Soft Rock 40 Frequency Doubler


I've been using a NJ QRP DDS card to make the SoftRock 40 more frequency agile. One problem in doing this is the Soft Rock needs a local oscillator that is 4x the frequency to be received. The NJ QRP DDS can only go to about 30 MHz without changing its output filters.

Frequency Doubler Circuit

The circuit below shows the circuit I got from "Experimental Methods in RF Design' to double the local oscillator frequency on my SoftRock 40.

The needed transformer was determined somewhat experimentally. I initially started with a 1:1 turns ratio, but that did not leave sufficient driver level to overcome the voltage drops across the diodes I used. Changing to a 1:2 ratio gave enough of a voltage step up that I get clean clocking signals from the LT 1719 comparator on the Soft Rock board.

I do not believe any of the components are particularly critical, my parts selection was mostly dictated by the parts I had on hand.


I'm happy with the performance I observed using this doubler between a DDS Daughter card and the Soft Rock 40. I've not done any formal measurements, so my results are only anecdotal.

Listening on 30 meters the doubler circuit worked well, was able to hear signals at comparable levels to what I can hear with an unmodified Soft Rock 40.

I also set up the local oscillator for 20 meters and the clocking signals looked good on a scope. I was not able to try it on the air on 20 meters as the input bandpass filter on the Soft Rock 40 does no extend to 20 meters. I'll need to change the input bandpass filter to see how well this setup really works on 20 meters.

Comments to: Bill Tracey (

Last Updated 5 September 2005

Copyright © Bill Tracey 2005