160, 80 and 40m Bandswitched exciter

[KM5TZ/6]

Here's a pic of my newly constructed exciter:

This is a bandswitched exciter for 160, 80 and 40m. It puts out ~120W (2 x FQA11N90). This will be used to drive a 4 x 5 FET bandswitched PA.



Here's a link to my website. Scroll half way down for more pics and details.

http://www.km5tz.netfirms.com/classe.htm

David.
KM5TZ

[W2INR]

SAWEET!!!

[frank carcia]

Nice job David. That exciter will drive a KW final so leave room to expand your finals. Great idea on the variable oscillator mixed with crystals.

[ke1gf]

Dave, any ideas where the Totem-Pole is crapping out on 40? Do the 4420's switch the (9)610 okay?

Maybe I could supply some simple modifications as to how to make it switch faster.

Possibly you could piggy back the 4420's and run fets with lower RDS(ON) maybe something like IRF520's and IRF9520's...

You've got me thinking possibly I'll build up a new driver deck and do some tinkering.

[KM5TZ/6]

Bill,
Yes it looks like the (9)610 is the problem. The gate drive waveform looks very messy on 40m but does look good on 80 and 160m.

I evaluated a couple of different 4420's. The Micrel parts are crap - good only to a few MHz above which they exhibit internal timing problem. The Maxim parts work very well - they cost a little more (~$4.50 each from Digi-Key) but are good up to 10MHz. I just got some parts from Intersil which also look good on paper but not sure I'll spend much time on them since I already bought more Maxim parts.

Since the driver FET's are not modulated, 11N90's are overkill for the Driver. Gate capacitance is what kills you on 40m so I selected a couple of lower Gate C FET's (11N40's) that I plan to drive with 3 parallel Maxim 4420's per FET. A quick breadboard test revealed very nice gate drive waveform on 40m. Each 4420 is dissipating just below its max 1W limit at this frequency.

David.

[ke1gf]

Sounds good. I look forward to reading up on your results Dave. Do your 4420's get screaming hot? Originally I super-glued little heat-sinks to the chips cause they were getting so damn hot, early on in the design phase. Sounds like you're on the ball. I'm thrilled that you got the thing working so far!

All the Best,
-Bill 'GF

[N9NEO]

David,

If you do decide to take a look at the Elantec/Intersil parts I'd like to hear your comments as how they stack up against the Maxim. I don't particularly like the turn-on and turn-off delay times of Intersil parts.

I'm running the Intersil parts into Zetex NPN/PNP configured as an emitter follower pair. That drives a capacitor coupled transformer. Small #43 binocular core. One turn primary and two turns on the secondary. I'm getting +- 10v to the gates. Difficulty I am having is driving big gates with transformer leakage reactance. Tends to slow down switching. Will probably have to go to a coaxial wound approach to drive the really big ones.

Zetex has decent gate drive application notes on their site.

You might be able to get away with emitter follower pair rather than multiple driver chips.

Good luck with design.

73
Bob

[ke1gf]

Dave if you want to check out some really cool mosfet drivers surf on over to IXYS's website and have a look

http://www.ixysrf.com/products/MOSFET_Driver_ICs.html#selection

I designed a 500 Watt Carrier SEPP Class E RF Deck for 10M using their devices. It's still a spice model, but it's something cool to have hanging around if a 10M AM group ever pops up in the future. These devices are pretty expensive but the amp is still well under $1/watt...

-Bill 'GF

[N9NEO]

Cool Bill,

I looked at them before. Big money, but you can build a nice mobile 300w transmitter just using a pair of them there gate drivers coupled into an antenna. No need to drive into mosfets at all.

ragheads,
Bob

[KM5TZ/6]

Updated pics of the new driver here (scroll down)

6 x MAX 4420 driving 2 x 11N40

http://www.km5tz.netfirms.com/classe.htm

David
KM5TZ