[time-nuts] Rb video

[Bob Camp]

On Aug 9, 2013, at 9:41 AM, Dr. David Kirkby <drkirkby at gmail.com> wrote:

> On 9 August 2013 14:09, Bob Camp <lists at rtty.us> wrote:
>> Hi
>> 
>> A few observations:
>> 
>> 1) He talks about using a heat sink on the front panel, but then never shows it / does it. The fan inside the box is not going to cool that Rb the way it needs to be cooled. You either need a pretty massive heat sink on the front panel with no fan or something smaller with moving air.
> 
> That was what I thought too. It does not look to clever to me.

It does get it all in one box and that was his stated objective. It just would be nice if he showed that the heat sink is pretty huge.

> 
>> 2) If the video amps are set up the way he shows (three outputs per amp) then when you put three terminated 50 ohm lines on them, the amp is trying to drive (50+50)/3 = 33 ohms. I suspect it's not to happy doing that. TI rates them at a  >= 100 ohm load resistance ….
> 
> I don't have the video in front of me, but I thought he was basically
> making the input a high impedance. 100 Ohms would be fine then if one
> used a 50 Ohm source impedance and a 50 Ohm load.

Ok, each op-amp drives three loads. Each load has a 50 ohm resistor in series with it. Each output could be terminated in 50 ohms. If they all are terminated, the op amp is driving three 100 ohm loads in parallel. The op amp sees 33 1/3 ohm at it's output in that case. That's way below the 100 ohms that the manufacturer seems to be recommending. 

You could leave all the cables open. The problem with that is the reflection from the far end. It will "wrap around" onto the outside of the coax shield. If the cable is long enough ( say > 10 meters) it will act as a pretty good radiator. Net result will be more 10 MHz crawling all over the lab. 

> 
>> 3) If you pick up the TDK power supply he's using, it's marked "air flow goes this way". He never seems to mention that. (maybe I blinked when he did….. the video sort of goes on and on).
> 
> I have my doubts there is enough air there.
> 
>> 4) If you are going to all that trouble, *and* have a big back panel, why not bring out the serial lines from the Rb to a DB-9? Then you could tune it on frequency ….
> 
> Interesting idea.
> 
>> 5) An Rb is sensitive to mag field. That should be part of what you consider when you place the power supplies and the Rb.
> 
>> 7) If you have that much room, and want to do it right, lock up a OCXO to the Rb and take care of it's awful phase noise and spur issues.
>> 
>> 8) I think a cutting disk on a Dremel tool would make short work of the standoffs that he's so worried about …
> 
> Yes.
> 
>> 9) There are many  examples out there of why video editing is a good idea. You could have chopped a *lot* out of that video ….
> 
> To be fair, not everyone wants to spend their time editing video, and
> its good of him to put it on the web.

I agree that it's good to share and we're better off with his information. 

> 
>> Overall, the heat sink on the Rb is the biggest issue by far. The practical approach is to put a fan on / near the bottom of the thing. If you are going that way, servo the fan and control the temperature.
> 
> My distribution amp arrived today. The more I look at it, the more I
> think it is best left as a distribution amplifier and the Rb is not
> put in the box. With no changes, one can put a signal into the first
> input, and take one of the 6 outputs to the input of the next input.
> Since there are 3 inputs, with 6 outputs per drive, one could use it
> as a 16-way unit with no changes at all, and leave the flexibility of
> using it for something else if the need arose. I personally would find
> 16 outputs sufficient.
> 
> I'm tempted to put my Rb in another box, with a linear supply and
> enough room for a GPS locking device. Then take the output of that
> other box and feed it into that distribution amp. Sure it is bigger
> than his, but I don't like the idea of a heat sink on the front, and I
> can't see how one is going to cool that very well. Maybe it will be
> fine if you have an air-con unit and have that moving air around the
> external heatsink, but I don't keep my aircon on all the time.
> Electricity is quite expensive in the UK - we are subsidizing all the
> next to useless wind farms. </gripe>

The problem with any sort of cooling is the air flow getting blocked. That can happen no matter how you do it. I've found it's more likely in my basement with passive (no  fan) cooling.

Bob

> 
>> Bob
> 
> Dave
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