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I have a stable reference oscillator on 15 MHz.? I could double it to 30 MHz without too much trouble.? Has anyone here tried feeding 30 MHz into the Si5351A on the U3S?? Hans, will the U3S software work with a reference frequency setting of 30.000000 MHz?? I intend to use it with WSPR and CW on 40 and 50 MHz, not 144.
Halden VE7UTS

You might try using your 15MHz reference directly.? I know the '5351A isn't exactly specified for this (other devices in the family are though), but I send a 10MHz 3V-logic-level clock to the '5351A on some of my designs and it works very well.
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Paul Elliott - WB6CXC

Hi Paul,
I just looked at your website and learned about the NB3N502.? Because of the concern about spectral purity that Hans mentioned, I'm thinking of using this chip.? I'd divide the 15 MHz by 2 and configure this chip to multiply by 3.333.? An alternative is to use the Cypress CY22800 or CY22801 to convert the 15 MHz to 25 or 27 MHz.? The advantage of the latter is that I have these and their programmer on hand.? But the NB3N502 has a better jitter spec.? Do you think jitter even matters when feeding an Si5351A?
Halden VE7UTS

I'm on the road today,? but tomorrow will share what data I have from my measurements on the pll. It definitely adds some broadband noise. The Qdx works well with my mod but I haven't evaluated the off-channel interference. It's not going to be horrible though.?
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Paul Elliott - WB6CXC

BTW the pll in the 5351 should (in theory) attenuate some wide band input ref noise, but I recall from an earlier project that using the nb3n502 to multiply 10 to 25 MHz wasn't as clean as just running 10 MHz directly to the 5351.
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Paul Elliott - WB6CXC

I found that my 15 MHz reference also has a 30 MHz internal signal.? So, I'll try 15 and 30 and see how it goes for feeding the Si5351A.? If the increased spurs are far from the fundamental, a bandpass filter might suppress them.? If close, then I'll have to convert the 15 MHz to something in the 25-27 MHz range.
Long ago, I installed a TCXO for the CPU clock which has enabled the unit to stay on time for months without GPS discipline.? Why not make it even better by using the 15 MHz reference to clock the CPU?? I could enter 15 MHz where one normally enters 20 MHz.? The CPU would run only 3/4 the speed.? Would the computations in the software still work?? The software uses the entered number to control its clock, so I presume the transmission timing will be okay.? Do any other internal calculations use the nominal 20 MHz instead of the actual number entered in the settings?? I anticipate transmitting WSPR and CW.
Halden VE7UTS

An update:
Since posting that question, I have used 15 MHz to feed the Si5351A without outputs at 40.68, 50.0565, and 50.2945 MHz.? I'm working with the understanding that by driving it with a reference oscillator outside the 25-27 MHz range, it might not work for some frequencies and it might give poor spectral purity on some frequencies.? Per my TinySA, there aren't any extreme spurs when used on the frequencies listed above. ?
Halden VE7UTS?