'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillator

[ms660]

Originally Posted by Al (astral highway)

Hartley? The inductor in the tank circuit is tapped halfway.

But then again, the stators of the tuning capacitor are also a voltage divider.

So what does that make it? A bit of Hartley and a bit of Colpitts?

I'm sure I saw a similar circuit in a piece of test gear once and it was called a Colpitts, but then again I might be mistaken.

Lawrence.

[Radio Wrangler]

Colpitts usually means a capacitive tap in the tank, Hartley an inductive tap.

https://en.wikipedia.org/wiki/Colpitts_oscillator

Then there is the Clapp oscillator which is like a Colpitts but the inductor becomes a series L-C circuit. The idea is that the colpitts capacitors are fairly large, and the series C largely dominates the tuning. It's a way of less strongly coupling the amplifier device into the ressonant tank, so that its Q is less damped.

There are lots of oscillator circuits and lots of names. It's more important to be able to see how they operate and how the compromises move around between different varieties than it is to remember the names. Older amateur radio handbooks threw loads of circuits at youu and told you the names but didn't fully cover the whys of the differences. More trainspotting than engineering!

The Clapp introduces an interesting concept. If the series L--C is dominating the tuning, then it's a 2-terminal oscillator.... so the 'Colpitts part' of the Clapp oscillator must be presenting a negative resistance to that series resonator, to cancel its losses.

David

[ms660]

Ref. my earlier post...I knew I'd seen an oscillator with a tapped coil and a tapped capacitance that was called a Colpitts....Leader signal generator type LSG-10, 11, 16 and 17, valve, transistor, and integrated circuit types.

LSG-10 description and schematic shown in the link below, looks to be essentially the same configuration as the oscillator in question:

https://nvhrbiblio.nl/schema/Leader_LSG10.pdf

Make of it what you will.

Lawrence.

[Radio Wrangler]

That's a different still style of RF oscillator. The tank is capacitively tapped and the tap grounded. At resonance, it acts as a phase inverter from anode to grid. The valve phase inverts form grid to anode, so there is a net phase shift of 360 degrees at resonance.... positive feedback!

The circuit is messier, so it's less visible in the audio oscillator area, but they use the same scheme.

There are so many oscillator circuits to choose from, most designers just use their favourites.

David

[ms660]

Originally Posted by Radio Wrangler

That's a different still style of RF oscillator. The tank is capacitively tapped and the tap grounded. At resonance, it acts as a phase inverter from anode to grid. The valve phase inverts form grid to anode, so there is a net phase shift of 360 degrees at resonance.... positive feedback!

The circuit is messier, so it's less visible in the audio oscillator area, but they use the same scheme.

There are so many oscillator circuits to choose from, most designers just use their favourites.

David

Yes but the manufactures called it a Colpitts, so do Heathkit and maybe other's do too, the circuit is essentially the same as the one in Al's link in which the capacitive tap is also grounded.

Lawrence.

[Radio Wrangler]

Notice that in the real Colpitts, the valve is operated as a cathode follower with the anode supply decoupled.

This circuit can be wrangled round to show a degree of equivalency with the grounded cathode circuit, but the strays and the effect of Cag is quite different. You also have to float the coil at both ends and practical matters prevent the topological similarity being a practical similarity.

David

[ms660]

Originally Posted by Radio Wrangler

Notice that in the real Colpitts, the valve is operated as a cathode follower with the anode supply decoupled.

I'm looking at a "Colpitts" circuit where the anode isn't decoupled and the cathode and the rotors of the tuning capacitors are connected to ground so what type of oscillator is that?

Lawrence.

[Julesomega]

Colpitts with grounded rotor, I guess. They're all the same really, a Hartley is a Colpitts with grounded cathode, or vice-versa or something.
I love the way they feed the heaters down the middle of the tuning line which thereby acts as the ultimate RFC!

[Al (astral highway)]

Originally Posted by ms660

Ref. my earlier post...I knew I'd seen an oscillator with a tapped coil and a tapped capacitance that was called a Colpitts

Good find, Lawrence!

Looks like there are a number of superficially similar rigs in which the differences look negligible but as David points out, go deeper than meets the eye.

[ms660]

Originally Posted by Al (astral highway)

Good find, Lawrence!

