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

[Bob kb8tq]

Hi

The chip is intended to be used with the divider engaged ( set to a divide of 
2 or greater). That should act as a pretty good buffer if the layout is reasonable. 

Looking at the spectrum analyzer plots, you either have crud on the control 
line ( ground it and see what happens … ) or on the supply. Either way it’s at
audio frequencies. Your regulator may have issues (switchers are not what you
feed a crystal oscillator with …..) or you need some caps in the 100’s of uf 
range on the regulator you have.

Pay attention to the max output C even when running the divider. You can drive a 
scope probe, but not a 50 ohm line with the device. For 50 ohms you will need
a pretty healthy ( = high current ) buffer. 

Bob

> On Sep 18, 2021, at 12:38 AM, Julien Goodwin <time-nuts at studio442.com.au> wrote:
> 
> 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.
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