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Old 24th Nov 2021, 5:44 pm   #1
6SN7WGTB
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Default Is this a Clapp AF oscillator in an RF sig gen circuit?

Could someone help me understand the AF oscillator circuit in the attached schematic please?

As you can see it provides AF modulation in a basic RF sig gen.

We have component values, bar the choke.

I've tried simulating it with a wide variety of choke values (1µH to 20H!) but cannot see any oscillation.

I'd also welcome the formula for the frequency of oscillation as it is not running at spec of 400Hz (it's ca. 1kHz).

Resistors are new, so i assume it may be a duff cap, but I don't just want to randomly replace components. Or can the choke have drifted? FYI the choke is pretty substantial - ca. 3/4" cubed 'transformer' looking.

Does the AF produced simply modulate the negative supply rail (which goes to the RF oscillator) across R10?

Thank you.
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Old 24th Nov 2021, 6:32 pm   #2
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

https://rickettslab.org/bits2waves/d...llator-theory/
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Old 24th Nov 2021, 7:28 pm   #3
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

This explains some subtle differences.

https://www.tutorialspoint.com/sinus...oscillator.htm
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Old 25th Nov 2021, 5:17 pm   #4
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Hi - I noticed that a couple of members had kindly posted links for you about Clapp oscillators - perhaps adopting the "teach a man to fish ...." approach, but not directly answering your three questions.

I'm far from an expert, but have taken a recent interest in Clapp/Colpitts/Hartley oscillators and I think that the jury is still out on your first question "is this a Clapp oscillator?". I does superficially look rather Clappy with those 3 capacitors and an inductor, but looking at the AF output C22 isn't even in circuit unless there is a load connected to that output socket, ie when just modulaing the RF which is its main function.

Offering an answer to question two, no the choke will not have drifted in value - it's just basically a length of wire. And finally question three - I would agree with your view that AF modulation is achieved via the power rails as you describe.

It obviously works, but if the mod frequency is annoying you then I would just try temporarily clipping an extra capacitor (maybe another 10nF) across C20 and see what new frequency results?

Perhaps an expert like Radio Wrangler, who basically wrote the book on oscillators, could cleer up any confusion regarding its heritage for the 140 plus following your thread?

Cheers
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Old 25th Nov 2021, 6:26 pm   #5
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

That's very helpful Chris, thank you.

I may well experiment with C20.

Initially the mod freq was about 950Hz, and when I found R6-9 25-40% over and replaced them I was a bit surprised to find it moved to 980Hz. But I now realise that this circuit is all about L and C provided the bias etc. is close enough.
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Old 25th Nov 2021, 6:47 pm   #6
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

The RF oscillator on the left looks like a Hartley oscillator on the RF lower ranges.

I think the AF oscillator is a simple RLC phase shift oscillator and I can have a go at describing a basic model for the oscillator and hopefully this will kick off some discussion. I don't know enough about Germanium transistors or AF chokes to model them very well so probably the first place to start is with a perfect transistor model and a perfect AF choke.

Also, it's worth mentioning that in order for this circuit to oscillate there has to be greater than unity power gain around the feedback loop along with zero (or 360deg) phase shift around the loop at 400Hz. Your circuit is designed such that TR2 provides the necessary power gain and some of the phase shift around the loop. The rest of the phase shift around the loop is achieved with the AF choke and the RC networks (C21 R8) and (C20 R7) together these components should ensure there is unity phase shift around the loop at 400Hz.

Note:
Normally this type of circuit uses three RC networks and a common emitter amplifier. The amplifier provides 180deg phase shift as it is an inverting amplifier (when feeding a resistive collector load) and each RC network provides about another 60deg phase shift each. 180 + 60 + 60 + 60 = 360deg around the loop which means input an output will be in phase at 400Hz. The triple RC circuit should then oscillate at as the transistor provides the power gain.

