[time-nuts] Re: Micrel (Microchip) PL-500 Low Phase Noise VCXO

[Julien Goodwin]

On 13/9/21 6:31 pm, Julien Goodwin wrote:
> https://www.microchip.com/en-us/product/PL500-16 (there's various other
> versions depending on the frequency you're after)
> 
> Haven't seen any discussion about this on-list, but the PL500 is an
> easily (well, in normal times) available VCXO control chip, for those
> who might want to make their own disciplined oscillator, especially at
> less standard frequencies. I had some arrive today and put the board
> I've designed as an OCXO, and was able to trim +/- ~3kHz (around 26MHz
> nominal in my case, pretty much the expected +/- 150ppm), all really easily.
> 
> I can't yet say much about quality as it turned out I'd put the wrong
> regulator footprint on the board, and with no local regulation the power
> rail was jumping all over the place, once I actually fix that and
> hopefully get it mounted in its intended enclosure for thermal control
> it'll be interesting how it goes (yes this was the project I was hoping
> to use my SR620 to monitor the other week).

The thermal and shielding situation is to improve, but I did at least
get local regulation fixed, and while improved, it's still not great.

With a 1mH inductor on the input (pre-regulator) and the local regulator
installed:
https://twitter.com/LapTop006/status/1439081534053515266

Traces are:
Yellow - Output signal (50-ohm terminated)
Green - Control voltage
Blue - 3.3v rail (main internal rail)
Red - 5v input rail

At a rough guess I either need more bulk capacitance on the 3.3v rail,
or, more likely, lower impedance decoupling caps (I'm currently using
100n 0603 of the "whatever I have in stock" variety). Would welcome
suggestions. I /do/ have an impedance analyzer that can handle this
frequency (goes to 500MHz), but I lack the SMD text fixture for it.

I suspect an output buffer would really help too, and on its own might
significantly improve things.