6 Sept 2011

Higher power on 137kHz

Today I did some work on increasing the output power of my 137kHz transverter used for WSPR, with mixed results. When driving the IRF540 PA with a decent square wave I can get around 24W out (was around 5-8W) from a 13.8V supply with decent efficiency. When using the drive from the mixed-down transverted signal I am having less success. The issue is driving the MOSFET gate from a really low impedance and ensuring it is as square as possible with the right mark-space ratio. Most 137kHz transmitters do this by driving at 274kHz with ICs designed to drive a pair of MOSFETs whereas I want to go in at 137kHz using discrete parts. This is a solvable challenge and I'll have this up and running in a few weeks. 25-30W would give me 6-8dB ERP improvement and more should be possible from a higher PA supply voltage. This would take my ERP up to around 0.5mW  ....heck that's high power for me!


David said...

Have you seen the gate drive app note?

That lists ideas for gate driving from TTL. CMOS and Linear circuits. I used one of the ideas for my 40m QRSS MEPT PA (that's only 300 mW though)

Dave G7UVW

Roger G3XBM said...

Thanks Dave. I have managed to fix the drive using the Schmitt trigger design from Dave Seargent. Now getting >25W using the full transverter.