6 Sep 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!

2 comments:

David said...

Have you seen the gate drive app note?
www.irf.com/technical-info/appnotes/an-937.pdf

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)

73
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.