[time-nuts] IC for 1 PPS Output

Robert Atkinson robert.atkinson at genetix.com
Fri Apr 7 03:46:32 EDT 2006

Hi Brooke,
Most of the Video buffer/driver type IC's should work with a suitably
scaled output resistor. Even a fast (high -slew rate) OP-Amp will work.
The output impedance should be low enough to get a good 50 ohm match
with just a couple of 100R in parallel. The out put impedance should be
in the data sheet for the device.
A good technique is to set the output of your buffer amplifier to a
higher voltage than the desired output and use a resistive divider (50R
output impedance) to attenuate the signal to the level you need, just
don't forget about the other 50R load! This is sometimes called a "soft"
output as the voltage will rise if it's not correctly terminated. 
See the TAPR TADD1 < http://www.tapr.org/~n8ur/TADD-1_Manual.pdf >,
Maxim Data sheet for the MAX477 <
http://pdfserv.maxim-ic.com/en/ds/MAX477.pdf >  and application note <
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1036 > or National's
LM6171 < http://cache.national.com/ds/LM/LM6171.pdf >for some ideas.
With the MAX477 just using a 50R output resistor will give good results,
not that it's not essential to have an exact resistance, many commercial
designs use 52R (preferred value) output resistors with out problems,
it's within 5%.


Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Brooke Clarke
Sent: 07 April 2006 05:14
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] IC for 1 PPS Output


I've got the TVB 1 PPS PIC circuit working and would like to add an 
output IC to drive 50 a couple of Ohm cables.
Need two outputs, the normal 1 PPS and also the 10 kHz output since I 
think it would work with the SRS app note for making 1,000 TI 
measurements in one second to better see small offsets.

I seem to remember that there may be an internal resistance already in 
some chips so putting a 47 or 50 Ohm resistor in series may result in an

impedance above 50 Ohms.  Any thoughts?


Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

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