bart said:
I would like to convert a 66MHz TTL output to a 66MHz +/- 5 Volt
square, triangle or sine wave. Would a high speed comparator be the
best solution for this? How about AC-coupling? Thanks in advance for
any advice..
It would need to be a very hgh speed comparator. The obvious candidate
would be the Linear Technology LT1016, but it doesn't look as if it
will do better than 25MHz.
There are faster comparators around - the original Advanced Micro
Devices Am685 could handle 100MHz back in 1972, and its many successors
have become progressively faster. The Analog Devices AD96685 comes to
mind - we used a bunch of them at Cambridge Instruments in the late
1980s. Analog Devices now sells even faster parts - their web site
threw up the ADCMP582BCP-RL7 which is made on a silicon-germanium
process, and they are not the only game in town.
Sadly, these faster parts produce an ECL compatible output. This is
easy enough to transform into a +/-5V square wave or triangle wave with
discrete transistors - back in the late 1980s at Cambridge Instruments
we did this with 5GHz wideband transistors like the BFR92 (NPN) and the
BFT93 (PNP) which are still ex-stock parts from Farnell.
Using these sorts of parts does take some care - you need to put around
33R of "base-stopping" resistance close to every base input, otherwise
the transistors are prone to oscillate, and the circuit should be laid
out on the basis that interconnections are terminated transmission
lines, running over a solid ground plane - but it is perfectly
practical.
Getting a good +/-5V sine wave out of the output could be trickier -
the obvious solution is to use a multi-pole low pass filter to clean
the higher harmonics out of the triangular wave. The harmonic content
of the triangular wave is already relativiely low, because their
amplitude decreases as the square of the harmonic number, so it doesn't
take much filtering to get rid of the hgher harmonics, but you might
need a multipole filter to get acceptable attenuation of the third
harmonic. If your original square wave has an exactly 50% duty cycle,
there won't be any even harmonics.