[time-nuts] Buffering the 1pps output from a Z3801A
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Mar 26 21:52:21 EDT 2008
Didier Juges wrote:
> The Thunderbolt's output impedance on the PPS output is much less than 50
> ohms, more like 5 ohms.
>
> If you terminate the other end of the cable with a good 50 ohms termination,
> you will be OK, even though it would be better to put a ~47 ohms series
> resistor right at the TB output before driving the cable. This will reduce
> the available amplitude, but will reduce ringing even more.
>
> Check this page:
>
> http://www.ko4bb.com/Test_Equipment/CoaxCableMatching.html
>
> I used the PPS output of the Thunderbolt as a fast rise time pulse generator
> for cable matching/reflections experiments.
>
> It will show you what to expect when you use that output to drive a long
> cable.
>
> Didier KO4BB
>
>
A 5V PECL to CMOS level translator capable of driving a 50 ohm load is
shown:
http://www.eds-fl.com/~bruce/5V_PECLtoCMOS_50_OhmDriver.html
<http://www.eds-fl.com/%7Ebruce/5V_PECLtoCMOS_50_OhmDriver.html>
Input transistors can be 2N3907 although they will be a little slow
since the transistor capacitances will be larger than usual because of
the low collector base voltages, however the input stage propagation
delay will only be a few nanosec. For a faster input stage use high ft
low capacitance transistors (eg BFT92/93).
The values of the output series resistors can be changed to 240 ohms to
provide a better match to a 50 ohm load if the load and consequently
lower reflections termination has significant shunt capacitance.
Bruce
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