开云体育

Hello group! Happy new year to all!
In PA3CJD's site??there's a good compilation of articles from QEX and QST:
? ?-> Technic -> Electronics -> Radio Tech -> Radio Amateur Magazines
I am specially interested in this:
? ?
Here he makes several IMD tests on a DBM, with matched and mismatched impedances both at the IF and RF ports. I was surprised that RF port matching has an important impact.
After reading this and other similar articles, there's a point that remains unclear for me: what are the specific frequencies that really demand having proper termination? RF+LO if I use RF-LO (or vice versa)? The LO feedthrough? Whatever?
Suppose I want to receive 7.00MHz, and there are strong undesired signals at 7.01 and 7.02. 3rd order IMD within the DBM will fabricate a fake 7.00 IF even with a resistive 50 ohm load.
Now, I place a diplexer after the DBM, and a xtal filter after the diplexer. The diplexer is much wider than the filter, so the DBM will still see a capricious Z at frequencies so close as 10 and 20kHz away from the desired IF, so I thought these ones would reflect back into the DBM, mix again, and worsen the overall IMD. But I must be wrong because if this was true then a diplexer would be useless.
Can anybody explain the IMD worsening mechanism?
Thank you!
Daniel Perez LW1ECP