[App_rpt-users] audio for direct VCO FM: Si5351B

[David McGough]

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 at 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
>