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
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Monday, May 29, 2006 9:33 AM
Subject: Re: [TenTec] FT-2000 Webpage
>
> What I don't understand is why they don't put in a second mixer that can
> handle the strong close in signals that got amplified in the high IF.
> Just a cost issue probably. I know in the Yaesu FT-736 on 6m, 2m, and
> 220 they didn't terminate the first mixer properly and cost extra mixer
> loss and take off 20 or 30 dB of 3rd order intermod capability. They run
> the mixer right into a narrow crystal filter where there ought to be a
> broad band termination to absorb the unwanted outputs from the mixer,
> exactly as done in many Tentecs. It isn't as if Yaesu didn't know
> better, they did better in the VHF modules for the FT-767 which predates
> the 736.
>
>
> --
> 73, Jerry, K0CQ,
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|