audio ouput. IP3 was calculated from those levels. The jump between -70 dBm and
-65 dBm
is rather abrupt, similar to the bump that was seen in the graphs I made for
the QEX article
sidebar. That sort of thing seems to happen with real receivers.
I don't feel real comfortable with this method because the receiver AGC is
changing by about
a half S unit between the reference-signal only state and the state of the
receiver when both
the reference tone and intermod product are present in the receiver output
simultaneously.
But this is the ONLY way to measure IP3 in a receiver into full ACG compression
with a
pinned S meter. :-)
It looks like that at some point above S8 or so, the IP3 starts to rise. I
will add that it is my
speculation, as evidenced by the spectral output of the receiver audio, one of
the strong
off-channel test signals is pumping the AGC a bit, causing the desired
reference signal level
to drop. At this point, IMHO, the IP3 measurement is being inflated.
Is it "real?" Yes and no. When the reference level was set to -20 dBm, about
S9+50 dB, the
signals that were causing the intermod were at -1 dBm, or S9+70 dB or so. So if
your receiver
is being overloaded by S9+70 dB signals, with an S9+50 dB interference S meter
reading, then
under those circumstances, the IP3 of the receiver is indeed at +8.5 dBm. At
interference
caused by S9+60 dB out-of-passband signals elsewhere in the band, as measured
above,
the IP3 is indeed +12 dBm. An approx S5 intermod product in the above table is
caused
by two S9+40 dB signals elsewhere in the band, pretty high yet, but at least in
the realm of
reasonable. In that case, the IP3 of this receiver seems about +5 dBm. And that
level seems
to hold reasonbly stable as the reference level drops, getting a bit smaller
than that as the
interfering signal product in your desired passband gets closer and closer to
the noise floor.
As I said in the sidebar, what is the "real" IP3 of this radio? In an ideal
world, the ARRL
Lab could do all of the charts and graphs, similar to the sidebar.
Unfortunately, Mike
already spends over a week in the screen room with a typical HF rig and it
would easily
be possible to double that. A few hams would love seeing additional data, I am
sure,
but for most, the eyes glaze over after the first graph. The Lab has to balance
what would
be nice to test against the available time to do it. I think that all in all,
we have a reasonable
balance.
73,
Ed Hare, W1RFI
ARRL Lab
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Internet: w1rfi@arrl.org
Web: http://www.arrl.org/tis
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