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Old 17th May 2019, 2:05 pm   #41
G6Tanuki
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Default Re: Franklin VFO ?

Point taken about there being a bit more thought behind the original enquiry than just 'building an oscillator to get the Squadcal on air' !

One thing I vaguely remember from the couple of hours of lectures about oscillators I underwent 40 years back (most of which was actually about generating ramps/staircases for video/radar stuff and multi-phase clocks for digital circuitry...) was that the noise-power contribution of an oscillator increases more-slowly than it's intended output as that output power is increased.

So - other things being equal - it could make sense to generate the oscillations at high level then attenuate them down to the required level, since the attenuation will also attenuate the noise component.

(The downside, of course, is that you've got greater power-dissipation so greater self-heating of the oscillator components which is a bad thing from the stability-perspective).

I've always pondered if this concept might have somehow been behind the design of the local-oscillator in the TCS receiver - which uses a 12A6 power beam-tetrode!
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Old 17th May 2019, 3:20 pm   #42
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Default Re: Franklin VFO ?

Quote:
Originally Posted by G0HZU_JMR View Post
However, I have to say that this lashup version of the VE3RF oscillator I have in the screened box is probably the most stable 5MHz oscillator I've ever made and I've changed nothing on it since the initial build. I did include the subtle phase correction in the initial build. Up at 16MHz I think the gain/phase error will be less because there will be some phase shift in the overall circuit anyway at the higher frequency.
This supports my feeling that the Franklin is potentially more stable - all other things being equal - than most of the other oscillator types. I suspected the trade-off would mainly be phase noise against absolute stability, and the comments so far seem to confirm that.

I don't need this to get the radio on the air - I'm there already. The points of the exercise are :

1 A hands-on learning exercise in oscillator design and implementation.
2 A more elegant and appropriate solution, rather than driving a small, simple radio via a high function synthesizer. (This works well, but offends my sensibilities a bit).

The discussion so far forms a very useful part of objective 1 - thanks all round for the thoughts.
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Old 17th May 2019, 6:04 pm   #43
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Default Re: Franklin VFO ?

For a hands-on learning exercise, you have to try building several alternative types and compare them - built to comparable standards to keep the comparison valid. One oscillator alone tells whether that one works and what its performance is like but doesn't tell you whether another approach might have been different.

Building a lot of oscillators, and taking care over each one shows that the chosen circuit, IE the name, doesn't matter a great deal. You achieve stability by using suitably stable components and by diluting the variability of active device parameters by light coupling both in the driving and the sense paths. This latter comes down to scaling of values.

In quite general terms, the design choices which favour low phase noise tend to worsen temperature stability, and vice-versa. Though, there are ways to screw-up in one area without getting a benefit in the other.

The amplitude governing method in an oscillator relies on either driving an active device into cut-off, or into significant compression. It is remarkably difficult to make an oscillator which runs at low levels - especially one which can be relied on to start.

Some oscillators have a specialised level controller, a detector and a levelling amplifier controlling an AGC amplifier.

The BBC bought a number of HP 8656 synthesised signal generators to act as the RF oscillators for their world service transmitters. There's footage of one being operated, somewhere on the net

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Old 17th May 2019, 9:06 pm   #44
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Default Re: Franklin VFO ?

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Originally Posted by Radio Wrangler View Post
The amplitude governing method in an oscillator relies on either driving an active device into cut-off, or into significant compression. It is remarkably difficult to make an oscillator which runs at low levels - especially one which can be relied on to start.
It is also remarkably difficult to find any text book which gives a method of predicting oscillator amplitude! There is a lot of info out there about calculating frequency and what affects it - but try finding anything about calculating amplitude or how to design for a given amplitude and you'll find zilch. F Langford-Smith says the most when he mentions 'the necessary experimental work.' And that's it!

Years ago I built a Wien-bridge oscillator at 50Hz. I also needed to ensure that oscillation had established before something else (a high voltage power supply) was enabled, so I put a time delay to inhibit the power supply. But how long for? I couldn't calculate how long the Wien bridge oscillator would take to build up from noise. I sent a letter to Wireless World (no UKVRR then!) and it seemed nobody else could either. So it came to experimenting with a slow-time base 'scope.

I'm not an oscillator expert, but I am familiar in principle with the Franklin circuit, basically it just has one connection (plus ground) to the tuned circuit. The overall non-inverting amplifier with feedback from its output to its input just works as a negative impedance in parallel with the tuned circuit. So for a good, high-Q LC circuit the coupling thereto can, as stated, be extremely loose.

