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The 24 MHz clock in that NanoVNA product that this group is not about is
likely used because it's easy to get a 48 MHz clock for USB from there.
A simple frequency doubler can be used.

The spec sheet for the Si5351A lists two clock frequencies at the
beginning, 25 and 27 MHz, and I believe the original PC software for
generating parameters for the Si5351A only worked at those specific
frequencies, but it has always listed a range from 25 to 27 MHz later in
the sheet. Lots of other programs and libraries will work for an Si5351A
that is used at other clock speeds, and a number of products
use non-standard speeds. The uBITX, for example, uses a 26 MHz clock. I
don't know why; perhaps the manufacturer has a cheap source of 26 MHz
crystals.

For a while, the 80 and 160 meter versions of the QCX from QRP Labs used
frequencies lower than 25 MHz, because their original method of using the
chip to generate quadrature clock signals required that change to work at
lower frequencies. Hans Summers later found a different method that does
not require the change in clock speed, and current production does not
use the reduced clock speed. One of the things that drove looking for
another method happened because there was a shortage of the Si5351A for a
while during the pandemic years, and so some companies (including QRP Labs)
switched to an alternative from China, the MS5351M. It's a drop-in
replacement in most cases, but it will not work at the 12 MHz clock
frequency that was used in the 160 meter QCX. Only one version of the
MS5351M is made, the 10 pin MSOP package that generates three clocks. There
are no equivalents to the Si5351A in larger packages that have more
outputs, nor to the Si5351B and Si5351C that have additional features.

Many products, including the original NanoVNA and NanoVNA-H that this group
IS about, also operate the Si5351A out of spec in another way. The
specifications only guarantee operation up to 200 MHz, but many products
push it to higher frequencies approaching 300 MHz. The exact frequency an
Si5351A can reach varies by chip, and also depends on temperature. The chip
may consume more power than the spec sheet says at those higher
frequencies, and specs such as duty cycle and jitter may not be met.

The original version of the chip, without the -B at the end, was only
specified to 160 MHz, which is why the Adafruit modules list that as their
maximum frequency, but that version of the chip hasn't been made for many
years and all current production modules use the version that is specified
to run at 200 MHz. The specific version used in most of the products
familiar to hams is Si5351A-B-GT (or GTR, meaning it comes on a reel rather
than loose), which is a 10 pin MSOP rather than a 16 or 20 pin QFN. (The
original was also offered in a 24 pin QSOP.) Variants like
Si5351A-Bxxxxx-GT are sometimes used because of unavailability of the
unprogrammed parts; those are pre-programmed chips that generate fixed
outputs at power-up, but they still respond to I2C commands to change the
outputs. Some of those also change the I2C port; watch out for that.