[time-nuts] Using VNWA3 to compare Frequency Standards

Dr. David Kirkby drkirkby at gmail.com
Sat Aug 10 17:21:59 EDT 2013


On 10 August 2013 19:59, Brooke Clarke <brooke at pacific.net> wrote:
> Hi:
>
> This is an interesting type of VNA where they independently change the LO
> and detector DDS clocks to fill in what otherwise would be nulls thus
> extending the frequency coverage.  20 minutes into the video they talk about
> comparing two Rb sources.
> HAMRADIO 2012 DG8SAQ VNWA UK HD (2012)
> http://www.youtube.com/watch?v=C94J4AutCRc

An interesting idea, though I am a bit sceptical. At such small
phases, things like changes in temperature of cables have an effect on
measurements of phase on a VNA. One can measure something as simple as
a short circuit, and will see some drift with temperature, yet I use
expensive VNA grade cables on a VNA costing much more than the VWNA.

The method assumes the only cause of a measured phase difference
between the two oscillators is their frequency drift, but there are
other reasons too.

I sold me 300 kHz to 3 GHz VNA last week. Someone picks it up
tommorow. A 10 MHz reference is too low for my 8720D, which covers 50
MHz to 20 GHz. I wonder if this would work on the inevitable harmonics
present, or if one put it into something like a mixer which is
non-linear.

I must admit I have been tempted to buy one of the VWNAs. I see some
results on an earler model, and they looked poor above about 600 MHz,
when harmonics of the oscillator are used. I belive they have improved
things, but in any case it would compliment my HP, so I really only
need something below 50 MHz.

> Brooke Clarke

Dave


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