Bruce Griffiths wrote: > Esa Heikkinen wrote: > >> Hi Bruce... >> >> >> >>> Try increasing C2 and C8 in the white emitter follower circuit schematic >>> to 100nF. >>> >>> >> Doesn't seems to change anything: >> http://www.amigazone.fi/files/gpsdo/544-15.png >> http://www.amigazone.fi/files/gpsdo/544-16.png >> >> When doing this measurement I noticed that the 3rd and higher harmonics >> level are changing! First I thought that the capacitor change was some >> effect on harmonics but then those peaks come back... >> >> Time domain analysis about 3rd harmonic level gave some explanation: >> http://www.amigazone.fi/files/gpsdo/544-17.png >> >> The EF circuit was turned on at start of sweep. Something is heating and >> raising the harmonic level. The bump and the end of sweep is a test >> where I momentary switch off the 12V feed to the EF circuit to see how >> it reacts. >> >> Temperature sensitivity was also verified with cold spray: >> http://www.amigazone.fi/files/gpsdo/544-18.png >> >> Well, that's not an issue but makes measurements harder because even >> without any changes the results can differ. However the temperature >> effect on the 2nd harmonic frequency was very small. >> >> > The switcher sidebands will still be there, they are just buried in >> > the spectrum analyser noise floor. >> >> Yes you're right. Just changed some settings and there they are again: >> http://www.amigazone.fi/files/gpsdo/544-19.png >> >> About 90 dB below carrier.. I would say that it's good enough! The LPRO >> for example, gives higher spuriouses, but far away from fundamental. >> >> > Does the board use the recommended LC filters and regulator for the >> > oscillator supply as depicted in Figure 3 on the 10544A data sheet? >> >> I haven't analyzed it fully but it seems to be just the datasheet >> circuit having LM723, 10 uH coil etc. But it's layout is totally wrong >> because oven switcher current runs via wrong trace. It would also been >> possible to have other side as grouding copper but this was not done. >> The PCB is manufactured by Cubic western data. >> >> > The required parts shouldn't be too expensive, however you may need >> > to wind your own inductors for the series tuned LC circuits. >> > Air core or powdered iron core inductors should be OK as long as you >> > use shields between filter sections etc. >> >> Sounds like hard. How it's possible that lower grade ocxo's (like in >> thunderbolt) output so much better spectrum? Is it all about ocxo output >> driver circuit? Would it be easier to modify the 10544A itself than >> trying to clean the distortion? Has anyone tried that kind of modification? >> >> >> > Esa > > The circuit that is used to extract and buffer the signal from the > crystal oscillator can have a significant effect on the distortion. > In the case of the 10544A the cascode buffer contributes most of the > distortion. > The emitter follower contribution to distortion with a 1k load is much > smaller, however it is sensitive to slew rate distortion with > significant capacitive loading. > Using a common base stage as in the 10811A can have lower distortion, as > the emitter current is filtered by the crystal. > If one uses RF transformers between stages then one can cascade common > base stages and increase the output signal current. > An output stage with a 50 ohm output impedance is useful in that the > connection to external circuitry can then be a 50 ohm transmission line. > > > Some of the early OCXOs even used a crystal filter to clean up the > oscillator output. > Modifying the 10544A circuit would require a new circuit board and the > mechanical space available would restrict what you can fit in unless you > you use some surface mount components. > You would need to reuse the trimmer capacitor. > > Bruce > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > Esa
Attached circuit schematic for the While emitter follower optimises the bias circuit frequency response. The bias circuit has a bandwidth of around 40kHz so that will reduce the effect of flicker noise should RF transistors be substituted for the 3 transistors comprising the white emitter follower. The modifications also remove peaking in the bias circuit frequency response. Bruce
<<inline: White-emitter-followerV3.gif>>
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