[time-nuts] Distribution Amp

Bill Hawkins bill at iaxs.net
Sat Sep 24 17:36:48 EDT 2005

Sorry, Mike, email allows the reader to assume any context they wish.

My post was born of curiosity about using a terminated transmission
line with high-impedance receivers. There was nothing academic or
esoteric about it. I use such a scheme for industrial process sensors
with 32 drops on a line. You raised the topic, and I responded with a
question about distribution. I don't know how you managed to read
criticism into that.

The relevant answer that I got from Robert Lutwak is that the high-Z
receivers have to have isolated inputs in order to prevent them from
putting unwanted signals on the line.

Mike, I hope you have me confused with someone else, because I don't
make negativism a habit. That's a poor way to live.

Bill Hawkins

-----Original Message-----
From: Mike Feher [mailto:mfeher at eozinc.com]
Sent: Saturday, September 24, 2005 3:01 PM
To: bill at iaxs.net; 'Discussion of precise time and frequency
Subject: RE: [time-nuts] Distribution Amp

Bill -

This thing does have 18 outputs, so yes, I did mean one piece of equipment
per one/each output. I do not know how that could have been misconstrued.
Since I was talking about frequency the simultaneous fits as I indicated
that a constant phase shift between outputs, to me is not relevant. If I was
using it for a timing reference then I would be more cautious. I am well
aware of mismatches occurring due to paralleling 50 ohm inputs or outputs.
My post was not intended to bring out an academic and non relevant debate. I
simply wanted to alert potential customers for a distribution amplifier to
this one which seemed pretty cheap for what it does. Since it is for HDTV, I
expect that all inputs and outputs are probably 75 ohms. That would explain
the loss I was observing as I was using a 50 ohm instrument to measure the
output power. I can live with the 1.5 to 1 VSWR at 10 MHz just fine. I
wonder if you even looked at the URL, or did you just start your negativism
out of habit. - Mike

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bill Hawkins
Sent: Saturday, September 24, 2005 12:54 PM
To: Discussion of precise time and frequency measurement
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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