[time-nuts] Thunderbolt spurs on 10MHz output at 100Hz and 200Hz from signal.

Attila Kinali attila at kinali.ch
Wed Sep 21 14:59:18 EDT 2016

On Wed, 21 Sep 2016 09:16:18 -0700
Dan Rae <danrae at verizon.net> wrote:

> On 9/21/2016 9:01 AM, David C. Partridge wrote:
> > Yes,  I fear a timepod is a bit out of my budget (I can dream tho').  Over here in the UK PN measurement kit is a bit thin on the ground too.
> >
> > So I looked at <http://www.wenzel.com/documents/measuringphasenoise.htm> but that seemed a little thin on details.  Or were you referring to something else?
> >
> The Wenzel note is at: http://www.wenzel.com/documents/circuits1.htm
> as a pdf under link: "Low noise amplifier for Phase Noise measurements"
> or try:  www.wenzel.com/wp-content/uploads/lowamp.pdf
> Pretty much all there, but the FET is hard to find now.

If you want to build such an amplifier, then the first stage (aka the
jFET input and the first opamp) is the critical component. There are
other, more modern descriptions for this kind of application, in case
you don't want to just replace the jFET by one if its modern cousins.

One well known is AN124[1] and it's more current cousin AN159[2].
For the later, Bruce commented that an jFET input stage would
probably be more quiet. Gerhard Hoffman has designed a similar
system[3] that uses a couple of paralel low noise opamps instead
of a jfet stage and claims a noise floor of 220pV/sqrt(Hz). All of
these would need to be adapted to imporve thier frequency response
up to 10MHz (or actually a bit higher), but then the input stage
gets also a bit simpler as you don't need the huge capacitors anymore.

I also recommend a look at [4] which lists a couple of issues with jFET
input stages for opamps and how to get around them.

			Attila Kinali

[1] "775 Nanovolt Noise Measurement for A Low Noise Voltage Reference",
by Jim Williams, Linear AN124, 2009

[2] "Measuring 2nV/sqrt(Hz) Noise and 120dB Supply Rejection
on Linear Regulators", by Todd Owen and Amit Patel, Linear AN159, 2016

[3] "A 220 pV/sqrt(Hz) low noise preamplifier", by Gerhard Hoffman, 2014

[4] "Some Tips on Making a FETching Discrete Amplifier", by George Alexandrov
and Nathan Carter, Analog Dialog 47-10, 2013

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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