[TenTec] More IP3 Stuff

[Jim Reid]

Hi again,

Am still thinking about the deltas between the ARRL
IP numbers as they report about the Argo V and the
Ten Tec claimed typical specs.  Not concerned,  however,
as TT reports pleasure with the ARRL report.  Also,
several users of the V during the last contest have
reported that it did very well in the crowded bands.
So,  will let that issue be settled.

I have one of TT's RX-340 rcvrs.  Was going to trade it
in toward my coming new Orion;  sent the 340 to TT,
they had it for awhile.  But,  after reading certain info
which Ulrich Rohde came forth with about the 340, decided
I did not want to part with it afterall!  Ten Tec folks being good
guys,  sent it back to me, no problem.  So it is happily
home on Kauai again!

Anyway,  if you go to the RX-340 spec page at the TT web site,
you will find two interesting numbers about the IP of that
rcvr.  Web site is:

http://www.tentec.com/rx340.htm

On down the page a ways,  you find a table of TT's specs
for IP:

With the preamp OFF,  TT lists the typical 3rd order IP as
being -30 dBm;  and since it is a "Mil-Spec" rig,  they must
also list a guaranteed number,  which they give as -25 dBm,  min. .
They also show what happens with the preamp ON,  and with
the attenuator active;  both change the IP's typs and mins.

Ten Tec does not list either the signal spacing,  nor the
input power levels used during the test for these numbers.

Elsewhere on the Ten Tec page,  Dr. Ulrich L. Rohde, KA2WEU,
is quoted as having written:

"In testing the RX-340, I find that this is the only affordable, 
totally DSP-based receiver where the digital AGC is as good, 
if not better than analog AGC systems.  The intercept point 
for reasonable spacing is very impressive, better 
than +35 dBm in all cases."

Again,  no signal levels nor specific signal spacing is given,
and I have no access to his actual data on the 340.  However,
Rohde did report specific numbers used in tests on two
amateur radios:  the FT-890 and the IC-746.

The powers he used in these test measurements (see
QEX,  Jan/Feb 2003,  pgs. 30 - 31) were -20 dBm,
or S9+53 dB (based upon Collins "standard" where 
S9=50 uvolts,  or -73 dBm) for the measure of the
FT - 890,  but -22 and -10 dBm for two measure runs
on the IC-746.  Why different input powers for the
two radios,  he does not say.

For signal spacings,  he has used  100 kHz when testing
the FT-890;  and he used both 100 kHz spacing and
25 kHz testing the IC-746,  again,  no explanation is given
for his choice of the two spacings used.

>From this,  I presume he used both different signal spacings
and input power levels from what Ten Tec used to generate
their typical and guaranteed minimum IP2 and IP3 numbers
for the RX-340 radio.  And,  I am sure that the ARRL Lab
used different input powers and signal spacings than did
Ten Tec to come up with the differing numbers they have
for IP3 and IP2 for the Argo V,  TT says nothing about 
typ IP numbers for 5 kHz spacing in that instance.

Interesting to me is that TT lists IP3 guaranteed for the 340 
as being at -25 dBm,  and Rohde prints, "The intercept
point.........is very impressive,  better that +35 dBm in
all cases."  !!  That's a 10 dB difference, with TT being very
conservative,  but, of course,  they have to stand behind
with  a guarantee.

As Ed Hare of the ARRL has written,  the IP numbers are a
moving target.  They depend upon where you measure in
the input power spectrum for a given radio,  what signal
spacings are used and what gain or attenuation loss is
used ahead of the first mixer in the radio.  To be meaningful,
and comparable from one radio to another,  all three
parameters ought to be specifically listed in any published
test report.  Only then can we really be comparing an 
apple to another apple;  and not a Pippin apple to
a Delicious apple either,  hi.  

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.

73,  Jim  KH7M