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

Hartley oscillator schematic

(Original source of schematic diagram above)

Redrawn with components I chose:

hand drawn schematic

And here is the scope output:

Yellow signal - point X of original schematic

Blue signal - point Z of original schematic

Purple signal - OUTPUT

photo of oscilloscope traces

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?

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  • \$\begingroup\$ Your Blue signal is point Y, which is ground? \$\endgroup\$ – Marla Feb 18 '18 at 21:48
  • \$\begingroup\$ All signals are grounded on circuit ground. \$\endgroup\$ – DannyS Feb 18 '18 at 22:05
  • \$\begingroup\$ Point Y (Blue) measured with respect to ground should be a straight line on oscilloscope ( 0 volts ). \$\endgroup\$ – Marla Feb 18 '18 at 22:08
  • \$\begingroup\$ Right, my mistake - blue signal is Z not Y. I apologize for confusion. \$\endgroup\$ – DannyS Feb 18 '18 at 22:15
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    \$\begingroup\$ @SamGibson, I edited the question. Thanks for good suggestion. \$\endgroup\$ – DannyS Feb 19 '18 at 9:33
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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.

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  • \$\begingroup\$ There is a mistake in my calculation - OUTPUT should be about 800 Hz not 3.5 Mhz. I do agree that breadbard is noisy but I feel like this is not the root of the problem here. \$\endgroup\$ – DannyS Feb 18 '18 at 22:18
  • \$\begingroup\$ What if your breadboard is one of your problems? Component values aside, Marcus is correct. A breadboard is no place for RF circuits. +1 \$\endgroup\$ – Sparky256 Feb 18 '18 at 22:30
  • \$\begingroup\$ Well there are projects on youtube which shows AM and FM transmitters mounted entirely on breadboard so I wouldn't blame it for everything. Although it is a big capacitor and absorbs a lot of energy. \$\endgroup\$ – DannyS Feb 19 '18 at 10:02
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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.

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  • \$\begingroup\$ Correct, I forgot about the square root in denominator. But even so the result I get are incorrect! Do you have the reason for that? \$\endgroup\$ – DannyS Feb 18 '18 at 22:09
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    \$\begingroup\$ You seem to show two separate 2.2mH inductances. The inductance should be a single inductor with a tapped winding. \$\endgroup\$ – henros Feb 18 '18 at 22:16
  • \$\begingroup\$ @henros that is profoundly incorrect. \$\endgroup\$ – Andy aka Feb 19 '18 at 9:48
  • \$\begingroup\$ @Andy aka Woops! That comes from staying up past my bedtime when the mind is not in gear. Sorry everybody! \$\endgroup\$ – henros Feb 19 '18 at 14:17
  • \$\begingroup\$ @analogsystemsrf, I don't understand the last two sentences from your post. I have inductors midpoint connected to GND, what other components you want me to ground? Could you please draw the schematic for it? \$\endgroup\$ – DannyS Feb 20 '18 at 9:10
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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: -

enter image description here

Picture taken from here.

Also, why not try simulating it?

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  • \$\begingroup\$ Thanks for suggestions. I think base biasing is good advice although it didn't help. Even worst, I tried to assembly colpitts circuit and got the same output which I think is just noise. Simulation is good advice too, I will check it too. \$\endgroup\$ – DannyS Feb 19 '18 at 20:35
  • \$\begingroup\$ Did you get this circuit working @DannyS ? \$\endgroup\$ – Andy aka Feb 24 '18 at 18:16
  • \$\begingroup\$ I tried colpitts using this schematic upload.wikimedia.org/wikipedia/commons/b/bf/… and eventually got it working. However I made some changes since thse circuit wasn't working as expected. Can I post extended story somwehere here? \$\endgroup\$ – DannyS Feb 28 '18 at 20:17

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