EMRFD Message Archive 7493

Message Date From Subject
7493 2012-05-09 08:01:57 Edson High Isolation RF Preamp
Hello everyone,

I am in search for a high isolation RF preamp to be used as the front-end of a sampling detector for a simple SDR HF receiver. The preamp should have a disciplined output impedance in order to maintain a stable input impedance for the detector. The high isolation is needed in order to prevent LO leakage to the antenna. Does anyone suggest a design that can be home brewed?

I am considering trying a hybrid cascode amplifier with a 6 dB attenuator pad at the output to discipline the output impedance. However, I see the hycas being widely used as a FI buffer, but rarely as a front-end. Has anyone tried using the hycas a front-end? Are there drawbacks?

Any comments will be much appreciated.


-- Edson, pu2mwd
7500 2012-05-10 16:56:18 JamesDuffey@comca... High Isolation RF Preamp
Edson - The cascode amplifier should work fine as a front end. The cascode circuit architecture was developed for RADAR front ends by the MIT Radiation labs during World War II. After the circuit was publicize in the Rad Lab books, it was picked up by RCA and made fringe TV reception possible in the early days of TV.

A dual JFET version was popular in the early days of 2M FM here as well. It should work fine as you intend it. You won't get the lowest noise figure possible, but for your application, that does not seem to be much of a problem. Reverse isolation should be good and you can model that in one of the many SPICE variants. - Duffey KK6MC

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