Isolating the active devices as much as possible from the LC circuit means that any variation in said devices will have correspondingly little effect. So stability, phase noise, harmonic content etc can be expected to be really low. The down side as I see it is that trying to couple a load to the tuned circuit will immediately degrade things. Taking the output instead from the middle of the amplifier will add the amplifier noise to the output - long-term frequency stability will be as good as the tuned circuit (obviously) but short-term phase noise will exist. How much, again, probably comes down to experiment, just as RW suggests!
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Old 18th May 2019, 12:16 am   #45
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Default Re: Franklin VFO ?

Some analysis & design material on oscillator amplitude in Clarke & Hess

https://www.slideshare.net/joseanton...larke-amp-hess

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Old 18th May 2019, 11:18 am   #46
G8HQP Dave
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Default Re: Franklin VFO ?

Quote:
Originally Posted by G6Tanuki
One thing I vaguely remember from the couple of hours of lectures about oscillators I underwent 40 years back (most of which was actually about generating ramps/staircases for video/radar stuff and multi-phase clocks for digital circuitry...) was that the noise-power contribution of an oscillator increases more-slowly than it's intended output as that output power is increased.
Yes, other things being equal, you get better noise performance from a higher power oscillator. Of course, you are also likely to get worse stability performance.

Quote:
Originally Posted by kalee20
It is also remarkably difficult to find any text book which gives a method of predicting oscillator amplitude!
The amplitude will be such that whatever gain control mechanism is used sets the loop gain to be equal to 1.

Quote:
Years ago I built a Wien-bridge oscillator at 50Hz. I also needed to ensure that oscillation had established before something else (a high voltage power supply) was enabled, so I put a time delay to inhibit the power supply. But how long for? I couldn't calculate how long the Wien bridge oscillator would take to build up from noise. I sent a letter to Wireless World (no UKVRR then!) and it seemed nobody else could either. So it came to experimenting with a slow-time base 'scope.
The amplitude build-up before you reach the steady-state condition depends on two things:
1. the bandwidth of the loop - which in many cases will be the bandwidth of the resonator
2. the excess gain - which will probably be varying as the amplitude builds up
I am not surprised that nobody could give you a satisfactory answer. A minor change to your circuit would give a different result; it might depend on temperature and supply rail voltage too. If you could produce a good mathematical model of how your circuit loop gain varied with amplitude then you could write down and (hopefully) solve a differential equation giving the time variation of the amplitude. It is essentially a non-linear feedback servo problem.
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Old 18th May 2019, 1:14 pm   #47
G0HZU_JMR
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Default Re: Franklin VFO ?

This may be of interest... many years ago I designed a basic 'trainer' 10.7MHz oscillator using a 50Ω MMIC gain stage and a (low) tapped resonator. It had a transformer splitter in the circuit that created an auxiliary 50Ω output port. This allowed the signal to be fed to a spectrum analyser. There was also a pair of 50Ω coaxial connection points that allowed the feedback path to be broken and the loop analysed on a network analyser. So direct measurements of gain margin, loaded Q, phase response and noise figure could be made using regular 50Ω lab gear . The option of fitting an attenuator and/or a delay line was possible at this point as well.

This allowed Leeson's equation to be demonstrated. I'm pretty sure I've posted about this old oscillator before on here and probably on Eevblog. Somewhere, I have some old phase noise plots taken of it with the company E5052A analyser. I dug it out yesterday and rebuilt it as I had pinched some parts from it a while ago. The plots below are for the group delay of the model of the resonator/oscillator and also of the real circuit. Both are measured in open loop at the 50R breakout point.

They agree very well and the loaded Q is about 41 with this resonator. By playing with the attenuator it is possible to control the loop gain. A loop delay can be added here as well (using coax cable) and the effect of spoiling the phase response can be explored. Because the MMIC is a well behaved 50R gain block with well controlled gain it is also possible to explore the region between regenerative gain and the onset of oscillation with this setup. I think I demonstrated it once as a regenerative detector but I had to tweak it down to the 40m band for this. It was possible to show the improvement in gain and selectivity as the loop gain was adjusted in tiny amounts.

It normally runs at 10.7MHz and this is twice the frequency of the Franklin oscillator. So that normally brings a 6dB penalty in phase noise. But because the MMIC has low flicker noise and because it runs at a higher power level and because I have the phase response optimised it is probably about 6-8dB better than the Franklin oscillator (in it current state) at offsets between 100Hz and 1kHz. It was only really designed to demonstrate Leeson's equation and it achieved this quite well I think. It wasn't designed to have ultra low phase noise or low drift.

This circuit is a bit like the Franklin oscillator in that it has two 180degree phase shift components in the loop. However, in this case the second one is a passive transformer and all the gain happens in the MMIC. I tap into the resonator at low impedance points in the resonator rather than tap into the top of the resonator. This seems to work really well and this suits the 50R MMIC. This setup makes it really easy to analyse and tinker with.
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