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

[Julien Goodwin]

On 18/9/21 10:26 pm, Bob kb8tq wrote:
> 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. 

I'm using the -17 variant, no divider, running at the 26MHz fundamental.

> 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.

First stage input is a Keysight N675xA supply, not the absolute quietest
supply out there but plenty good enough (it's also currently the best I
have, my last two linear supplies died, and I've not replaced them), the
second stage is an MCP1802 LDO, now with 1mH on the input in series. I'd
have expected 10uF to be enough bulk capacitance, but I can absolutely
try chucking some more on and seeing if that does help.

When I was just running the DAC without the oscillator the lines were
dead quiet, I suppose it's also possible there's leakage from the
control signal running too close to one of the lines to the crystal
(just a hair under 1mm between the traces)

> 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. 

It /should/ be able to handle driving into 50 ohms per the spec sheet,
but I agree it's not doing a great job of it.

>> 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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