Amateur Radio Pages
updated November 2014
Web Site By W G Taylor - Created Using Serif WebPlus X6 - Copyright ©  All Rights Reserved
LF and MF Experiments etc - 15
Harmonic Amplifier  (Frequency Doubler)
Wishing to transmit modes on 137kHz which normally would require a linear transmitter system I looked at the possibility of generating a signal from my HF transciever to drive my Class D transmitter at it’s times two (x2) input frequency ie. For 275.400kHz input the output will be 137.700kHz.

I browsed various Internet sites and found several circuits which looked promising. I constructed a few IC based frequency doubler devices but was unable to produce the desired output. So I started from scratch and constructed a tuned frequency doubler from circuits studied in the G-QRP Club journal “SPRAT” (Small Power Amateur Radio Transmitters). For the first time I was able to output a reasonable looking doubled frequency. The single stage low power amplifier output was tuned to 274kHz, so 137kHz input gave 274Khz output. After making several variations of the doubler I finally had a circuit which I could connect to my LF/MF transverter output and drive my Class D transmitter to operate with a few modes which previously I could not use, for instance, WSPR, Pactor, Amtor, MFSK, Domino, Olivia, Throb, WSJT etc. The device would not allow the use of amplitude modulated modes like BPSK, PSK, QPSK and WOLF etc.
My first version (v1.0) worked reasonably well but was very critical to adjust and was effected by the load placed upon it and variations of the tone frequency of multi-tone modes.  This resulted in regular re-setting meaning that it could not be reliably left un-attended. After several modifications to the circuit I finally got a reasonably stable design (v4.1) which I could run without continuous monitoring.

Before running completely un-monitored I did of course transmit many times under varying conditions.

Continued next page.
Harmonic Amplifier v1.0
Harmonic Amplifier v4.1
Harmonic Amplifier v4.1 inside and outside
Frequency Doubler PCB v1.0
LF & MF experiments 16 LF & MF experiments 14