Trio TR2200GX RX Oscillator

[Jon_G4MDC]

The TR2200 was a well known crystal control rig from the 1970s.
It went through a few different revisions the last of which was the GX which had provision for an external VFO as well as crystal control. I had one as my first rig in 1977 or so.

It has always puzzled me that they had no frequency adjustment for the RX crystals. They just said buy the genuine ones from a Trio dealer or they might come out off frequency. They are 45MHz third overtone types.

Today I thought I might look into the oscillator to see whether it would be possible to add some adjustment. Crystal availability being what it is I wondered if anything in the junk box might be useful for an extra frequency or two. A pulling coil might be useful.

Now I look at the oscillator circuit it doesn't make sense to me. I expected to see a Colpitts. Well is isn't. Does anyone recognise how that works? The diodes for the VFO/XTAL switching are clear enough. I can see it would work OK with VFO input but I can't see any feedback to make the crystals oscillate.

Any ideas please?

[Cruisin Marine]

I have seen this before, but can't remember where.
They seem to rely on the inherent C-E capacitance that the transistor has.
Why they don't add a few pF to ensure the oscillator always starts I don't know.

[Jon_G4MDC]

That was the only thing I could think of. Pye always did it like this for 3rd Overtone crystals and I expected something similar but with L1 fixed and so the same for all channels.

[G6Tanuki]

The Trio circuit shown is essentially the bipolar-transistor version of the Miller-Pierce valve oscillator, where the collector-to-base capacitance and circuit-strays provides the feedback, but it's multiplied by the gain of the transistor.

https://electronicsdevicesbyravi.blo...scillator.html

[Jon_G4MDC]

I suppose the collector parallel tuned tuned circuit makes that point pretty high impedance so there can be lots of swing.

I think it might be an idea to knock one up and see what happens.

[Cruisin Marine]

It looks to me like a mistake, if C56, went to the C57 on the emitter, it would then be a normal Colpitts Osc. (The clue is the lowish values of those two caps) I wonder if the schematic diagram or whole design was wrong by mistake?
Without that connection I would hope to see a small capacitor (1-3 pF) from C to E to maintain the feedback loop.

[dmowziz]

Originally Posted by Jon_G4MDC

Now I look at the oscillator circuit it doesn't make sense to me. I expected to see a Colpitts. Well is isn't. Does anyone recognise how that works? The diodes for the VFO/XTAL switching are clear enough. I can see it would work OK with VFO input but I can't see any feedback to make the crystals oscillate.

Any ideas please?

Please can you explain the diode switching part?




This is an interesting "class" of oscillators. I actually did a little study on it last week. Like Cruisin said earlier, it relies on inherent data.

Here's what's been observed. (This is my understanding for now)

Firstly, for oscillation c58 (The collector capacitor) is not needed for oscillation. But it has an importance in the crystal oscillator (we'll come to c58)..

Funny enough, C57 and R48 are not necessary for oscillation at some frequencies...




A transistor has a stability factor (k) associated with it.
ANY frequency in which k<1 means that if the emitter is grounded then we can add a reactive element at the collector to get a negative resistance at the base.
Negative resistance at base means we can oscillate.



Using 2N3904BU
G0HZU measured the S-parameters of 2N3904BU's and posted an image of k and Gmax. First image

We can see k <1 below 50 MHz and between 600 MHz and 1.2 GHz
Therefore, we can ground the emitter, add an inductor or capacitor at collector to get negative resistance at the base

He posted the S-parameters file of the 2N3904BU (7V, 10mA) from 30 - 300 MHz.
I simulated the k, can see k<1 only below 40 MHz
So, with the emitter grounded, we can never get oscillation between 40 and 300 MHz



Looking at the schematic I posted (2nd image). With 2.7 uH inductor at the collector, we can see that the input impedance at 30 MHz is a series resistance of -3.135 ohms and series capacitor of reactance -18.767 ohms.

I added the 1pF at the collector, so can view on a Spectrum Analyzer.
and the base 3.9k ohms, makes things slightly worse but it mimics the bias resistance in prototype

So all we need to add to the schematic to get oscillation, is to add an inductor of 99.6nH at the base.




I biased the circuit, and added a series capacitor to this 99.6 nH to disturb the bias...
The circuit oscillates but unfortunately, not at the intended 30 MHz, but at 39 MHz... (I think due to the unmeasured collector inductor-)



Continuing analysis in next post. If you have a 2N3904BU (from farnell), you can have a go.

[dmowziz]

At 30 MHz, the negative resistance is just -3.135.

If we have a crystal that works at 30MHz, there will not be oscillation if the ESR of the crystal is greater than 3.135.
That's the need for C57 at the emitter. It reduces the value of the negative resistance seen looking into the base and it also makes k<1 at frequencies where it is greater than 1 (with grounded emitter)


C58 at the collector is just for tuning with L22.. To prevent the crystal from operating at the lower modes. (if k<1 at 44 MHz, it will likely be less than 1 at 22 MHz)
Without C58, the crystal will operate at 22 MHz

I'll do more tests maybe tomorrow. Especially to oscillate at 300 MHz, with an emitter capacitance.

Please, can you explain the diode part?
Thanks

[Cruisin Marine]

D4 is turned on via an RFC (L21) etc. on the Receive 9 volt line, that makes it conduct and allows the connection to the to crystal make a path, at the same time that voltage seems to block via L20, and reverse bias D5, I don't know what that feed line is though as the diagram is not inclusive.

[dmowziz]

Originally Posted by Cruisin Marine

D4 is turned on via an RFC (L21) etc. on the Receive 9 volt line, that makes it conduct and allows the connection to the to crystal make a path, at the same time that voltage seems to block via L20, and reverse bias D5, I don't know what that feed line is though as the diagram is not inclusive.

Ah, I see now. Thanks

But I'm a bit confused. If the RX line is not on, shouldn't it be when the crystal is part of the circuit on the right of C50?
Like, when RX line is 9V and makes D4 conduct through L19, it's providing an AC short through D4 and maybe C49 so that the crystal is not part of the circuit to the right of C50

[Cruisin Marine]

I guess when not in RX mode there is bias applied to D5 and this reverse biases D4, therefore blocking the Crystal path.
I don't know what that mauve coloured line is, it isn't obvious from the diagram shown.

[Jon_G4MDC]

As written in the original diagram mauve line is low on VFO.

D5 conducts and blue (VFO RX LO) is connected to Q10.

I also wondered if there could be a drafting mistake with C56.

[dmowziz]

Originally Posted by Jon_G4MDC

I suppose the collector parallel tuned tuned circuit makes that point pretty high impedance so there can be lots of swing.

I think it might be an idea to knock one up and see what happens.

Hi G4MDC,
Did you still attempt in making one?

The 1st attachment is the schematic of a crystal oscillator (10 MHz and 12 MHz).

As can be seen, the only capacitor needed is that to be used to view on a spectrum Analyzer. The crystal is acting like a series RL
It does not look like an oscillator but it sure does oscillate.
The base voltage is from a potentiometer

You can attempt if you have a genuine 2N3904 (Although I used a 2N3904BU but any genuine 2N3904 should work)

Alot of harmonics : The negative resistance is alot more than crystal ESR

[Jon_G4MDC]

No, I have not built it but I might open up the 2200 to check the layout around that oscillator conforms with the circuit as it is drawn.