In your case there are only two RC networks. It looks like the missing phase shift is made up using the AF choke in the collector. The perfect transistor will act as a voltage controlled current source or, in other words, an AC voltage at the base controls an AC current at the collector. Normally we think of common emitter amplifiers as voltage inverting amplifiers but this is usually with a resistive load. Here we have a huge choke as a collector load. The perfect transistor will simply push AC current into the choke. However, voltage leads the current in a choke (google 'Eli the Ice man' as a neat way to remember this)

Quote:
What is ELI the ICE man?
ELI the ICE man is used to remember the relationship between current and voltage in an inductor and capacitor. ELI the ICE man stands for the
fact that voltage [E] leads current [I] in an inductor [L] (that’s the ELI part) and current [I] leads voltage [E] in a capacitor [C] (that’s the ICE part).
So the current is inverted by the transistor but the voltage will be shifted another 90degrees because the collector load is inductive. There will also be loads of voltage gain here. Following this are two RC phase shifter networks. These provide a bit more phase shift each and this will be just enough to achieve 360degrees (unity) phase around the loop at 400Hz.

A simulation using a perfect transistor model and a perfect AF choke predicts the AF choke has to be 15 Henries for oscillation to occur at 400Hz although I suspect it could be a bit less than this (maybe 10H) because this analysis uses perfect models for the transistor and the AF choke. I'm not sure what the ESR of the choke is but it must be quite significant.

This really is a crude analysis but it is a reasonable starting point I think. I did also model the AF oscillator circuit in the Simetrix Spice simulator using a regular transistor Spice model and the circuit oscillated at about 400Hz but it needed an AF choke of 10H to achieve this (not 15H). The 400Hz AF waveform didn't look very good though. it wasn't a nice sine wave. it had some harmonic distortion on it.

See the simple model of the AF oscillator below. This is open loop because you can see the circuit is broken open at ports 1 and 2. If you prefer to see a transistor in place of the VCCS I can post up a simulation screenshot. I think the phase shifts around the loop will be fairly similar but it will probably require a 10H choke to get unity phase around the loop at 400Hz.

In the plot below there is a plot of open loop gain and phase around the loop.
You can see this plot shows unity phase around the loop at 400Hz (tick).
It also shows much greater than unity gain around the loop at 400Hz (tick). Both ticks mean it should oscillate at 400Hz when the loop is closed.
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Old 25th Nov 2021, 6:55 pm   #7
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Note that the 12k port impedance of port 2 replaces R7 (12k). This is why R7 isn't connected. You can see in the 400Hz sine wave plots on the left that Vout (dark blue) is in phase with Vin (red) and Vout is bigger than Vin. So when the loop is closed (port 1 and 2 connected together) it should oscillate at 400Hz.
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Old 25th Nov 2021, 10:08 pm   #8
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

If you want to see a classic triple RC phase shifter circuit that is easier to analyse and understand I had a quick go at designing a 400Hz RC oscillator. This uses three identical RC phase shifting sections each of 10nF and 15k.

The initial crude analysis once again uses a voltage controlled current source model for the transistor. The collector current for the design is 1mA so gm for the VCCS model will be -1/26 = -0.038mho.

You can see in the green phase trace in the lower left graph the phase shift for just the three RC sections on their own is about 180deg at 426Hz.

The transistor provides 180degres phase shift because the collector load is mainly resistive. So these two phase shifts combined should provide unity phase shift at 426Hz. The same graph shows the open loop gain and phase around the loop for the combined transistor model and three RC sections and this does indeed show unity phase at 426Hz and plenty of excess gain around the loop. So this circuit should oscillate when the loop is closed.

I put the same RC values into a Spice simulation using Simetrix and used a BC547B transistor model. This shows the circuit oscillates at about 466Hz and this is very close to the oscillation frequency the crude VCCS model predicted with an open loop analysis.

Note that the last 15k resistor is made up of 18k and 100k in parallel.

