RF preamp

Woody KZ4AK says “My µBITX has always been a little deaf (couldn’t figure out why) so I built a RX front-end addition”.

Woody also wanted AGC so it became an amplifier followed by an attenuator. As with prior AGC / RF Gain mods, it inserted in the RX antenna trace (Ripley, et.al.). As built, it provides a maximum gain of ~ 8 db (+12db from mmic – 4 db from atten) with a 3.5 db NF, which can then be attenuated to more than (about) 50 db down.  Woody has not measured this exactly… but it really wakes up the RX!

Attenuator Control: >5 volts for minimum attenuation, approaching zero volts for maximum attenuation (at 22 to 5 ma, depending on attenuation setting). For manual control would use a potentiometer and voltage follower for current buffer.

Woody made no attempt to minimize the current draw. It is a bit of a pig, consuming 70 ma + control current (22ma).  Also, no attempt was made to minimize the board physical size.

The circuit uses a MAR3 SM MMIC followed by a HSMP-3816 pin diode array attenuator.

If anyone is interested,  he has a to-scale PDF for the board layout, a parts placement image, full resolution schematic, etc.  Woody can ZIP up a package for you …

He is also currently working on an audio derived AGC amp/controller for this thing. Have it sort of working, but he is not happy with the performance yet.

Reference

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