[WriteLog] Icom 756ProIII SDR 1st IF pick off point when INRAD filter installed

[Bruce King]

Some time back I had the INRAD roofing filter installed in my trusty
756ProIII and have been pleased with the results. I also set up the
RTL2832u SDR dongle and brought the 1st IF signal out using the "no mods
required" on-board test jack. Not so pleased... The test jack now sees
the post INRAD filter and significantly limits the SDR bandwidth in the
bandmap.  I had been eyeing the IC7610 as a replacement for the ProIII,
but based on a recent posting in our local contest club which pointed
out that the ProIII receiver seemed to be as good as the 7610, the 7610
goes on the back burner until there's a significant step function change
(or until I read about what I missed).

 

But that got me thinking again about the SDR displayed bandwidth. I
decided to take one more pass through the circuits and see if there was
a better point to tap the 1st IF that was accessible without having to
do any significant soldering around SMC items. I did find a couple of
points, only one of which was visible w/o taking off filter covers or
removing the board. But being visible is not the same as being
accessible! So I resigned myself to buttoning up the rig and living with
what I had. Then I had the "DOH" moment and realized that Hey, wait,
where does INRAD get their IF signal from, and where does it go? Sure
enough they tap a good spot, and bring it to a jack on their "floating"
PC board which has life-sized components on it. I tapped the jack input
with a 50pf cap (smallest I could find in my junk box), scavenged a
small coax with a SDR matching connector on it, and brought it out
through the back of the rig. Purists may want to actually mount a jack
on the INRAD board. 

 

Bottom line, no change to 756ProIII performance or screen display, and I
now see the entire desired spectrum on the bandmap; and I still have my
rig play money!

 

73 Bruce W1CSM

w1csm_1 at comcast.net