Looks like there are a number of superficially similar rigs in which the differences look negligible but as David points out, go deeper than meets the eye.

No problem, actually found that last night after remembering the Tech signal generator circuit, anyways here's the original (in effect) Colpitts circuit as per the man himself (Fig 3):

https://worldradiohistory.com/hd2/ID...%22colpitts%22

Lawrence.

[ms660]

The R1155 receiver also used a Colpitts oscillator for the BFO, a series fed type, the coil in series with the anode, the junction of the tuning capacitors tied to the cathode and the cathode bypassed to ground with a 0.5uF capacitor, the frequency of the BFO is half the IF so as to avoid the BFO being locked by the IF.

Lawrence.

[Al (astral highway)]

Originally Posted by ms660

No problem, actually found that last night after remembering the Tech signal generator circuit, anyways here's the original (in effect) Colpitts circuit as per the man himself (Fig 3):

Ahah, what stands out for me at least, is how he explicitly uses (for the first time?) inter-electrode capacitances as the voltage divider/tap.



Hence he is designing something inherently capable of oscillating at a frequency influenced largely by the physical properties of the valve.


Neat.

[Al (astral highway)]

Quick update folks,

I discovered the Acorn RCA955 I installed had previously been stressed around the join between one heater pin and the envelope, so the valve was gassy.

Here is its replacement, this time on the proper ceramic base.

Pic shows it tuned to 100MHz, with the signal coupled via a loop to my 'scope. This is the bandwidth of my 'scope.

This frequency is likely below the design frequency of the circuit, since I'm using a considerably larger tuning capacitor than the 10pF + 10pF on the cct. diagram

I may reconfigure it to see how easy it is to get to 200MHz with the 955 and a conventional tank circuit.

[Ed_Dinning]

Hi Al, you should still get a trace on the screen, even at 300 MHz, however, have a look out for a GDO that goes up to that frequency.
Hold it near your tank circuit and tune for max meter illumination

Ed

[M0FYA Andy]

The glow from that 955 looks as I expect. I confess I was quite worried by your first picture where it is lit up like a torch bulb, presumably an effect of being gassy?

Andy

[Al (astral highway)]

Originally Posted by Ed_Dinning

Hi Al, you should still get a trace on the screen, even at 300 MHz...

Hi Ed, as you predicted, a trace even at 303MHz - below 1m wavelength. It's good in amplitude but starting to degrade in quality.

I think it's rolling off too fast now to be reliable at any higher frequencies, but I'm sure the 955 is easily capable of a lot more. I could try lecher lines, I suppose. 450MHz would be a pleasing result.

The tank circuit is now a tapped 1cm single turn w/ 10+10pF tuning capacitor. The inductor is hidden by the coupling loop above it.

I think this is now higher than the design frequency of the circuit printed in 1940, which looks to have been targeted at 200MHz max.

[Al (astral highway)]

Originally Posted by M0FYA Andy

I confess I was quite worried by your first picture where it is lit up like a torch bulb, presumably an effect of being gassy?

Hi Andy, that was a good instinct and right on the money.


I did notice the crack before I put the valve into operation, but I didn't realise it was arleady fatal. But all is great with its replacement, very impressed with this valve as an oscillator.

[Radio Wrangler]

How do you know whether it's the scope rolling off, or the oscillator?

Those holders for 955s put a lot of force on the pins when you try to fit or remove the valve, and the force is in exactly the direction you don't want. They are notorious for cracking seals.

David

[Al (astral highway)]

Originally Posted by Radio Wrangler

How do you know whether it's the scope rolling off, or the oscillator?

That's a very good question, David! I don't know. Bandwidth is 100MHz,so the rol-off is by now quite severe, but I don't know exactly what it is.


I suppose I could increase the resonant frequency of the oscillator to 350Mhz and see what happens to the trace. But then I still won't know which is which. Any ideas?

Originally Posted by Radio Wrangler

Those holders for 955s put a lot of force on the pins when you try to fit or remove the valve. I can see how that could happen. The previous one I fitted with little crimps, but it was already U/S.

[ms660]

You might be able to monitor the grid current (bottom end of the grid leak resistor) with a meter.

Lawrence.