[time-nuts] Some thoughts about Crystal Oscillators

Bob kb8tq kb8tq at n1k.org
Sun Apr 15 14:28:43 EDT 2018


Hi

> On Apr 15, 2018, at 1:01 PM, Gerhard Hoffmann <dk4xp at arcor.de> 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 I did not test the buffer stage because the BFG540 is obsolete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 :-)
> 
> 
> 
> 
> 
> 
> 
> 
> 
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