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 <[email protected]> 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 > <48A59499.ZIP>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
