[time-nuts] fast freq. synthesis schemes

John Miles jmiles at pop.net
Thu Oct 15 01:08:24 UTC 2009

> Ooohh.. Sampling Phase Detectors or Harmonic Mixers.. The problem
> is that you have to hit them with a lot of power on the reference
> port (+20dBm wouldn't be unusual)
> Depending on your application,
> making that much LO power that is suitably quiet is a challenge.
> Presumably, though, you're not DC power limited, so that helps.

Yes, if the goal is to breadboard something at home the constraints are
looser in some ways and worse in others.  The Hittite parts are all QFNs
that are annoying at best to dead-bug.

The LO side isn't typically a problem in a sampler loop with a fixed drive
frequency.  Mini-Circuits has a useful monolithic amplifier chip, the
HELA-10, that yields 1 watt from HF to 1 GHz with a 3.5-dB NF.  You can feed
it with a typical +15 dBm OCXO and put ~+25 dBm into your sampler for $20,
and they even throw in the baluns.  Or for $75 they will sell you a
gold-plated, connectorized and enclosed eval board.  One of the real
unheralded bargains out there, as far as broadband driver amps are

> Getting a PLL with simple single integer N is pretty easy with
> the Hittite parts, especially if you can tolerate N that is a
> multiple of 4 or 8.  There's a paper out there by S.K.Smith, et
> al., that describes a breadboard PLL we did at 8GHz, where we
> drove the reference input with the output of a DDS mixed with a
> fixed signal.
> http://tmo.jpl.nasa.gov/progress_report/42-166/166A.pdf
> take a look at Figure 6 and 7
> However, that won't change in less than a microsecond (the loop
> bandwidth is too narrow).. you could widen up the loop bandwidth,
> but the reference source would need to be quieter (not a
> challenge.. we didn't take any special efforts to make our DDS
> quiet, etc.)

In most cases you'd want a switched loop filter, one time constant for
acquisition and another for lock.  Pretuning is a big help there as well.

-- john, KE5FX

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