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

[Bob kb8tq]

Hi


> On Sep 18, 2021, at 9:12 AM, Julien Goodwin <time-nuts at studio442.com.au> wrote:
> 
> 
> 
> 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.

You need a quiet linear regulator feeding the chip. Any switcher will drive it nuts. 
Indeed a lot of LDO’s are not very quiet. They also may be very poor at isolating
you from switcher “crud”. You need to be careful about what you pick.

> 
> 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.

The spec sheet shows output current of 4 ma at full output and something in the
9 ma range with reduced output. You normally want to be at the < 4 ma point on 
a chip like this.  3V into 50 ohms gets you to 60 ma. 

The same loading issue can come in from a scope probe that has a bit too much 
C.  A 12 pf probe is right at the edge / over in this case. 

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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