Omni VI 2nd order IMD Mod

Del Seay
Thu, 28 Mar 1996 10:16:49 -0800

H. L. Serra wrote:
> Del- I think there are two separate Tentec mods you all are discussing
> here, both involving only a few parts. The first is the mod to effect
> better second and third order IMD rejection, a quick and dirty version of
> Rohde's suggestion in QST to change all the front end switching diodes to
> PIN diodes, which have a higher resistance to rectification of strong
> signals. Tentec's answer to this is to switch out the resistors which set
> the switching voltage through the exisitng Tentec diodes. Their engineer
> told me that raising that voltage on the existing diodes substantially
> increases the reluctance of the diodes to rectification-- it's provable
> according to a formula. The second mod also uses just a few parts, and is
> meant to increase the resistance of the product detector to IMD from two
> or more strong signals LEGITIMATELY within the passband of the OMNI. G4PKT
> or someone produced a daughterboard to fit right on the OMNI product
> detector board, and which contains a higher order product detector than
> the one on the OMNI, which is bypassed. I installed the daughterboard and
> it does work better than the old one-- even with strong signals within the
> filter passband. For example, on CQ Alley on 20M and 40M during CW tests,
> you can hear all the signals separately, even weaker ones. Tentec claims
> that their mod using a couple of parts works just as well at no expense
> (the daughterboard is about $80-90, and installation takes a few hours). I
> really cannot judge how well their mod works, since I only have the G4PKT
> daughterboard. 73, Larry N6AZEHmmmm, looks like I didn't do my homework! But what you say makes a
great deal of sense. The resistance change in the Bandpass filter
area is the one I am referring to. Didn't look up the article in
FOCUS to see what that was about, just asked Ten Tec when I ordered
the new Firmware.
Now that I look at what was done, I don't see any reason for a
gain reduction at the rf stage, and don't understand why the change
in the 'Ringing' that a lot of us suffer from.
 Damn, Larry- now I hafta find my glasses so I can look at this
Thanx  --Del, KL7HF--