Hi Steve, We're getting way off topic for the mailing list, but, I thought I'd post this reply here if there are any other interested hams out there, perhaps who are also experimenting in this area. I haven't played with the Si5351B version of this interesting chip, yet. I wonder whether the voltage control input of this chip would be suitable for AF FM modulation? Assuming that you could modulate the VCXO at audio frequencies (the datasheet isn't completely clear to me about that), an immediate issue would be that everytime you change the fractional-N division ratio, you'll change the modulation index. This will have to be compensated for, one way or another. I've been playing with several boards with Si5351 chips. They're pretty neat. They're also pretty noisy! ...With both spurious and phase noise. The phase noise can be improved somewhat with a better external crystal oscillator, the spurs seem more challenging. Using the on-board VCXO with the 5351B, you can't use a high quality external oscillator, so phase noise and overall frequency stability could be an issue, too. Anyhow, for my applications, I'm following the Si5351 with a secondary clean-up VCO/PLL, which was already in the radios. I just needed a frequency-agile reference oscillator, so to speak. I think the Si5351 will work well for this, based on my preliminary tests! (I'd certainly like to hear the experiences from anyone who has already been down this road!) I guess you're not finding any local sources of surplus radios? Do you have any 6m power amps available? 73, David KB4FXC On Mon, 6 Jun 2016, Steve Wright wrote:

On 04/06/16 04:00, Joe Leikhim <rhyolite@leikhim.com> wrote:

...

Assuming the VCO is part of a PLL, the PLL will tend to "correct" the modulation, [....]

Was planning to use the Si5351B - has VCO input.

S