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. >>> On Sep 18, 2021, at 12:38 AM, Julien Goodwin <[email protected]> >>> 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. >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] -- To unsubscribe send >>> an email to [email protected] >>> To unsubscribe, go to and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send an >> email to [email protected] >> To unsubscribe, go to and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
