EMRFD Message Archive 9579

Message Date From Subject
9579 2013-12-21 13:53:41 bob_ledoux Success: Clean, Simple Local Oscillator

My efforts to get a clean sine wave from a single transistor Colpitts local oscillator were not successful.

However, I found a nice circuit that will provide 7 to 10 dbm from a single transistor. However it requires winding a transformer.

The circuit is available in the following articles:

“A Band-Imaging CW Receiver for 10 and 18 MHz” See, 1990 ARRL Handbook, Chapter 30; or QRP Classics, page 28.

The circuit is a Hartley oscillator with the crystal in series with the feedback tap on the coil. The transformer primary winding is part of an LC circuit. A trimmer cap in this circuit allows adjusting for a clean sine wave form and power level. My prototype provides a clean wave form between 7 and 10 dbm into a 50 ohm load.

The circuit appeared slightly different as Figure 7 of the “Progressive Communications Receiver” article found in the EMRFD disk. In that earlier configuration power was provided to individual oscillators using a band switching scheme.

Make the following changes to the Progressive Receiver schematic to derive the newer circuit mentioned above: Disconnect the 100 ohm resistor from the transformer secondary winding and remove the 0.01 capacitor. The bottom of the secondary winding is now grounded. Feed power into the 100 ohm resistor.

I tried my prototype with a number of different microprocessor crystals and they all worked reliably with a clean wave form. But the circuit uses a tuned LC so its more effort than un-tuned oscillators. Table 2 in the Progressive Receiver article provides details for frequency specific components.

9580 2013-12-21 16:03:53 John Marshall Re: Success: Clean, Simple Local Oscillator
Good work, Bob,

That circuit also appears in EMRFD, page 4-15. I've always had good results with it. The tuned circuit helps clean up the waveform (aka reduce harmonic content). It will work with fundamental or overtone crystals. You can even make a crystal intended for fundamental mode use oscillate at its third or higher overtone or an overtone work at its fundamental or an overtone other than the intended one.

You should adjust the output tune capacitor for highest output (with the intended load connected) consistent with reliable starting and not use it to set the output power. If you need to adjust the power a little, try adjusting the emitter resistor.

Another way to get clean output from an oscillator is to take the output directly from the crystal as shown on page 4.13 of EMRFD. This one will require an amplifier stage to bring the output up to 10 dBm but it will provide low harmonic content and low noise without the need for an LC tuned circuit.

John, KU4AF
Pittsboro, NC

9581 2013-12-21 16:32:07 bob_ledoux Re: Success: Clean, Simple Local Oscillator
Thanks for finding that.  I was looking for a newer source but missed Figure 4.29 while looking through EMRFD.

I prefer to deal with the tuned circuit rather than mess with multiple transistor stages I've found with other circuits.  I also like the fact that it is already matched to a double balanced diode mixer.
9582 2013-12-21 23:22:22 victorkoren Re: Success: Clean, Simple Local Oscillator

If you are willing to use dual polarity and a current feedback operational amplifier you can check this:

For a crystal oscillator, change the inductor with a crystal in parallel of a 10K resistor (to provide bias to the input pin) and decrease the resonant circuit capacitors from 1nF to lower value that will cause the oscillator to oscillate. It can be used to drive +7dBm or more easily with excellent waveform.

Victor - 4Z4ME