To get lots of voltage gain at the collector (to overcome the significant passive losses in the three RC sections) the collector load resistor is 2.2k ohm. This is probably a bit marginal with a real transistor. 3k3 at the collector would give more loop gain. I've not tried to build this circuit for real but I'd hope it would work OK. The supply voltage is 9V.
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Old 25th Nov 2021, 10:26 pm   #9
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Here's a screenshot of that first attempt at simulating the RLC phase shift oscillator from the sig gen using the Simetrix Spice simulator.

There's a fair bit of distortion on the AF output but I'm not using the same transistor type and the choke model is very basic. The ESR is probably huge for that choke and 50 ohms ESR is probably a bit low.

It does seem to oscillate fairly close to 400Hz using a 10H AF choke. I'm also using a modern silicon PNP transistor although I've turned down the current gain a lot in the model. Germanium transistors are a bit leaky and I've not accounted for that.

The 9V power supply in the simulator is obtained using a fast pulse up to a steady 9V DC. This simulates the power up state. Have you done this in your simulation? It might not start up if you don't do something like this. I think LT Spice can simulate the power up state in various ways.
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Old 25th Nov 2021, 10:33 pm   #10
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

I think you're right that it's just a phase-shift oscillator (with the L and one of the C's taking place of two RC stages), but... with other oscillator circuits available, why
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Old 25th Nov 2021, 11:12 pm   #11
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

I did wonder that myself. I think it might be partly due to the requirement for low overall power consumption for the whole sig gen. It only has a budget of 3mA.

The reactance of the 10H choke at 400Hz is 400 x 6.2832 x 10 = 25132 ohms. This means the BJT only needs to run at maybe 200uA collector current and it can still produce enough gm to get lots of voltage gain at the choke. There is no voltage drop to worry about in the AF choke. It also provides maybe 60 degrees of phase shift around the loop for free.

I think (at just 200uA collector current) it will be harder to achieve high voltage gain and a good phase response with the lossy triple RC circuit. It will be harder to maintain the gain margin around the loop. It could be the case that the attempts to use a high value resistance in the collector will introduce undesirable phase shift around the loop that will fight against attempts to get more loop gain.

The phase slope at unity phase will be steeper with the RLC oscillator although I'm not sure if that matters much here. It might improve the consistency of the AF oscillation frequency as the 9V battery discharges. I guess there will be a fair bit of energy sloshing/stored in the RLC tank and this may help with the task of modulating the RF oscillator without upsetting the AF oscillator too much. At just 200uA the AF oscillator (when selected on its own) could run for months and months on a small 9V battery.
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Old 26th Nov 2021, 9:15 am   #12
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Really interesting analysis, thanks Jeremy.

I tried in iCircuit (a simple Mac sim) but it would only oscillate briefly on switch on, and so this wasn't any use for analysis.

I've attached two screenshots of the actual AF output, the slightly lower frequency one is with the RF off, and the ca. 1007Hz with RF on and being modulated.
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Old 26th Nov 2021, 10:47 am   #13
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

You can't rely on all simulator software to show oscillator circuits start up and run. For many years it's been their major blind-spot. Some will do it, but the results can also be misleading as well. There are methods of applying transients as a kick to get them going and some may start if the turn-on of power supplies supplies the transient.

In real life, some oscillators are biased to start as class-B or C amplifiers and have a dead zone in their transfer characteristic. Without a kick, these can just sit there and look at you, their only chance of starting is at a turn-on transient. Other oscillators are biased to start as class-A and these have high gain once power is there. The turn on might kick them into life, but if not, the cumulative gain on circulating noise will start them from nothing.

Unless the simulation includes noise, and can handle the fine time steps needed to handle the noise as well as the long time periods of its progressive growth, this start-up mechanism doesn't get represented and you have to use a transient to kick them into oscillating.

