[time-nuts] NLTL comb generators going away...

[John Miles]

FYI, I don't know how many others on the list are interested in microwave PN
measurement with gear like the HP 11729B/C or 70420/E5500 series, but I'm in
the (slow) process of upgrading my own 11729C setup to lower its measurement
floor and thought I'd mention something I learned the other day in case it's
helpful to anyone else.

Briefly, the HP noise test sets all seem to use low-noise comb generator
modules fed by 600 or 640 MHz from a quiet OCXO-based multiplier chain, with
filters to select the desired comb harmonic to downconvert the signal under
test.  Since I'm replacing my 11729C's 10811A-derived 640-MHz source with a
much-quieter ULN OCXO-derived source, I also wanted to upgrade its
step-recovery diode-based comb generator with one of the nonlinear
transmission-line parts produced by Picosecond Pulse Labs.  I've been
plotting this upgrade for awhile, but there was no incentive to do it until
upgrading the OCXO.

NLTL comb geneators are awesome.  They have 15-20 dB better residual phase
noise specs than the SRD multipliers that HP was using in the 80s and 90s,
much faster output edges, *and* their inputs are much more broadband in
nature than SRD-multiplier inputs usually are.  They work well with input
frequencies over the better part of an octave, whereas the 11729C's
multiplier is very peaky around 640 MHz.  All of these attributes make them
ideal for use in high-performance PN downconverters.  If you were building a
high-end phase noise test set today, I don't know what else you'd specify.

Unfortunately, when I called Picosecond Pulse Labs to order my first choice
(the 7103 model), they told me they'd shut down their fab in Oregon and were
no longer producing NLTL multipliers.  They have a few left in stock and are
selling them for $975 each regardless of the part number.  The connectorized
7103s were gone when I called, so I bought their last connectorized 7123,
which is one of the higher-spec parts (input=+29 dBm at 800 MHz-1.5 GHz,
output=0 dBm at 50 GHz).  It works fine with inputs down to 640 MHz, so
it'll be OK in the 11729C, although it has a 2.4-mm output connector rather
than SMA.

So this really sucks... hopefully somebody will buy that fab and continue
production, but it sounds like they just didn't sell enough of the comb
generators to be worthwhile.  I haven't been able to find any other
off-the-shelf multipliers with additive jitter specs anywhere close.  If
this sort of thing is your sort of thing, get 'em while you can.

Specs:
http://www.picosecond.com/product/category.asp?pd_id=22

Residual PN measurement notes:
http://www.picosecond.com/objects/RPN-Measurement.pdf

-- john, KE5FX