Hi > On Apr 15, 2018, at 1:01 PM, Gerhard Hoffmann <[email protected]> wrote: > > > > Am 17.03.2018 um 00:57 schrieb Ulrich Rohde via time-nuts: >> https://synergymwave.com/articles/2013/04/full_article.pdf >> This may be still of interest... happy weekend, 73 de Ulrich, N1UL >> >> > > Since the weather did not look so nice this weekend, I canceled a > 2-day motorcycle tour and tried the much applauded Colpitts > instead. > > I think it is not possible to make it closer to the published circuit. > OK, I dismissed the 0.5fF C0 modifier which is probably only a > placeholder for simulation experiments. I had no 200nH parts, so I took > 220nH, 150 nH did not make a difference other than less output voltage. > I did not test the buffer stage because the BFG540 is obsolete > and I could not find anything that fits its weird pinout. > > From the Altium Designer to .pdf, to Laser printout on foil as a > photo mask and the contact copy to the unpopulated pcb can be > done in a good hour, so it's a nice weekender. > > Circuit: < > https://www.flickr.com/photos/137684711@N07/41468657011/in/album-72157662535945536/ > >
For a conventional overtone Colpitts, you should have a bypass cap on R9. Without the bypass, the “tank” L4 + C11 will have very low Q. It is also likely that some portion of the “R” will get into the resonant loop. Bob > Layout: < > https://www.flickr.com/photos/137684711@N07/40754928234/in/album-72157662535945536/ > > > Hardware: < > https://www.flickr.com/photos/137684711@N07/41427204622/in/album-72157662535945536/ > > > > It turned out that the circuit, as it is, leaves the choice of the overtone > to the crystal. > From a collection of 100...120MHz crystals not one worked at the intended > overtone. > There were results from 20 to 70 MHz. > The test point is where the BFG540 emitter would be. > > < > https://www.flickr.com/photos/137684711@N07/40754932194/in/album-72157662535945536/ > > > < > https://www.flickr.com/photos/137684711@N07/41468662151/in/album-72157662535945536/ > > > < > https://www.flickr.com/photos/137684711@N07/40754931924/in/album-72157662535945536/ > > > > I forwent measuring the phase noise. > > Call me amateurish again, but I'll stay with the Driscoll. It has the tank > circuit to enforce > the correct overtone and I can adjust the delay so that 0 degrees around the > loop > coincides with maximum dp/df. Maximum Q helps nothing when it does not happen > on the > oscillation frequency. > > When I'm asked in a design review how I have made sure that the frequency of > a single point > of failure VCXO is not off by 30 to 80 percent, then I like to have an answer. > > > > And WRT vastly changing transistor parameters: They don't need to. If one > looks into the emitter > of a BJT at 10 or 20 mA bias, one sees 1 or 2 Ohms. That depends only on Ic, > K, T, q and some > minor parasitics such as RE. > The emitter is fed by the crystal in Driscoll's case with 50 to 70 Ohms on > resonance, which takes > nice care of the transistor's shot noise. Off frequency it gets even better, > esp. with C0 compensation. > > The base is fed with a huge step down ratio, so the base voltage is stiff. > The cascode isolates from the > effects of the tank circuit and it can easily be bootstrapped. If one > dislikes Schottky limiting, one can > duplicate the cascode transistor and divert some of the RF current to nirvana > and not to the transformer, > whenever the AGC voltage wants that. > So the environment of the crystal and the sustaining amplifier can be very, > very time-invariant. > The whole thing is just a current conveyor belt with the crystal enforcing > the current trough the > whole cascode to the tank/load. The output IS the resonator current. > > That is the very same structure as the proposals to extract to output > "directly from the resonator, > where it has the best SNR". > Given the little bit of power that we can steal from the resonator: must we > really split it between > extraction amplifier and sustaining amplifier right at the location where the > signal is weakest? > > When we can build an extraction amplifier that adds less noise than the > sustaining amplifier, why don't > we take the feedback also from there and cut out the sust.amp. and the signal > power it requires? > The Driscoll does that. > > > > regards, > Gerhard, DK4XP > > (the loyal heretic :-) > > > > > > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
