RF Drive from digital OSC...

[Dave]

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!

 

[Tim Wescott]

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!

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

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[John Larkin]

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!

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

 

[Tom Bruhns]

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!

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