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
reference.
-RL
------------------------------------------------------------
Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
(978) 232-1461 Voice [EMAIL PROTECTED] (Business)
(978) 927-4099 FAX [EMAIL PROTECTED] (Personal)
(339) 927-7896 Mobile
----- Original Message -----
From: "Bill Hawkins" <[EMAIL PROTECTED]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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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