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 attached circuit schematic is for a slightly modified white emitter follower that reduces the temperature dependence of the input transistor collector current. It also increases the loop gain slightly. Bruce
<<inline: White-emitter-followerV2.gif>>
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