[time-nuts] [hp_agilent_equipment] a bit OT: Video Dist Amps Questions

[J. Forster]

> Hi all,
>
> I am close to choosing a video distribution amp for a GPSDO for use in
10 MHz reference signals for various test equipment (HP equipment.so
somewhat OT) and had a few questions.

> 1.       50/75 ohm question.  I know that putting 50 ohm source into a
75 ohm device will cause SWR. I know that the total power delivered to
the device will be the difference between forward power and reflected
power. So that doesn't seem bad, just might have a lower power input
into the DA. However, what about distortion? Will the sine wave be
distorted somewhat? And what if any adverse effect could a distorted
sine wave have on various instruments using the 10 MHz reference?
>
> 2.       Many of these amps have LOOP through connections to expand the
number of outputs (in ->loop->in).  I often see 75 ohm terminators at
the end of the loop.  Since we are mostly using 50 ohm sources (GPSDO)
would there be any benefit in terminating these loops with 50 ohm
terminators? I realize that the output of the DA's are still 75 ohm, but
I am thinking if a
> 50 ohm source terminated in 50 ohms might have less distortion (question
above) going into the amp.

Hi Gene,

Most DAs are high input impedance amps with internal or external
(loop-through) 75 Ohm terminations. You should be able to change the
internal terminations or put on an external 50 Ohm terminator w/o issues.
Most RF sources will drive 50 Ohms.

> 3.       Some of these video distribution amps can be loaded with different
> cards. Some cards are straight VDA cards. Others have some form of "EQ."
According to one description the card comes ".with the addition of
equalization to compensate for attenuation and phase errors associated
with
> long cable runs."  And I was wondering, could some form of this EQ
compensation be useful in a 10 MHz ref distribution system?

TV is concerned about amplitude flatness across a wide frequency range.
You are working at a single frequency. I'd not bother with equalization.

> What are the
> effects of long cables runs around a lab when distributing the reference
signal?  What is the effect of not maintaining the same phase across all
instruments in a lab?

I'd be concerned about stability. To have equal phase at every location,
all the cables to the DA have to have equal electrical length (or differ
only by integral number of electrical wavelengths at the operating
frequency)

Unless you are planning to do interferometry or something like itr, don't
worry.

Best,

John


>
>
>
> 73 Eugene W2HX
>
>
>
>
>