a little RF power, ham-ish question

Safe way is to use a stronger transmitter with a power attenuator at the output.

That looks nice. The schematic is helpful.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Nice, Thanks. Opamps for everything, makes life easy.

George H.

try using this search term

"20 meter QRP"

mark

I just bought a couple of those from Farnel/Element14 last month to upgrade the crappy 3092I dual opamp version Feeltech had used in their cute little FY6600-60M AWG to cure the sine wave distortion when driving

50 ohm loads at the 20v p-p setting (10v p-p on load in this case) when close to the 20MHz limit before it limits the output to 5v p-p.

In this signal generator application (DC to 60MHz), their outputs have a

50 ohm series resistor to create a standard 50 ohm generator impedance so the resulting rms output from a "20v p-p" output becomes a mere 250mW (5v amplitude half watt peak, thus 250mW rms). Obviously, the wasteful matching resistor can be eliminated using a transformer over a 10:1 frequency range, say 5 to 50 MHz. By using a pair in bridged output, they should comfortably generate the required 2 watts output using +/- 15v supply rails.

Unless someone comes up with a hack to do away with the 5v p-p limit imposed above 20MHz, a pair of THS3491s are rather an overkill solution. A pair of THS3095s would have sufficed in this case but the 3491s didn't cost much more and I didn't want to miss out in the event of a hack materialising to let me use my investment at the full 60Mz sine wave output limit. :-)

--
Johnny B Good

But it's got a maximum current limit of 100mA. If you short circuit it for long enough it breaks itself.. or something? (just reading the spec sheet again.)

George H.

also there is a trick using current sense feedback to use a lower physical series resistance and have the feedback raise the effective output Z up to 50 Ohms.

mark

THS3491 ?

first page says 420mA current limit and thermal shutdown protection

Yeah, read absolute maximums (table 7.) note 3. (3) Long-term continuous current for electro-migration limits. Someone mentioned this part in a previous thread. (JL maybe?) And said the current limit was really a heat thing and if you kept the supply voltage low it wasn't a problem.... but I don't know.

It would be nice if they said what 'long term' is.

George H.

That's pathetic. Top-level chip metal (aluminum-copper alloy or just copper) is good for about 1E6 A/cm**2, a fact every baby chip designer knows. How did they get that one past the design review?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I've never used the THS3491.

Electromigration is a bit worse at high temps but it's not primarily a thermal thing. Electrons are just knocking metal out of the way. Running at lower voltage won't help much.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Sure.. sorry. I've never used it either. probably some other chip.

George H.