[time-nuts] Why using HP5370 ext-ref is (maybe) a bad idea

[Magnus Danielson]

On 03/03/14 14:14, Bob Camp wrote:
> Hi
>
> *IF* I understand the plot (and that’s a big if, it’s early and I’ve had limited coffee): The period is shifting with phase. We trust the 3336 to be on frequency. The likely answer is that the trigger point must be changing. The question is whether it’s changing because the 3336 is doing something (small waveform changes) or because the 5370 is doing something.
>
> I would make up / dig up a coax cable that is 8 degrees at 10 MHz. Something around 1/2 meter long should do the job. If you see the same period shift when you use it, the problem is more likely in the 5370 than in the 3336.
>
> Next step would be some sort of filtering between the 3336 and the 5370. That would help rule out harmonics and spurs from the generator as the source of the problem. I’d try a lowpass filter first since I have them in my junk box. My junk box and yours may not be stocked with the same stuff :)
>
> My only concern is that we spend time chasing 5370 issues and not subtle gotcha’s with the signal source. I’m looking for some quick / easy / cheap ways to narrow things down. If you have a toggle switch based line stretcher, by all means use it instead of making up a cable.

Well, considering that you make 6 cycles in 18 degrees of the 10 MHz, 
this means that you have 6*20=120 cycles over 10 MHz or 1,2 GHz.

Another approach would be to consider it as the 6th overtone of the 200 
MHz signal.

Poul-Henning, you sure have given us something to think about! :)
YES! :)

I think I will have to figure out how to duplicate your measurement. 
Realize that I need to work on getting some GPIB programming done so I 
can get some scripts going.

Cheers,
Magnus