On Mon, 2006-05-29 at 11:23 -0700, Ron Castro wrote:
> My understanding is that IMD damage gets done in the first IF. Once the IMD
> artifacts are created, no amount of narrow filtering later on down the line
> will remove the ones that fall into the desired passband. Hence the 15 dB
> difference in close-in selectivity performance between the Orions and the
> '7800 or any of the other high performance up-converting radios out there
> today.
>
> Doug Smith details that in this article:
>
> http://www.doug-smith.net/orion.htm
>
> Look at the text below Figure 3.
>
> Ron N6AHA
>
Yes, the close in intermod happens in the first IF and the second mixer
of the yaecomwood receiver. It can be prevented, at some cost, by being
careful with the mixer terminations, the mixer power capabilities, and
the first IF stages. Yes, they begin to look like transmitter parts and
take transmitter driver power but it is possible. Yaesu's FT767 did good
mixer terminating while the next generation FT736 did not. The 767 has
at least 10 dB better sensitivity on 2m and most significantly, a much
better intermod performance (out beyond 20 KHz). The 736 loads the
double balanced mixer with a crystal filter. I'm publishing a mod in the
next Central States VHF conference proceedings correcting that, but I
have only a few days to also prove it works better. The cost is under $5
in parts, but 60 to 100 ma DC current for that added broad band MMIC
stage.
I'm not so sure about the terminations for FET mixers unless they are
ring switchers that need the same attention as diode double balanced
mixers. The key in a diode double balanced mixer is to not let signals
be reflected from any port at any frequency because its those reflected
signals that can increase the conversion loss while lowering the IMD
performance and increasing the mixer noise output.
Stephen Maas' tome on mixers goes into the details, but he doesn't
discuss FET mixers to any significant degree.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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