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Hi ive just built a closed loop uhf mosfet modulator.
Posted: Tue Aug 23, 2016 4:39 pm
by G1BFK
ive tested it with some surplus russian devices get about 50w on 70cms. i used 3 x 19v 4 Amp laptop supplys avaliable cheap on ebay. one -s a bit of a over kill because its only used for the -19v for the opamp. I used 3mm brazing rod for the anode line. mounted obout a cm obove the ground plane. Heres a link for the circuit.
.Regard Roger G1BFK
Re: Hi ive just built a closed loop uhf mosfet modulator.
Posted: Wed Aug 24, 2016 8:32 am
by G8GKQ
Hi Roger
A novel approach! Any chance of a photo of the construction and a plot of the output spectrum?
Dave
G8GKQ
Re: Hi ive just built a closed loop uhf mosfet modulator.
Posted: Wed Aug 24, 2016 8:58 pm
by G1BFK
Hi Dave, Could probably do with a harmonic trap. Only used it on dummy load so far.The modulater drives a loud speaker well,all the advantages of dc coupling and all that just up the feedback resister a bit.
Re: Hi ive just built a closed loop uhf mosfet modulator.
Posted: Wed Aug 31, 2016 2:18 pm
by G1BFK
Hi there a mistake on the drawing the 10k negative bias resistor on the IRF10's should go to ground not the source of the devices. you cant see much on my construction because i solder copper clad board round it for screening i am doing another build with a bigger heatsink because it gets hot very quickly i will do some intermediate picys. DONT BE TEMPTED TO TWEAK THE THE TRIMER CAPS (THEY ARE HOT!) TRIM THEM USIND A GRID DIP METER. Regards and good constructing Roger G1BFK.
Re: Hi ive just built a closed loop uhf mosfet modulator.
Posted: Wed Aug 31, 2016 2:36 pm
by G1BFK
Ther are alot of fast op amps avaliable but they probably wont work. The LM715 has a cascade input which compensates for propagation delays in the loop. Regards Roger G1BFK