[TenTec] More IP3 Stuff

[Hare,Ed, W1RFI]

> 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 at arrl.org
Web: http://www.arrl.org/tis

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