Combining Two Waves Experiment

[Tractor Beam]

This is my first post here so I hope everything is ok. - I read the rules.

Do rf sign waves have a small attraction for each other when they are near the same frequency? I thought that electronics men might know why this happens. My experiment shows that they have some attraction for another when two frequencies are very close.

I’m doing a physics experiment with heterodyning of two slightly different frequency sign waves. I set one frequency generator at 1,000,000 Hz. and the other at 999,100 Hz. The two outputs are connected together. The scope shows there is one convoluted heterodyned wave. Then when the lower frequency is slowly moved up near the higher the scope reveals the waves snap together like magnets (so to speak) and have combine into one sign wave as seen on the oscilloscope. Checking the frequency of both I find they are still not quite equal. The attraction seems to hold a nicely formed single sign wave until the frequencies are separated by 7729 Hz approx where they suddenly snap back apart. A very apparent snapping together or locking of the two waves into a single nice sign wave occurs well before the two generators become equal frequencies.

Two separate toggle switches are required at each generators output so to connect/disconnect the generators separately during the experiment. In addition an audio amp w/speaker picks up the heterodyned low frequency (this is an option). The audio amp simply provides an audio version of the output and I can hear the wow as the frequencies get closer. The toggle switches allow the same frequency counter to be used for both generators.
I've tried re-arranging the componets and removing counters and the audio amp but the results are the same. Both generators are set at 5v output.

EXPERIMENT: ATTRACTION BETWEEN TWO RF WAVES OUTPUT FROM TWO FREQUENCY GENERATORS

RESULTS: Locks into a single clean sign wave well before the generators are equal frequencies.

Base Generator Frequency reading 1,000,109 Hz. Only Generator #2 is slowly adjusted until the lock and unlock occurs. At the moment of lock and unlock generator #1 is toggled off to check #2 frequency.

GEN #1
BASE FREQ Constant
1,000,109 Hz

Experiment Step 1
GEN #2
Low to Higher Freq movement
Locks into Single Wave At:
992,967 Hz Diff: 7142

Step 2 After they have locked together.
GEN #2
High to Low Freq movement
Waves Break Apart Separating At:
992,380 Hz Diff: 7729


Why does this happen I don't recall reading anything on this in my studies?

 

[BradtheRad]

This sounds something like the AFC (automatic frequency control) that used to be on FM radios. If you tuned slightly off your frequency, the AFC would center on it. Don't know quite how it worked.

It's possible for an oscillator to be influenced by another in contact with it. At 1 MHz you're in the radio spectrum, so it's hard to be sure to what degree it's due to direct electrical influence, versus radio wave influence.

 

[Tractor Beam]

Yes I've heard this before being infulenced.
Both generators are old tectronics tube type but are very well built and stable.

 

[DartPlayer170]

I suspect it has to do with the Q of the tank circuits in the oscuillators.
The tank circuit is resonant at some width of frequencies which depends on the Q of the tank.
So, when the two frequencies are close enough, the tanks begin to amplify the energy from the other oscillator circuit.
When the frequencies are far apart, the tanks attenuate the energy from the other oscillator because it is outside the bandwidth of the tank.
You should be able to test this out by adding resistance into the tank circuits and see if the oscillators lock at larger frequency seperations.

In fact, this may be a way of measuring the Q of a tank circuit!

 

[zorro]

This effect of oscilator locking is related to coupling between circuits and with nonlinear effects.
If the sources are not coupled (i.e. none of the two oscillators receives some signal from the other that influences it) the effect does not appear. If they are well isolated and buffered, you could see a low frequency beat.
Harmonic lockimg is possible too: two oscillators have a common armonic at which they lock.
Regards

Z

 

[RF_Jim]

"EXPERIMENT: ATTRACTION BETWEEN TWO RF WAVES OUTPUT FROM TWO FREQUENCY GENERATORS"

Experiment we conducted many years ago with two HP/Agilent 8640B signal generators with outputs into a common hybrid combiner and tuned to, at, and near each other's frequencies as part of the test to witness the vector addition of their output energy into an amplifier under test (we were doing two-tone tests) ... and NEVER did we see any 'locking' phenomenon at all ... and how could we have? The __oscillator stages__ in each 8640B are buried well within the 8640B generator themselves and well shielded, the 'frequency' lock must occur at the oscillator stage, not at any point 'down the line' past the oscillator stage where the sine wave is 'output'.

Suggest that maybe your two generators are not as well-shielded as required in the area of the oscillator stage?

That is one possibility that explains the 'locking' that one might see.

Jim

 

[zorro]

Yes. Lack of shielding gives a possible path for one oscillator injecting signal in the other (or reciprocally each one into the other).
Another possibility is conducted path: if s12 of te amplifier-buffer from the output of the oscillator is not small enough, it carries signal from the output to the oscillator.
Regards

Z

 

[betwixt]

I think the effect you are seeing is "injection locking". It happens when one oscillator circuit sees a tiny amount of signal from the other, the start of the oscillation cycles is pre-empted by the other signal and they lock together. It will only happen if there is a path for the signals to get back to the oscillator stage so I suspect what is being seen is to do with lack of buffering or screening or, just possible, a ground loop carrying the signals.

Brian.

 

[rahul.6sept]

hi,

i would like to know the steps that might give proper sheilding of generators as in this case to avoid such locking.
Plz.might help me aswell.

Regards,