You can take a simulation which doesn't start and add a tiny amount of swept sinewave source. This allows you to probe other parts of the hoped-for oscillator and you should see a peaked response at the frequency you're aiming for.

The obverse to the old saying that all amplifiers turn out to be oscillators is that oscillators too can sulk, and they do it by not starting or jumping onto unwanted modes.

The AF oscillator in that Nombrex is a variant of a circuit called the phase shift oscillator (but all oscillators are phase shift controlled anyway)

It needs a 180 degree shift in a feedback path to combine with an inversion in an amplifier, to give a frequency-dependent overall 360 degrees.

A phase shift oscillator normally employs three concatenated RC meshes. The meshes can all be 'high-pass' series C, shunt R as in this Nombrex, or they can be 'lowpass' with series R, shunt C. Both work, but the former can save a few blocking capacitors. Nombrex were trying to make a useful sig gen for hobbyists, but price was their prime objective.

So, needing 180 degrees, you need three phase shift meshes, three concatenated RC sections. One mesh will give you 45 degrees at its 3dB attenuation point. You need an average of 60 degrees per RC stage, so the attenuation will be approaching 8dB per stage for the normal 3-stage RC variant. Now, you can get 90 degrees of phase shift from two stages, but theoretically this is only at infinite frequency, where the loss per stage is also infinite. Some phase shift in the oscillator can bring this down to more finite numbers, but it needs a lot of gain and operation is not very dependable.

This circuit in the Nombrex uses an inductor and this allows for resonance and an off-axis pole, so this becomes a bandpass element with phase shift related to its resonance curve width (1/Q) rather than its centre frequency, or the single-order response of an RC mesh.

Jeremy mentioned that the inductor gives a faster change of phase versus frequency. This is usually a very good thing in oscillators. Pure RC timed oscillators act as if they had a timing network Q less than unity and this shapes the noise they make. LC oscillators can do much better than this. The other advantage seen here is that an LC mesh can give the pase shift of two RC meshes, and with less attenuation. It also in this case provides the DC bias for the collector without much DC voltage drop, and doesn't load the oscillator's output. So maybe Nombrex' bean counters thought it a bargain.

Notice that the collector current doesn't go straight to ground, but there is R10 in the way, so some oscillator voltage will be expressed across this, and will modulate the voltage of the - terminal of the battery. It'll go through the battery and modulate the power rail to not only the RF oscillator but also to the AF oscillator. So here's an additional feedback path to make things extra-complicated. Note that C1 also comes into play, 3nF.


there's plenty written on oscillators. Most of it is on the naming of different circuits. People get really really concerned over whether a circuit is called this or that. This is just a matter of labels. Whether a circuit works well can only be seen by understanding it and how it works. Plenty is written on the frequency control of oscillators (and how to improve it)

Something's been left out. It is kind of important that the oscillator works at the amplitude you want (and even oscillates at all). Very little has been written on amplitude control mechanisms in oscillators. An ideal circuit would just build up and up until it hit infinite amplitude and melted the universe. In reality it'll smash into power rail voltage or current availability saturation long before that. More subtle oscillators have the active device start to drive itself into cutoff over part of its cycle. The bigger the amplitude, the bigger fraction of time it spends cut off. This gives a progressive throttling-back of gain (averaged over a full cycle) and smooth amplitude control. However, it only works really well in a circuit dominated by a true second-order resonator, like an LC circuit. The resonator acts as a flywheel smoothing the output waveform to reduce the on-off transitions of the device.

Here in a mostly RC circuit, there is little filtering effect and the output waveform will probably be poor.

But it's a cost conscious sig gen, it needs some modulation so you can see a receiver demodulating it and some sound passing through its audio stages. It may sound awful, but it shows continuity in the receiver. These generators are for fixing sets which aren't working at all. If you want to debug a reciver which distorts, you need a distortion free, more expensive, sig gen.

