[time-nuts] New topics (was Re: He is a Time-Nut Troublemaker....)
Lux, James P
james.p.lux at jpl.nasa.gov
Tue Dec 23 12:05:31 EST 2008
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of John Ackermann N8UR
> Sent: Tuesday, December 23, 2008 8:41 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] New topics (was Re: He is a Time-Nut
> Magnus Danielson wrote:
> > My intent is to get some stuff done in the lab during the vacation.
> > (Desperatly trying to get some more on-topic discussions going).
> Here are two questions that have been running around my head:
> 1. Following on from the discussion last week about trying
> to synchronize multiple oscillators to improve phase noise,
> I've wondered about a simpler tack: take, for example, two 5
> MHz atomic standards and mix their outputs together, using
> the 10 MHz result to drive a time scale. Assuming the
> standards were of relatively equal quality, would this
> provide a better time scale than using one of the standards alone?
> 2. Several measurement techniques require a given phase
> relationship (e.g., quadrature) between DUT and reference.
> For HF frequencies (ie, 5 or 10 MHz) is there a *practical*
> phase shifter design covering 180+ degrees that doesn't
> involve switching various lengths of coax in and out of the line?
Surely, you don't have a problem with a precision trombone line for 10 MHz, do you?
You could use a water dielectric and slow the waves WAY down so it's of practical length (if not impractical in other ways)
But more seriously.. How accurate and stable does the phase shift have to be? There are some little 10MHz phase shifters down in one of our labs that are (guessing) some sort of transformer inside. They're about the size of the Helipot style 10 turns (i.e. a couple inches in diameter and a couple or three inches long, with a 1/4" shaft to adjust phase. (I'll go down and see if I can find one and the mfr & part #.. They're probably not made any more, though)
A more classic approach is to take a 2x or 4x frequency and run it into a suitable divider/decoder to generate quadrature.
Once you have quadrature, you then need some way to combine the two with varying amplitudes.. Sine/cosine pots, etc Or some MDACs driven by a lookup table. (the commercial phase shifters use a 90degree hybrid and some sort of variable gain thing.. PIN diodes, mixers, what-have-you)
Some form of goniometer springs to mind as a way to do it. The stability of couplings, etc., are going to be determined by mechanical tolerances, and clever design might make it fairly temperature immune. Vacuum capacitors are quite stable with aging (since the dielectric doesn't change)
James Lux, P.E.
Task Manager, SOMD Software Defined Radios
Flight Communications Systems Section
Jet Propulsion Laboratory
4800 Oak Grove Drive, Mail Stop 161-213
Pasadena, CA, 91109
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