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RF Drive from digital OSC...

I am using a small SMT OSC module at 100 MHz that has good harmonics to ~400 MHz to drive several sources on a board I have designed. I need the output to be capable of driving a couple of 50 ohm loads to 0 dB. I cant find anything that will take a 3.3V CMOS CLK input that can drive a pair of outputs into a 50 ohm load that will preserve the harmonics fairly well. Ideally, I was looking for a single IC that could do this, even if it was one channel... I could use two of them. Any solution has to have a minimum of parts and be very cost effective, in the dollars range. Any help would be great! By the way, I can find plenty of LVDS differential drivers that can do the job nicely into 50 ohms, but I don't know how to "single end" a differential output without a transformer that I'm not allowed to even suggest, and most of them require a differential drive, which I do not have.

Many thanks!

Reply to
Dave
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Define "minimum parts".

A class-C transistor amplifier could be done with a transistor, a cap or two, a resistor or two and an inductor. That's not too many parts, and you should be able to get at least 10 to 20dB out of each stage if I remember my practical RF correctly.

How much power can you take off of the oscillator module? Is there a sufficiently fast LV CMOS mini-logic part that you can use for a buffer? With care you can get 7dBm out of a complimentary pair of 74HCxx outputs, although not at those frequencies and not at 50 ohms unless you use a transformer.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

Most of the LVDS-to-CMOS receivers have screaming fast outputs, sub-ns. Bias one input maybe a volt off ground, and drive the other single-ended.

I also like the NL37WZ16 triple buffer, with all sections in parallel. It will go nearly r-r into 50 ohms, with roughly 600 ps edges. The downside is the terrible US8 package. We're paying 26 cents. There's a schmitt version, too.

John

Reply to
John Larkin

Should be duck soup -- though I realized on re-reading your posting that it's not clear if you want the total square wave power delivered to each load to be 0dBm, or you need 0dBm at particular harmonics. I'm assuming just square wave power in the first part below.

Note that the 3.3V CMOS output is capable of considerably more than

0dBm, normally. Assume it can drive +/- 24mA to within 0.5V of the rails. That's in excess of 1V@24mA symmetrical square wave. That's 24mW, or +13dBm, and that's into just over 40 ohms. You could just use CMOS buffers if you want several isolated outputs. The only parts you need, though, to get 0dBm at the fundamental at least into a couple of 50 ohm loads are a capacitor and a couple resistors: the cap to block DC and the resistors in series to each load to keep from loading the oscillator output too much. (By the way, expect the harmonics to be predominantly odd: 300MHz, 500MHz, etc.) If you want to select a particular harmonic, you'll need a selective circuit like an LC tank, of course; then an MMIC could be a reasonable way to get 0dBm at, say, 500MHz.

There are also frequency doubler and tripler circuits that use diodes to reasonably efficiently generate specific harmonics.

Cheers, Tom

Reply to
Tom Bruhns

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