[time-nuts] ***SPAM*** Re: 5>10 doubler

[Andrea Baldoni]

On Sat, Jan 31, 2015 at 01:37:47PM -0500, Charles Steinmetz wrote:

> Andrea wrote:
> >the square-law characteristic of devices should be avoided, so the
> >configuration of the doubler must be some sort of "ideal" full wave rectifier
> 
> I disagree strongly with this, at least where push-push JFET
> doublers are concerned.

I see. This configuration is in effect a common gate B-class (or AB, or
"barely A") amplifier and the rectification is a side effect.
But, what is the advantage between it and a couple of diode-connected
transistors with a full A-class (more linear, so less spurs) amplifier in front
of it?
I know that the circuit originates at NIST and thus there surely IS an
advantage. Are it trading more spurs (that you can cancel out with filtering)
for less phase noise (that you cannot recover anymore)?

By the way, it's interesting that Richard Karlquist notice that 40MHz coming
from distortion and 40MHz coming from multiplication are in effect not
cleanly adding. Shouldn't happen the same degradation effect with the push-push
circuit, where 10Mhz coming from rectification superimpose with the ones
generated by nonlinearities in the active devices?

> ones designed by Ed Oxner and the later "H-mode" mixers work).  See
> below for a schematic of an Oxner mixer using a quad JFET (but note
> that commutating mixers generally use MOSFETs).

This is sort of "synchronous rectifier" the kind is used in SMPS.

> So no, running the FETs in Class AB or B does NOT confer a material
> noise advantage compared to running them in "barely Class A," as my
> design does.  It does, however, create an exponential explosion of
> odd-order distortion products that must be removed if the circuit is
> to be useful for time nuts purposes.  So in my view, the "barely
> Class A" push-push JFET doubler is clearly superior to its Class AB
> or B cousin.

And Bruce added:

> One issue is that to reduce flicker phase noise the 5MHz input signal output
> impedance should be sufficiently large to ensure that the JFET common gate input
> impedance is significantly smaller. ie there is significant series RF feedback
> in the JFET source circuit.

Adding negative feedback linearize further the "barely Class A" amplifier; so,
it's good to sacrifice part of the gain of the push-push stage to reduce
flicker noise (and thus add less phase noise) and at the same time spurs.

If it's so, why use a nonlinear (or barely linear) gain stage to rectify?
Using just one stage means in general less phase noise output (but with
probably more spurs that can be filtered out), versus a more stage linear
amplifier (perhaps with strong negative feedblack) followed by a rectifier?

Best regards,
Andrea Baldoni