[time-nuts] Distribution Amp

Robert Lutwak Lutwak at Alum.mit.edu
Sat Sep 24 13:22:28 EDT 2005

A good distribution amp will give you >100 dB isolation between the ports, 
i.e. you can inject a signal into one port and it will show up on an 
adjacent port at <100 dB of the injected level.  Even a cheap RadioShack 
cable-TV distribution amp will give you >40 dB isolation.  The principal 
downside of daisy-chaining multiple high-input impedance instruments is that 
you give up this isolation between the devices.

You might think this is no big deal (you're not going to deliberately inject 
a signal into the output of your distribution amp), but some instruments are 
pretty sloppy about isolating the reference input from other parts of the 
circuit.  In particular, low-end synthesizers might dump -30 dB of output 
frequency and various harmonics thereof onto the reference input.  Another 
nefarious contraption is a counter with an internal OCXO which is left on 
and free-running when you apply an external reference.  Some of the older HP 
counters did this, I usually open them and cut the power to the internal 
OCXO.  The newer models PLL the internal oscillator to the reference so it's 
not quite so bad.  In some beatnote heterodyne measurement applications I 
recommend isolation transformers on both sides of the distribution amp as 
well to keep the final (50 Ohm) termination from contaminating the 


Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
(978) 232-1461   Voice           RLutwak at Symmetricom.com   (Business)
(978) 927-4099   FAX             Lutwak at Alum.MIT.edu  (Personal)
(339) 927-7896   Mobile
----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Saturday, September 24, 2005 12:53 PM
Subject: RE: [time-nuts] Distribution Amp

> Mike Feher said,
> " It will be ideal to drive a bunch of test equipment simultaneously
> from the same source. "
> You mean one equipment per output, right? Paralleling 50 ohm inputs
> will result in the wrong termination resistance for a cable. This may
> not be a problem for a 10 MHz sine wave, as long as the direct coupled
> emitter follower can handle the load.
> I have used networks where the devices are high impedance. The cable
> sees 50 ohms at both ends. Some test equipment has an optional 50 ohm
> terminator, for use as a high impedance device.
> Suppose I have 3 frequency synthesizers that have optional terminators.
> Are there any ill effects, except phase shift, from putting them all on
> the standard's output, if only the end one is terminated?
> Has anyone tried the high impedance distribution scheme?
> Seems like "simultaneously" would be relative, unless the distribution
> cables are all of the same length.
> Bill Hawkins
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