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Old 23rd Apr 2021, 8:13 am   #1
crackle
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Default RF oscillator circuit

I wonder if you could please help me and explain the working of this circuit for a Unikit RF Oscillator.
I find it difficult to follow.
It has been mentioned that it would not be viable as drawn.
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Thanks
Mike
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Old 23rd Apr 2021, 8:40 am   #2
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Default Re: RF oscillator circuit

The valve is a battery heated tetrode, strapped with g2 to A to act as a triode.

It isn't clear that the HT battery -ve os connected to chassis, there is no dot where the wires cross, but in that era a cross, without a looped 'hop' being drawn, was assumed to be connected.

HT flows to the valve anode via one of the coils and the band switch. This is just a coupling winding to the main winding on that coil assembly. The main winding feeds the grid of the valve through the other pole of the bandchange switch, and is resonated with the variable capacitor. Note the DC block capacitor on the way to the grid - it's very important but not obvious what it does. You get DC isolation from the HT on the anode by transformer action in the coils.

So it's a transformer-feedback circuit, with the transformar connections in the right phase for positive feedback. The loop has gain at the resonant frequency and so it will oscillate.

The blocking capcitor and the 5 meg grid leak resistor allow the grid to self-rectify the signal there and to create its own negative bias as the grid's positive excursions lead to grid current, while the negative excursions are free to swing as low as necessary.

This mechanism stabilises the amplitude of the oscillation. Without an amplitude stabilisation mechanism, oscillators would just bang themselves alternately against the end stops, making hellish harmonics and quenching the Q of their tank circuits which would lead to very bad noise content.

So tha artfulness in this circuit is in allowing the grid tuned circuit to have high Q, not damped by too much coupling to the anode coil, and not damped by a low-Z self rectification level stabiliser.

Tons has been written on oscillator frequency stability, but damn all on amplitude control mechanisms. You really need both for a successful oscillator, and you need tounderstand both in order to see what an oscillator's up to.

David
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Old 23rd Apr 2021, 8:40 am   #3
lesmw0sec
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Default Re: RF oscillator circuit

I don't see why it should not work - it looks like a standard Hartley type with feed back inductively coupled from anode to grid. The 'bias' would be due to the voltage developed on the grid. Not too different from the operation of many local oscillators in use in superhets at the time.
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Old 23rd Apr 2021, 8:50 am   #4
crackle
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Default Re: RF oscillator circuit

Thanks for your comments that's very useful.
There is another diagram I have just noticed which shows each of the HT coils connected to ground. So yes the HT battery -ve is connected to chassis

Mike
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Old 23rd Apr 2021, 10:05 am   #5
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Default Re: RF oscillator circuit

Series fed tuned grid, aka an Armstrong/Meissner oscillator, very common in domestic radios.

Lawrence.
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Old 23rd Apr 2021, 4:38 pm   #6
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Default Re: RF oscillator circuit

Yes, standard Armstrong circuit: Tuned LC circuit to grid, and a feedback coil between anode and HT+.

Quote:
Originally Posted by lesmw0sec View Post
I don't see why it should not work - it looks like a standard Hartley type with feed back inductively coupled from anode to grid. The 'bias' would be due to the voltage developed on the grid.
Not Hartley, IMHO - Hartley oscillator has a tapped inductor, such that the feedback winding is part of the L in the tuned LC circuit. This (Armstrong) has feedback via a separate coil.
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Old 24th Apr 2021, 10:01 am   #7
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Default Re: RF oscillator circuit

Quote:
Originally Posted by kalee20 View Post
Yes, standard Armstrong circuit: Tuned LC circuit to grid, and a feedback coil between anode and HT+.

Quote:
Originally Posted by lesmw0sec View Post
I don't see why it should not work - it looks like a standard Hartley type with feed back inductively coupled from anode to grid. The 'bias' would be due to the voltage developed on the grid.
Not Hartley, IMHO - Hartley oscillator has a tapped inductor, such that the feedback winding is part of the L in the tuned LC circuit. This (Armstrong) has feedback via a separate coil.
I was speaking of the generic term. We were always taught: Tapped inductance = Hartley type, tapped capacitance = Colpitts - regardless of whether or not the feedback winding contributed to the tuning.
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Old 24th Apr 2021, 3:11 pm   #8
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Default Re: RF oscillator circuit

Note that the Hartley uses a cathode follower or emitter follower active device, with less than unity voltage gain. The amplifier drives the tap and the full coil drives the amplifier, so the oscillator uses the tapped coil in the resonator as a voltage step-UP to compensate for the low voltage gain in the amp.

The colpitts uses a capacitive tap in the resonator to perform the same trick.

Don't worry about the names. Everyone fixates on the names! but look instead at how they work - that's the important thing.

David
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Old 24th Apr 2021, 3:25 pm   #9
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Default Re: RF oscillator circuit

Quote:
Originally Posted by Radio Wrangler View Post
Note that the Hartley uses a cathode follower or emitter follower active device, with less than unity voltage gain. The amplifier drives the tap and the full coil drives the amplifier, so the oscillator uses the tapped coil in the resonator as a voltage step-UP to compensate for the low voltage gain in the amp.
I'm looking at a Hartley circuit that's not a cathode follower.

Lawrence.
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Old 24th Apr 2021, 3:47 pm   #10
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Default Re: RF oscillator circuit

I've never understood why, unlike most circuit designers, inventors of oscillator circuits were privileged to attach their names to circuit topologies; Hartley, Colpitts etc - even Wien. Did they make a useful income from royalties? I just know that their circuits have since fed endless technical arguments as to which is which. Those arguments create a mystique which unnecessarily complicates the subject. I guess at one time it facilitated the setting of examination questions.

All we need is frequency-selective positive feedback round one or more gain blocks with a means of regulating amplitude.

Martin

P.S. Was there ever a designer called Mr Phase Shift? Or even Mr State Variable?
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Old 24th Apr 2021, 4:32 pm   #11
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Default Re: RF oscillator circuit

I have never heard of a Mr Blocking ;

https://worldradiohistory.com/BOOKSH...ors-Schure.pdf
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