[time-nuts] Neat toys on eBay for PN measurement
grant at ghengineering.co.uk
Tue Jul 10 15:38:55 EDT 2007
The 8662A derrives it's 640MHz o/p from a (very) good 10MHz crystal osc.
which is then doubled 6 times and filtered at the 40MHz and 160MHz
stages with crystal filters. The problem with using so many multipliers
is that the noise floor increases at every doubler stage, which is why
the filters are necessary to reduce the the noise floor down to
something close to thermal.
An alternative is to use a higher frequency crystal osc., such as 80MHz.
I guess that this is what Wenzel would do for a 640MHz osc, as 160MHz
crystals would probably be 7th overtone which have lower Q and therefore
higher phase noise than 80MHz 5th OT xtals. That's the route that I'm
exploring at the moment, and I think it should be possible to get close
to the performance that Wenzer quote. A crystal filter at 160MHz would
be a bounus, an inverted MESA xtal filter at 320MHz or a SAW filter at
either 320MHz or 640MHz would be better still, but I think this would
start to add serious $$$ for small quantities.
An 80MHz crystal osc. would probably have worse phase noise very
close-in (100Hz or less) but better far out and with a lower noise
floor, compared to a 10MHz osc. But as I'm not testing radars, I'm not
after the ultimate close-in performance, I'm more interested in the
performance at 3kHz-1MHz.
John Miles wrote:
> Picosecond Pulse Labs quoted me $1,000 for one of the NLTL comb generators a
> couple of years ago. They are nice parts, no question about it, and
> substantially better than SRD multipliers.
> In quantity, they would be cheaper, since I was just asking them for a
> one-off price to some random guy with a credit card.
> However, the residual noise floor of the 11729x is still lower than its
> absolute spec when driven by an 8662A's 640 MHz output (see page 42 of
> 11729B-1). So unless you plan to use a quieter 640-MHz driver than an
> option-3 8662A, there seems little benefit in improving its comb generator.
> Likewise, the noise at the 8662A's main output still limits the floor of at
> least some microwave measurements with the 11729.
> That said, I am trying to get a price quote out of Wenzel for a 640 MHz ULN
> This is the only OTS source I can find that's substantially cleaner than the
> 'typical' figures quoted for the 8662A's 640-MHz output. It would sure be
> nice to install one of these as an an internal source in the 11729, and
> drive its electrical tuning line from the quadrature PLL.
> -- john, KE5FX
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