Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

Roy Lewallen, W7EL

Joe McElvenney wrote:

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me. My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter, pi
-section filter appears to do the job but it was just my day for being
scientific :-(


Cheers - Joe

Roy Lewallen wrote in message ...
Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

often, u might find a need for a diplexer at the output of the 74HC
series! quite like terminating the HEXFET power amps. I would suggest
that u include a 22 ohms resistor in series with the output and about
20-30 of capacitor to smoothen out the jitter on the square wave
edges. we used to need this to prevent the memory IC from blowing up
when driven by fast TTL chips with a large fan out.

- farhan

Roy Lewallen wrote in message ...
Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

often, u might find a need for a diplexer at the output of the 74HC
series! quite like terminating the HEXFET power amps. I would suggest
that u include a 22 ohms resistor in series with the output and about
20-30 of capacitor to smoothen out the jitter on the square wave
edges. we used to need this to prevent the memory IC from blowing up
when driven by fast TTL chips with a large fan out.

- farhan