[time-nuts] Distribution amps for Thunderbolt

[Didier Juges]

These are handy little boxes for the money. Unfortunately the outputs are
not isolated from each other (galvanically and signal-wise) so be mindful
that if you connect/disconnect one port, the adjacent port will be
affected. Unless you need to, it is probably best to wire all 3 inputs in
parallel (with a single 50 ohm termination) and use only one output from
each buffer. That still gives you 3 or 4 channels with reasonable
isolation. The next step would be to replace the output BNCs with
plastic-body types and install isolation transformers for full galvanic
isolation (no ground loops).

Didier



On Sun, May 19, 2013 at 3:58 AM, Charles P. Steinmetz <
charles_steinmetz at lavabit.com> wrote:

> Chris wrote:
>
>  I have been offered one of these:
>>
>> ADA 6 300MX HV HDTV RGBHV 1 x 6 Analog Video Distribution Amplifier
>>
>> I *THINK* it should meet my needs except the HV bit worries me. It
>> seems this means it has not got unity gain, but a gain of 0.7 or 1
>> volt, switch selectable. Could anyone comment if it's suitable to use
>> my Trimble Thunderbolt with several pieces of test gear at the same
>> time? Thanks. Here in the UK the ADA-3-80 seems a rare find.
>>
>
> "HV" just means that it has an additional sync channel ("Horizontal and
> Vertical").  I think "MX" means selectable gain.  I have two plain ADA 6's
> (no MX, no HV) that I used before I built my own iso/DAs.  The gain
> switching components are present, but the switch itself is not populated,
> so it runs at unity gain overall (the amplifier runs at x2 and each output
> is back-terminated).
>
> My ADA 6's use 6, CLC409 amplifiers and, when modified with 50 ohm input
> and output terminations, each section (3 jacks) can drive 3, 50 ohm outputs
> to +13dBm with ~1dB of headroom (I generally only fed 2, 50 ohm loads from
> any one section).  Like any video DA, it should have much smaller coupling
> caps installed to restrict its low-frequency response to HF.
>
> You need 21, size 1206 (SMD) 50 ohm resistors and some 100 nF capacitors
> to do the mod (I used NP0 ceramics).  You also need a deep socket (9/16", I
> think) to get the PC card dismounted from the back panel (it is mounted by
> the BNC connectors).
>
> As supplied, there are 3, 1x6 DAs in the box.  You can parallel inputs
> internally as desired (deleting duplicate input terminations) to make one,
> 1x18 DA or one each 1x12 and 1x6 DAs.  You really shouldn't have more than
> one HF reference frequency running around inside a DA, so the 1x18
> configuration is probably the most useful.
>
> I was quite happy with mine for routine distribution of 1/5/10 MHz.
>
> Best regards,
>
> Charles
>
>
>
>
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