The AM Forum

Play on words, we have “what’s your favorite oscilloscope” so want to do What’s your favorite oscillator! Think I have to say that the crystal oscillator is my favorite right now with the RC Phase shift being the least favorite, and the Wien Bridge being the least understood.
Any Colpitts or Hartley supporters out there?
I am also somewhat found of the Monostable Multivibrator for its ease in synchronizing so going to give a shout out for that although relaxation oscillators are not something that you usually think of in transmitters.
For people doing PDM/PWM modulation on there AM transmitters what’s the preferred oscillator for generating the PDM waveform?

  I built a two-tube pine board transmitter a few years ago, using an 807 driven by a 6AG7 oscillator. I based the oscillator on Greg Latta's excellent technical description, here: https://www.frostburg.edu/personal/latta/ee/wing2e26xctr/schematic/oscillator/2e26oscillator.html
  It's essentially a series-tuned colpits. Very stable; delivered gobs of swing—more than enough to drive the 807 to 50 watts output. I'd use the same scheme again in future projects.

Pierce.

Simple.

73DG

http://www.amwindow.org/tech/htm/series.htm

Imagine the whole RF transmitter using just one tube -  plus the frequency is variable +- 3KHz.  Called a VXO (variable crystal oscillator) or in this case, a "variable crystal power oscillator."


This little 6AQ5 series modulated rig could be made even simpler using a tube  screen modulator or a MOSFET screen modulator.   Or, make it TWO 6AQ5s as the series modulator to make it more robust.

"No modulation transformer is required. HUGE, clean audio peaks. This rig is capable of clean audio from 10 Hz to 15 kHz. The VXO can "pull" the crystal up or down a few kHz with good stability. Only 3 or 4 crystals could cover the entire 3870-3890 AM Window on 75 Meters. For complete crystal stability short out the VXO cap and inductor to ground. The crystal will now determine the frequency and not be able to move the crystal +- 3 KHz as before. Recommend using a filament transformer for the final amp tube rated at 1500 VDC insulation. Be careful. Remember, the cathode of this tube is at 300 volts or more."

VXOs are my favorite oscillator.  They are "fun oscillators." I still have this rig in the shack, ready to go. I plan to match it up with my little nine tube homebrew superhet.

A close second is a variable Hartley creating a DFO - drift frequency oscillator --    or Timtron's "Sideband Eliminator" circuit.


T

Way Cool! Series modulated 6AQ5! Harris use to have a series modulated 5 kW AM transmitter, the MW-5 Most people who had to maintain them will tell you they were a real pig. Not only series modulated with something like a 3CX5000 output tube but also just for fun they were one of the first generation PDM transmitters. The modulator had its cathode grounded and the plate ran thru the PDM filter to the cathode of the PA tube so the entire PA assembly floated around three or four thousand volts above ground. The 3CX5000 had a 10 volt filament transformer that looked like a baby Austin Ring transformer with a good six inches or so isolation from the primary to the secondary so that the filament would be isolated from the AC line or ground. Imagine that you want good HV isolation on that filament transformer on the 6AQ5 rig and ground although maybe you can get away with having a big voltage difference between the filament transformer and ground.
At one point I was taking care of three different MW-5 transmitters, developed a test of if a person was a broadcast engineer or not, if they can tell you the power control system and how it worked on a MW-5 they were an engineer.
Surprised we have not had any TNT transmitter people but don’t imagine that circuit went very far in AM?

I guess the oscillator in my Globe V10 VFO!

Maybe the Gunn Oscillator? doing yeoman's duty opening doors for us...

73 - kevin

Far Out!
The Holman Oscillator rules!
Quick disconnect, cleaning plug, over ninety two thousand views and over forty two hundred followers this makes me think my life in electronics and radio has been a waste, time to drop all that and follow the Holman!
Going to run out and buy one and put it in the front yard and worship it!

We are the hollow men
We are the stuffed men
Leaning together
Watering our dry grass

(TS Eliot)

My favorite is the one that oscillates on the designed frequency every time, rather than sits there drawing current doing nothing, or oscillating at some other frequency.  Usually it is the third or fourth iteration of the design before a favorite emerges...

Ed

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I completely agree. It's  great one!

Mine's a little 282.8THz pulsed job.

That sprinkler would last about a week in service before the minerals in our water clogged it up solid. :P

I wonder of it could stand immersion in vinegar to 'fix' it?

73DG