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

[Julien Goodwin]

On 20/9/21 5:37 pm, Julien Goodwin wrote:
> 
> 
> On 18/9/21 11:12 pm, Julien Goodwin 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.
>>
>> 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.
> 
> ... no it shouldn't, my ability to do mental ohms-law is apparently
> terrible, thanks for those correcting me off-list.
> 
> Some more experimenting today actually pointed out that the output
> clears up almost entirely once I get the breadboard out of the picture,
> which I guess isn't too surprising, but is annoying. I don't normally
> use breadboards at all, as I'm normally working with SMD parts and it's
> often quick to knock out a partial board as a test.
> 
> I'll design up a carrier PCB with proper connectors for power & output,
> I'll also chuck a buffer on for driving 50-ohm targets as a second
> output which will help make clear if it's worth integrating one for
> general use.

Between various shipping delays and my own schedule I only got around to
soldering up one of these boards yesterday, only to discover that the
sockets I'd been intending to use wouldn't work. A quick trip to the
local electronics hobbyist store today to pick up some replacements and
I was in business.

I'm still not happy with the amount of ripple on the 3.3v supply, with
~330mV peak to peak, however everything else seems solid and the output
is much cleaner through a local buffer that can actually drive 50-ohm loads.

Sadly, and fittingly for this project where every few steps forward
seems to take a step back, just after turning it on today something
(probably a cap from the smell) has gone inside the 53310A and so I
won't have a clear picture of output stability until that's resolved.

For those who want pictures:
https://twitter.com/LapTop006/status/1449616018846871558

Then I get to build some more test units and ovenise them. I have the
cans and heaters, so this should be easy once I've managed to build some
more units with working temperature sensors.

>>>> 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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