[time-nuts] High rate, high precision/accuracy time interval counter methods

Bob Camp kb8tq at n1k.org
Thu May 5 20:16:24 EDT 2016


Hi

Be careful of isolation specs on some of these combiners / splitters. Often they are deponent on the return loss
of the signal source. An OCXO that presets a 12 db return loss is doing ok. One that is past 20 db is doing quite well.

Bob

> On May 5, 2016, at 7:00 PM, Mike Monett <timenuts at binsamp.e4ward.com> wrote:
> 
> 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
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