I was wondering about converting it for a different reference input but
have so far just had a cursory look inside at the non-RF side of the module
(it has a lot of hex headed screws holding it together!). The 26.25MHz
reference oscillator is followed by what appears to be the beginning of a
multiplier chain - presumably driving a harmonic mixer. This is borne out
by the fact that the offset frequency, 10500MHz in this case, is an exact
multiple of the reference (400 times) so my plans to replace with a 10MHz
input and change the R divide ratio look to be a definite non-starter.
Wonder if its possible to change the reference/step size to something
nicer than the rather odd 416.666kHz? The BridgeWave (That's what the
PCB is labelled, but it has been referred to as Broadcom) module I also got
going recently http://www.g4jnt.com/Bridgewave_Synth.pdf is a lot nicer
to do things with. It has a more conventional design, starting off with a
1400MHz Z-Com VCO and multiplying up to the final frequency. The 1400MHz
is directly synthesized using an LMX2326 device so any reference and
comparison frequency is possible within reason.
The simplest way to change the (effective) step size would be to replace the
synthesiser chip with a Frac-N part, such as one of the AD415x series. With a
few changes in loop filter constants, it's possible to obtain almost any step
size, particularly with the AD5157... Modern Frac-N synths using sigma/delta
interpolation can produce quite remarkable results, and I wish I had the time
to complete some of the designs I have in mind for amateur radio projects. The
AD 514x series are more-or-less pin-compatible with the LMX23xx parts, while
the code needed isn't that different...