Hartley oscillator problem

Question

For a couple of days, I have been trying to build Hartley oscillator on my breadboard based on this schematic:

(Original source of schematic diagram above)

Redrawn with components I chose:

And here is the scope output:

Yellow signal - point X of original schematic

Blue signal - point Z of original schematic

Purple signal - OUTPUT

Theoretically we have sine wave on the output, and the voltage on the two inductors is 180 degrees out of phase, but I feel like this is not OK. First of all, the amplitude of the output is too small and second, even more important, I expected about 800 Hz no 100 MHz (as you can see on the scope).

So the question is: What is wrong with my circuit?

Answer 1

Beyond ca 10 MHz, breadboard simply becomes unusable due to high parasitic inductance and capacitive coupling between adjacent rows.

Build your circuit again on a piece of perfboard or better yet double-sided copper clad board with traces cut out on the top side only.

Answer 2

The total inductance is 4.4 milliHenry. The tank capacitor is 10uF. Fresonate is

1/[ 2*pisqrtL*C) ] = 0.159 / sqrt(4.4e-3 * 1e-5) = 0.159/sqrt(4.4e-8)

Fresonate = 0.159/2e-4 = 10,000 * 0.08 = 800Hz.

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Large inductors have lots of parasitic capacitance, such as between layers.

And your "breadboard" will have suspicious inductors, such as 10nH or 20nH, all over the place.

Alter your connections, by tieing the inductor midpoint the other 3 components also tied to GND.

AND ADD A 0.1uF capacitor from this new single-point GND to the VDD pin.

Answer 3

The problem is your biasing resistor values - they are much too low and the oscillator isn't er... oscillating. Try resistor bias resistors that are ten times higher so they do not load the coils and capacitors too much. Try making R1 about 5 to 10 kohms and R2 about 2 to 3 kohms.

The 100 MHz you see on your o-scope is very low level I suspect and not related to the oscillator working unless you have component values shown incorrectly in your diagram.

Look at this Colpitts oscillator design (very similar to the Hartley) and note the values of the bias resistors: -

Picture taken from here.

Also, why not try simulating it?