EMRFD Message Archive 2650

Message Date From Subject
2650 2009-01-28 14:23:44 rjtmkiii Causes of Hartley harmonics?
Dear list,
I have recently built a simple 7MHz Hartley VFO to learn about
temperature compensation, frequency range, etc, but in the course of
testing it, my frequency counter will sometimes show a reading above
14 MHz -- the second harmonic. (I understand that it is common for
harmonics to appear with only a single Pi filter, but since I don't
have a spectrum analyzer or oscilloscope, I can't determine whether
this is the only issue.)
I was particularly intrigued to note that the apparent doubling only
occurs above a certain capacitance setting. I guess that this is
because at a low enough capacitance, the filter is sufficient to keep
it on target. Unfortunately, the maximum 'stable' setting is still
more than 8 MHz and I wanted a 7Mhz VFO!
The circuit comes almost directly out of the EMRFD version of the
binaural rx's vfo (sorry, don't have my book with me at the moment).
The only difference is that I omitted the "6-hole" RFC on the power
supply. My power supply is a 6V battery of four 'C' cells.
Final thought is that my vfo fet (J310) only has a 10 Ohm resistor in
the drain supply -- should I change this? Add a zener/3-terminal
regulator?

Any and all comments / suggestions welcome.

Best regards,
Ross, NS7F
2651 2009-01-28 15:43:33 Chris Trask Re: Causes of Hartley harmonics?
>
> I have recently built a simple 7MHz Hartley VFO to learn about
> temperature compensation, frequency range, etc, but in the course of
> testing it, my frequency counter will sometimes show a reading above
> 14 MHz -- the second harmonic.
>

You may actually be overloading your frequency counter. However, the
potential for harmonics with the Hartley oscillator is something that is
common for most oscillators. It's due to the fact that just about all of
them go from saturation to cutoff, even with a high-Q resonator.

One way to avoid such a problem is to use an oscillator such as a
differential Butler and add an automatic level control (ALC) to keep the
level low. That's what I ended up using in my crystal test set so that the
measurements would be free of distortion (ie - harmonic) products:

http://www.home.earthlink.net/~christrask/Paper008.html

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_
2653 2009-01-28 16:07:04 leon Heller Re: Causes of Hartley harmonics?
----- Original Message -----
2655 2009-01-28 21:51:07 cwfingertalker Re: Causes of Hartley harmonics?
Hello Ross,

When I experiment with oscillators I try to keep a load on the output.
If your pi network does not have a load resistor to represent the
next stage things can get weird fast. Figure out what that should be
and use the appropriate carbon resistor value as the load.

I use a small value capacitor to tap into the circuit for the
frequency counter. Maybe 10 or so pf. Just enough to get a signal
for measurements. If you connect the frequency counter directly you
might be overloading the counter.

Bill N7EU
2656 2009-01-29 12:07:38 Wes Hayward Re: Causes of Hartley harmonics?
Hi Chris and gang,

We all have our favorite circuits. Among mine are the Hartley
oscillator and single transistor cascadable feedback amplifiers.
Guess that's no secret!

If you take a look at the Hartley oscillators in EMRFD, and there are
probably a lot of them, you will see that most have a catch diode at
the gate. The operation of this diode is explained in Chapter 4. It
is an AGC system and the FETs are NOT slamming from cut off to full
on. Rather, the catch diode restricts the gain during oscillation
such that there is a few volts bias on the D-S of the FET over the
full operating cycle. This goes back to experiments reported in a
Dec 1970 QST paper. The operati
2659 2009-01-30 07:02:32 Chris Trask Re: Causes of Hartley harmonics?
Wes,

>
>If one seeks that illusive pure sine wave, whatever that might mean,
>you can enhance the waveform purity by extracting the signal directly
>from the resonator. This can be done with a pair of series capacitors
>from the hot end of the resonator. In one rig with a Hartley VFO at
>2 MHz I use 33pF in series with 680 pF to ground. Harmonics are
>then 40 or 50 dB down.
>

That's something that I hadn't thought of. I'll have to put that into my notebook of sneaky circuitry.



Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_
2660 2009-01-30 07:51:46 Chris Trask Re: Causes of Hartley harmonics?
Wes,

>
>If one seeks that illusive pure sine wave, whatever that might mean,
>you can enhance the waveform purity by extracting the signal directly
>from the resonator. This can be done with a pair of series capacitors
>from the hot end of the resonator. In one rig with a Hartley VFO at
>2 MHz I use 33pF in series with 680 pF to ground. Harmonics are
>then 40 or 50 dB down.
>

Now that I've thought about this, I should have recalled that there is a similar method used by crystal oscillator designers. Thay make use of the modified Butler oscillator where the crystal is connected from the transistor base to ground. What they do is connect the grounded end of the crystal to the emitter of a common-base amplifier. You get a very clean output, and the noise easily approaches the theoretical -165dB minimum.



Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_