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Old 26th Nov 2021, 2:08 pm   #14
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Yes, it can be frustrating sometimes when trying to model oscillators using Spice. I find that the PSU transient pulse method with Simetrix to be very reliable. However, I also have to use advanced settings for the transient analysis and this takes some experience to set up. Also, you have to allow enough analysis time for the oscillation to begin. It is a fairly fiddly and clunky process and patience and some determination is often required.

Up at RF, especially VHF and UHF the Spice models can become a bit dodgy. I prefer to use a small signal s-parameter model taken of the active device using a VNA. This gives good insight into the small signal startup frequency but it can't model large signal behaviour. I've successfully used this technique to design BJT oscillators up to 16GHz although I used the manufacturers s-parameter models for that oscillator. It was just a fun based research design to explore the accuracy and limits of the technique.

I suspect that this 400Hz oscillator is the lowest frequency oscillator I've ever analysed like this! I was a bit nervous to look at it but I think the key thing to take into account is that the transistor is best considered as a voltage controlled current source for this particular circuit. The voltage isn't inverted (180degrees) at the collector because the load is inductive. If there was only the AF choke there and no feedback the collector voltage would be 90degrees out of phase with the phase of the base voltage when driven with a 400Hz signal. Most people would still expect the collector voltage to be 180 degrees out of phase because that is what they are used to seeing with a traditional resistive load at the collector.
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Old 26th Nov 2021, 5:03 pm   #15
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

With the inductive load/bias the load line goes elliptical which can make life interesting if you come at it from that direction.

Synopsis: Oscillators are awkward b*****s

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Old 26th Nov 2021, 5:38 pm   #16
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

I am surprised they didn't make the (expensive) inductor into a transformer (auto ?) and do a simple tuned collector/base circuit. Doubtless I will find out in a later post.
 
Old 26th Nov 2021, 7:33 pm   #17
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

Quote:
Originally Posted by Radio Wrangler View Post
Notice that the collector current doesn't go straight to ground, but there is R10 in the way,
It doesn't though - surprisingly! I couldn't follow the diagram, everything is upside down or back to front. So I redrew the oscillator - see below.

Quote:
Originally Posted by Radio Wrangler View Post
so some oscillator voltage will be expressed across this, and will modulate the voltage of the - terminal of the battery. It'll go through the battery and modulate the power rail to not only the RF oscillator but also to the AF oscillator.
It does look as though the supply rail to the RF bit is modulated by an extremely small amount of voltage wiggle. But it doesn't modulate its own supply line. With an already-small voltage on the base, this AC signal is further divided-down by R9, R10.

I could scarcely believe this, but I checked my sketch against the OP's pdf, and it is a correct re-draw. So I'm wondering if the manufacturer's circuit diagram is actually correct!

Quote:
Originally Posted by Radio Wrangler View Post
Note that C1 also comes into play, 3nF.
Though, with R10 being the value that it is, it probably makes negligible difference to the oscillator. I'll bet it's just there for RF bypassing on the RF oscillator supply lines.
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Old 26th Nov 2021, 8:29 pm   #18
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

The model 27 circuit in this thread has an equally obscure modulation method:-
https://vintage-radio.net/forum/show...05&postcount=3
They connect the AF output socket between the modulated supply and the base circuit (no cap values that I could see)
Model 41 ( RM site ) seems saner.
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Old 26th Nov 2021, 8:42 pm   #19
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

It's the same oscillator circuit, they obviously liked it! This one, presumably, came first (really old-fashioned transistor symbols).

It's just the coupling to the RF bit that's different - by a secondary winding on the choke, rather than RC coupled.
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Old 27th Nov 2021, 10:11 am   #20
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Default Re: Is this a Clapp AF oscillator in an RF sig gen circuit?

I need to get the PCB out and look again as I’m now wondering whether the ‘choke’ might be a (auto) transformer.

As I had the schematic I didn’t bother reverse engineering it.
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