At 09:46 AM 1/1/98 -0700, you wrote:
>Stan and Leon; In regards to your comments about passive mixers having
>resulting in recievers with "very high noise figures";
Stan, Leon and James:
I have been using an SBL-1 DBM for over 10 years as the basis for my
receive converters for frequencies up to 432MHz (could be higher frequency
limit with DBM's suitably rated).
The key to minimum noise figure is proper termination of the DBM ports (IF
especially) and low-noise post (IF) amplification. Referring to some of the
information sources in K5BDZ's e-mail is essential, since any good
DBM-based receiver design is going to use a diplexer at the DBM IF port,
prior to the post-amp. Seems that the design I used came out of an ARRL
Handbook from the early to mid-1980's and utilizes a JFET as the postamp
(28MHz IF frequency). I took the thing to a VHF conference shortly after I
got it built and, if memory serves, it measured about 9.5db noise figure
(on a commercial H-P meter).
The minimum noise figure of any DBM front-end will be at least the
conversion loss of the mixer...nothing you can do about that...but good
design can keep additional noise figure deterioration down to as little as
2.5 to 3db.
Hope you proceed with your project...the SBL-1 (or similar) is a good choice.
72/73,
Jack Parker, W7PW
Fernley, NV
QRP-L #1310
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