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SPICE-ing things up

A project log for Simple Digital Mode Transmitter

A low-power, all-digital transmitter.

RobGRobG 05/14/2021 at 17:370 Comments

Had a bit of a muck about with LTspice, but can't seem to work out a way to gang the DAC outputs together to get more power - the balun approach doesn't seem to work*.

However, what I did find is that the filter supposedly on the module is a bunch of junk - certainly not suitable for driving a notionally 50ohm antenna. A better one might be:

C2=C4=C6=22pF, 

C1=C7=33pF,

C3=C5=82pF,

L1=L2=L3=100nH

together with R4=51ohms, R5=no fit, R6 (Rset)=1.95kohm. Oh, and a series DC blocking cap somewhere.

LTSpice then gives the output power around 20mW (+13dBm) and rolls off at about 70MHz. When I next get into work I'll swap all the components over, and stick the output on a power meter and see...

(*update: oh hang on, maybe I've just worked out a way to get ~38mW (+16dBm) out?)

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