Gerhard wrote:

> I just  imagine a gang of 16 MTI-260s, each slooowly locked  to an
> external ref  with  their outputs wilkinsoned together  to  make a
> make a  really  low  phase noise 10 MHz source.  I  have  bought a
> pretty number  of those Lucent 24386 units without  GPS  for their
> MTis. :-) I mean, we cannot get those BVAs anymore.

> regards, Gerhard

I have been thinking along the same lines, to combine multiple OCXOS's to
obtain lower phase noise. But an N-way Wilkinson could get tedious. After
you calculate the impedances for each leg, you then have to convert them to
lumped-element equivalents to run at 10 MHz, as shown here:

http://www.microwaves101.com/encyclopedias/lumped-element-wilkinson-splitters 

The contribution from each OCXO may not be perfectly balanced, and it may
be difficult to calculate the relative contribution of each, especially
when you consider component tolerances. The isolation between units can be
fairly low; perhaps -13dB to -20dB. This could have serious effects when
running into a PLL. The Wilkson is a relatively narrow-band device and
could not be used for other oscillator banks at considerably different
frequencies.

I propose using a simple resistive combiner. The loss is -6dB, which can
easily be made up in the distribution amplifier. The balance is as perfect
as the resistor tolerances you use. Depending on how you model the OCXO
output stage, the isolation can be better than -40dB from a single unit.

I am attaching LTspice files to show the loss from each oscillator to the
output, and the isolation between oscillators. I used 10 units to help a
little in mental calculations for sanity checking.

The noise contribution is determined by the resistor network. In this
example, it is 5 Ohms in parallel with 5 ohms, or 2.5 ohms. The
contribution of the combiner is then negligible. A resistive combiner is
inherently broadband, and could easily be used over a 100:1 frequency range
with a little care in construction.

I think isolation is the main deal. When you have a bank of oscillators at
the same frequency feeding a pll that is supposed to respond to only one,
the effect of the other units could be significant. It may be necessary to
further isolate the units by providing separate outputs to feed the pll and
the combiner.

But in principle, I think it is a very good idea.

Mike
