On Wed, 21 Sep 2016 09:16:18 -0700 Dan Rae <dan...@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 http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf [4] "Some Tips on Making a FETching Discrete Amplifier", by George Alexandrov and Nathan Carter, Analog Dialog 47-10, 2013 http://www.analog.com/library/analogdialogue/archives/47-10/discrete_amplifier.html -- Malek's Law: Any simple idea will be worded in the most complicated way. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.