[time-nuts] Advice on 10 MHz isolation/distribution (Bob Camp)

Garry Thorp GThorp at pascall.co.uk
Mon Feb 22 18:15:38 UTC 2010


Hello Bob,
 
My 15mA estimate for the output stage was based on AC coupling the
output, which is how I connected it. The IC is then sourcing 25mA into
100R for half the time and sinking 25mA the rest of the time, so the
mean supply current drawn is 12.5mA. The extra is to allow for the IC's
dissipation capacitance x 5V x 10MHz.
 
As I was just doing this as a feasibility check, the 74AC163 was on a
board on its own, powered from a linear bench supply with loads of RC
filtering plus local decoupling, so no problem with interaction with
other components. The input was biased to half the supply voltage, and
AC coupled to the oscillator which gave ~18dBm sine wave into 50R. Slew
rate wasn't much of an issue as it was being driven at 100MHz.
 
Garry
 
Hi

Yup, that works. 

A couple of *very* minor points:

-------------

My guess is that your output stage will be pulling a bit more than 15
ma.

This assumes you are running a 50 ohm load and a 50 ohm output impedance
to

deliver 13 dbm (~ 2.5 V p-p). 

More or less, 100 ohms from +5 to ground would be 50 ma half the time.
The

stage likely will pull 25 or more ma. (Says Bob who hopefully hasn't
done

any typos in the last minute or so). 

You might also want to parallel two (or more) gates to get the output

current into the "safe" region. 

--------

The Datum LPRO manual on page 18 shows some data for AC gates driven
with

very simple circuits (just bias the gate ...). The data they show would
be

adequate for the proposed application (if I remember the numbers right).

Power supply bypassing is indeed an issue no matter how you do this.

--------

AC gates create some *major* supply line spikes. Depending on how you
look

at ground bounce, it might be an issue in terms of isolation. Good
layout

practices required....

--------

The gate approach also lets you generate an "almost differential" signal

without a lot of crazy effort. One 74AC86 should do the trick. 

--------

Not real sure that Clay is a big fan of lots of harmonics or of filters.


 

Bob

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