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

[Bob Camp]

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. 

Bob



On Mar 3, 2014, at 2:26 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> In message <E7A494F6-78F1-4568-8BD3-D94A32DEBB8B at rtty.us>, Bob Camp writes:
> 
>> Do you have any idea how 'clean' your 200 MHz signal is? The manual 
>> suggests getting it to -65 dbc for subs using a spectrum analyzer. That's
>> pretty far down. I seem to recall the adjustment process being a bit tedious
>> (lots of back and forth). 
> 
> My estimate is that all harmonics are at least -50 and most probably
> -60 down.  The 10MHz is probably the worst.
> 
> My Lab is not really set up for RF work, so this is probably an
> area where one of the hams could do lot more competent job than me.
> 
> 
> I've attached a plot of one of the runs yesterday, beause things
> are more complicated than I initially thought.
> 
> The vertical bars are AVG +/- STDDEV of 1000 sample TI on a 10MHz
> from my HP3336 in start-com mode.
> 
> The X-axis is the phase offset set on the HP3336 in degrees, and
> represents the phase difference between the ext-ref and start+stop
> signals on the HP5370 plus some arbitrary offset due to cables etc.
> 
> Obviously, the phase difference has no systematic meaning for the red
> bars, since it is free running on the OCXO at some frequency offset
> from the input signal.
> 
> Yet, it is quite evident that there still is a periodicity in the
> data, which peaks around 0, 5, 10 and 15 degrees.
> 
> The green bars however...
> 
> The initial artifact I noticed when I just plotted the STDDEV is
> still there, around 9 degrees where both the average and the stddev
> take a hop.
> 
> But that blib is peanuts relative to the 3-degree periodicity
> for which I have absolutely no explanation, and the equally
> evident 18-degree periodicity.
> 
> The 3-degree periodicity cannot be a simple harmonic, it it were
> it would be a 1.2 GHz signal.  (360/3 * 10 MHz = 1200 MHz)
> 
> But what then ?
> 
> As in a canonical scientific paper, I have to conclude that more
> research is clearly needed, and I'd really love to see what results
> other people might get.
> 
> In the meantime, run you 5370 on internal clock, and rely on the
> law of big numbers.
> 
> Poul-Henning
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> <intext.png>