[time-nuts] bad ocxo or measurement artifact ?

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 16 20:57:58 UTC 2010


Stanley Reynolds wrote:
> Looking at the output of Greenray Industries Y-313-16A 160 MHz OCXO I discovered a new feature of my Hp 54501 scope. It split the signal into 4 traces each with a different amplitude and they are phase shifted from each other. P to P levels are 2.8V, 116mv, 53mv, and 9mv. Not much difference running the oxco on a battery or substituting the scope probe with coax cable rg58 6' with a tee and 50 ohm termination.
> 
> Could this be a aliasing effect of the AD sampling the as the scope is a digital 100Mhz scope ?
> 
> No I don't know what I'm doing :-) But I would like to know.

The HP 54401 has 100 MHz bandwidth, but only 10 Msa/s sampling rate at 
the most. Channel 1&2 mux into a common T/H and A/D and similarly for 
the channel 3&4. I could not find the fact in the manual, but I suspect 
that enabling channel 2 and 4 lowers the sampling rate. It's quite common.

Regardless, sampling a 160 MHz signal with 5 or 10 MHz sampling rate 
causes alot of aliasing phenomenes. The scope is undersampling for all 
signals above 5 MHz. Undersampling scopes can be confusing until you 
master it. It's a fine compromise, but the burst-sampling rate is 
getting very high now.

Cheers,
Magnus



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