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 <[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. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
