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I built this Hartley oscillator (taken from the November 1990 edition of Popular Electronics) as part of an AM modulated signal generator project. But, it steadfastly refused to oscillate.

enter image description here

I was careful about layout and built it on copper clad board, triple checking all connections and component values. I even clocked the confusing reversal of drain and source. Still nothing.

So, I modeled it in LTSpice and it doesn't oscillate there either, even after playing around with the values. Is there something I'm missing here? I'd still like to get this oscillating, since I've put some effort into its construction.

For parts I used an air blade variable cap (10 - 317pF) and an old RF transformer that I tuned to 255uH (to give me the low end of the MW band) on the primary (the tapped coil is about 7uH). I wound a new secondary to give me the 7uH I needed according to the 707VX spec (which has a 6:1 turns ratio).

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  • \$\begingroup\$ Oscillators simulated with SPICE often fail to oscillate, and need starting off with a stimulus, such as a couple of cycles of the required frequency. \$\endgroup\$
    – Leon Heller
    Commented Mar 31, 2017 at 13:32
  • \$\begingroup\$ Fair enough, but it doesn't oscillate in real life either;-) I've built plenty of oscillators in spice and it's rare I can't get them to oscillate somehow. This one is different. \$\endgroup\$
    – Buck8pe
    Commented Mar 31, 2017 at 13:36
  • \$\begingroup\$ I have used MPF102 before. The Idss specification ranges from 2mA to 20mA (normal). I found biasing a bit tricky. Perhaps you could experiment with value of R3. Perhaps increasing R3 to 1M. Bias current can be measured by measuring volts across R2. References from your LTspice. \$\endgroup\$
    – Marla
    Commented Mar 31, 2017 at 13:39
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    \$\begingroup\$ R3 is way too high. Try 270 ohms rather than k. \$\endgroup\$
    – Andy aka
    Commented Mar 31, 2017 at 13:57
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    \$\begingroup\$ @pipe Answer included. \$\endgroup\$
    – Buck8pe
    Commented May 8, 2017 at 10:50

2 Answers 2

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I'll take that credit LOL: -

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R3 is far too high for this to begin oscillating. Try 270 ohms rather than 270 kohms.

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It turns out that the original Popular Electronics schematic had an error where R3, the drain resistance, was 1000 times bigger than it should have been. It was printed as 270k when it should have been 270.

However, even with this error corrected, I found that the circuit performed poorly in reality. Biasing was difficult and the resulting sine wave was distorted. A much better circuit includes a diode at the gate to shift the DC level and bias the FET into its linear region. The details of this circuit (along with a description of the diode function) can be found in this stackexchange question.

The issues I had with fake MPF102s, whilst annoying, had no bearing on the issue or its solution.

Finally, 100% of the solution credit is owed to Andy Aka.

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