Quote:
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?
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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.