[TenTec] More IP3 Stuff

[Jim Reid]

Ed,  W1RFI,  reported on Friday results from the ARRL
Lab screen room:

> 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 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 output. IP3 
> was calculated from those levels.......

> 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. :-)

Oh my, now some added lights have been turned "on"!
I now have a whole bunch of questions/thoughts,  hi.

First off:  an epiphany!  I had assumed all was being done
in the linear region of the rcvr to audio transfer characteristic;
that is,  NO AGC action at all!!  It had not dawned upon me that
the AGC would be active during a test for a rcvr's linear
characteristics and IP numbers....

Now that the horse has changed colors,  I have gone back
and read in great detail again the previously ref. pieces by
U.  Rohde (two most recent QEX's) and by  Doug Smith in 
the past Summer's QEX with the detailed side bar by Ed
Hare.  I just had overlooked the author's various mentions
about the AGC action and impact....reading too fast I guess.

However,  I am beginning to doubt the applicability of
any meaning to what is being done to use the IP numbers,
using the techniques reported especially, in finding any real
meaning at all in use comparing different radios!  I just don't
see how.

As Ed reports from his screen room experiment:
"to measure IP3 in a receiver into full ACG compression...
it looks like that at some point above S8 or so, the IP3 starts to 
rise."  And it certainly does,  because the AGC is causing the
rcvr system to behave exactly as it would were ATTENUATION
being added at the front end!  In fact,  Rohde plots just such
behavior,  I now see,  in Fig. 45 of the Jan/Feb QEX piece,
where he illustrates with a curve,  and adds this text of
explanation under the curve,  "A plot of intercept point
behavior for the receiver system (then he refers to a complete
rcvr block diagram,  his Fig. 43).  The RF attenuation activated
by the AGC voltage IMPROVES [increases] the third order
intercept point."  He shows that the IP3 of this particular
rcvr is a constant +20 dBm with no AGC  from the MDS level
up to the area of between S7 and S8 input signal pair level.

AGC action begins,
in the rcvr,  at about -88 dBm,  after which the IP3 improves
at a constantly increasing rate of 0.5dB improvement higher
for each added dB of input signal level all the way up to about
1 mv signals (-47 dBm or S9 + 26 dB).  At this level,  again in
this particular rcvr,  another part of the AGC system "kicks in"
which continues to increase the IP3 level another 1 dB for
each added dB of signal increase all the way up to where
the IP3 has grown clear to +60 dBm with the RF preamplifier
switched OFF,  and the input signal levels now at about
7 mv,  or some -29 dBm.

So as Ed speculates in his note,  yea verily, IP3 will increase
with increasing signal pair level when the AGC  system is active,
because it REALLY does increase!

Because the AGC systems in every radio are different --
how much the gain is reduced vs. a particular input
signal level --  we can gain no true comparative information
of value between rigs once the AGC system activates!
At least that is what I now believe (I could be all wet also!).

I suggest,  as Ed Hare has already suggested,  that the 
IP numbers collected,  and intended for use comparing
various radios,  be based only upon input signal pair
levels,  at stated spectrum spacings (20 kHz and 5 kHz)
as Ed has plotted in his side bar piece last Summer, which
will generate "spur" signal amplitudes only between
the MDS level and probably
no higher than -97 dbm (S5 on the Collins standard
scale where S9 = 50 uv,  or -73 dBm.  With that data
plotted, ( the same as Ed did in his Figs A, B,  and C,
pages 50 and 51,  Jul/Aug. '02 QEX ) determine the IP
numbers from a best straight line fit.  Both the RF preamp,
if any,  and the AGC system are to be OFF !

If the AGC system must be ON in order to handle the needed
input power to generate third order "spurs" at the MDS level,
or a few dB higher,  then the test report must tell us about
the AGC system characteristics!  How many dB is the gain
REDUCED in that particular radio for,  say each 3 dB increase
in intermodulating signal pair level up to that needed to reach
a level where the "spur" amplitude can be read and measured.

So,  in addition to the power levels used to measure the IP
numbers,  we should also be told the intermodulating signal
pair spacing,  whether either or both are in the rcvr's
instantaneous operating bandwidth,  or not (if not,  is
one of them still inside some earlier "roofing" filter BW
of the particular rig,  or even outside a front end band
pass filter (that is simulating international broadcast signals
in any of the 9.6, 15.2, 17, or 21.5 MHz broadcast bands.)
Why,  because the amplitude of one of the intermodulating
pairs will be reduced by whatever the attenuation of the
filter is outside the instantaneous operating band width.

Maybe I am worrying about all this too much,  hi.  But as
Ed has said,  and Rohde has clearly written,  tis a complex
situation,  and are rig comparisons of such numbers collected
in the screen room really meaningful to us?  Not sure just now.
If so,  it seems to me that we need to know the details of each
radios AGC system.

73,  Jim  KH7M