n6gn

I sometimes run a common GPDSDO derived external 66.660 MHz clock to correct and synch frequency.

But for time I have no solution that gets multiple kiwiSDRs exactly synchronized.

Yes, there's a chance. On my list of interesting projects is a fast-switching block converter. Essentially this would be a transverter (up/down converter) to an IF above the highest frequency of interest, perhaps 3+ GHz. This uses an LO synthesizer of reasonable performance, (e.g. Harris chip set) speed, phase noise etc paired with isolation stages to drive two mixers with BPF filtering. Picture a, say, 3.0 GHz transceiver that converts 10-32 MHz from a Kiwi or other SDR up/down.
That 3.0 GHz IF is then used with another pair of mixers and the same chipset as the fixed LO synthesizer but this time that LO is stepped in [20 MHz] steps from [3.5 - 6.5 GHz] . This is paired with another similar mixer and converts the original 3.5 GHz IF back *down* to 0-3 GHz. Because LOs are identical and use the same reference, phase noise contributions largely cancel and the result is an all-band receiver using the Kiwi as the last 'IF'/detector/demodulator moving anything the base HF KiwiSDR can do at HF to 0-3 GHz. With stitching it becomes a wide band spectrum analyzer as well.

Synthesizers that can do this are pretty inexpensive as are mixers and gain/isolation stages. Other than BPF & LPF filtering, done between planes in the PC board there isn't much to it. Since all LOs can have high PLL bandwidth, close in spurs and phase noise can be that of the base clock which is perhaps coherent with and also providing the 66.660 MHz Kiwi external clock.

A nice side effect of all this is that it can transmit at low power, as well as receive. Low noise pre-amplification and PA stages can be added as desired (or not).

I think it would make a nice open source hardware/software addition to the KiwiSDR and even Apache/Red Pitaya ... style SDR transceivers. With only 20 kHz of information bandwidth, the Kiwi might not offer every sort of mode that *could* be desired at VHF-microwave, it's not going to receive WiFi or WBFM, but it would still be good for a lot of interesting uses.

It's not a small project but a prototype could probably be running in only a few months...

Glenn n6gn

I have found it very worthwhile to use toroidal 'choking' in the form of a Guanella balun simultaneously on all lines going in/out of my KiwiSDRs.

as shown in the bottom portion of the figure.

Like the situation so nicely described by G3TXQ's graphic, suitable cores need to be used, possibly several of them to cover the broad frequency range of the Kiwi. By winding equal turns in the same direction with each line on a single core, the effective current that can flow through the receiver is reduced by the "transformer action" of the cores. Current that would otherwise flow into and through the SDR creates common flux within the core that causes an opposite current to come out, thus pushing total current toward zero and effectively creating a much higher impedance than can be the case when each line has only a dedicated core.

From my experience, CM currents through the entire KiwiSDR structure produce IZ drop that appears across the effective ADC input, which is a complex function of PCB layout and conductivities - the "attenuator problem". For this reason, I believe, treating all lines as common mode lines, in addition of course to also eliminating differential noise across pairs of individual conductors of those lines, e.g. power line or inter-pair CAT5 noise, can be very effective in improving the KiwiSDR noise floor.

Glenn n6gn

How about a spectrum instead, there's information in that display that is hard to see in the waterfall.

I think this is correct, that's how I generated these spots, by running on a moderately capable desktop. Judging from the relative performance of the KiwiSDR WSPR extension and other systems, I think asking an FT8 extension on the KiwiSDR to keep up with a band like 40m is lately is probably not realistic. I posted this in response the "Doable on the Kiwi?" question at the top of this thread. I suspect it really isn't....
Glenn n6gn