[time-nuts] Using commercial video amplifier for 10MHz clock distribution.

[Dana Whitlow]

Hello Taka,

As long as you stick to CW sinewaves *and* the distribution amp specs are
honest when they say "buffered out puts", the problem is fairly simple.  If
you're not seeing distortion under your operating load and signal
amplitudes,
it's OK and I'd not bother to change anything.

By "honest ..." I  mean that each output has its own independent output
amplifier.  However, this device looks to be fairly inexpensive and it's
possible that the mfr uses a single amplifier with a low output impedance
and distributes that through three 75-ohm resistors.  In this event there is
a possibility that with lower than 75-ohm loads, the amplifier's drive
current
capability might be exceeded, leading to distortion.

So, I'd suggest that you keep an eye on distortion levels on each output,
especially after making any system changes.  And, I'd take care to load
any unused outputs with a termination whose resistance equals the
impedance of the cable you're using for distribution.  If you fail to do
this,
and the *electrical* length of any *unterminated* cable is anywhere near
1/4 wavelength at 10 MHz, the impedance seen by the afflicted distro
amp output could be much lower than the intended 75 ohms, again
leading to risk of significant distortion.

I'd try to avoid distributing non-sinusoidal signals unless you're
careful to make sure that the cable impedance matches the output
impedance of the distro amp *and* the input impedance of whatever
your load is.  Otherwise the situation can get more complicated in
a hurry, with the prospect of both nonlinear distortion and edge
distortion due to reflections bouncing around in the cable.  But if
you must, the key is again to check the waveforms at the far ends
of the cables with the intended loads attached.  If the waveforms
look just like what you're expecting (with clean transitions at the
leading and falling edges), you should be OK.

One other issue with a poorly-buffered distro amp is that something
about a given output (like it's phase and/or amplitude) might change
depending on what loads are connected to the other outputs (and the
cable lengths involved).  If this is a concern to you, then you should
check for this effect to establish the viability of your system.

I hope this helps.  Sorry to say, nothing is ever really simple.

Dana



On Fri, Oct 18, 2019 at 3:17 AM Taka Kamiya via time-nuts <
time-nuts at lists.febo.com> wrote:

> I am using Extron MDA 3V for distributing 10MHz at several places in my
> lab.  MDA3V is a one port in - 3 port out device made for VIDEO market and
> rather low end at that.  Input and output is 75 ohm and bandwidth extend to
> 150MHz or so.  Looking at 10MHz sine wave through it, it looks good.  No
> visible distortions.  Gain is unity between input and one of the ports.
>
> I also have distribution Amps from Spectracom.  These, of course, are made
> for clock distribution purpose and I don't question usage for the purpose.
> The level of "accuracy" of distribution is that I need frequency stability,
> but time delay in coax, slight phase shift in coax, and other small nuances
> are not my concern.  (should they be?)  I'm not running a competitive
> service to NIST and this is not for commercial purpose.  I'm just a junior
> time nuts.
> Couple of questions though.  This device is 75ohm.  It's easy to add
> 150ohm in parallel to a matching resister and make it 50ohm.  However, I'm
> guessing 50 ohm / 75 ohm mismatch is not really a big deal that I'm not
> doing anything about it.  Input and output cables are RG58 (50 ohm)  VSWR
> is 1.5, For external sync port, many equipment is not exactly spec'd for 50
> ohm, but often only spec'd for voltage or at db level without any specific
> impedance mentioned.  Recommended cable is 50 ohm though.  I typically shy
> away from consumer grade gear but looking inside, it's just an op-amp and
> registers.  If I use square wave which contains higher frequency
> components, would this be inadequate?
> There are many web pages by people doing exactly what I'm doing with
> usually bigger Extron devices.  I'd like to consult the Time-Nut community
> of sanity in what I am doing.  Is this really not an issue, or "it's not
> simple as THAT" type situation?
> Thank you very much
> ---------------------------------------
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
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