[time-nuts] Rb video

Bob Camp lists at rtty.us
Fri Aug 9 09:09:38 EDT 2013


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. 

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 ….

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).

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 ….

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. 

6) I think I might have kept the bottom board and run something through it. 

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 …

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 ….

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.

Bob


On Aug 9, 2013, at 5:19 AM, Dr. David Kirkby <drkirkby at gmail.com> wrote:

> On 9 August 2013 03:50, Mark Sims <holrum at hotmail.com> wrote:
> 
> Hi Mark,
> 
> I'm not sure why you needed to start a new thread - why not just add
> to the old one?
> 
>> I also use a blinking status LED controlled by an ATTINY13 AVR chip.  It also reads a temperature sensor and controls a PWM'ed fan.  The LED blinks out the temperature.
> 
> In Morse Code? Now that would be a bit of fun!
> 
> If has he says that has an open-collector output, I can't understand
> why one would not just add a resistor and LED and use it.
> 
>> BTW,  the guy made a lot of bad decisions in his build.  FOr instance,  he should have brought out the Rb frequency/PPS signals to BNC connectors and patched them externally into the input BNC's.  That way he could use any unused amplifier channels for other things.
> 
> I mentioned some things I felt were wrong, and one in particular (BNC
> terminals) I appeared to be wrong about. But I also wrote I felt it
> was unwise not to use the 1 PPS output.
> 
> But if you have other thoughts, about where he went wrong, apart from
> those I mentioned in my post yesterday, I would be interested.
> 
> 
>> If you want to get fancy, you can run a speed controlled fan on the heat sink to regulate the temperature. You still want to pick a temp that's as low as you can practically get it for your control point.
> 
> My unit arrived today. It looks OK. I've not tested it yet.
> 
> 
> Dave, G8WRB
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