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Fabian
Post subject: Highly symmetric oscillator Posted: Wed Oct 05, 2005 8:43 am

Lieutenant

Joined: Wed Oct 05, 2005 8:23 am
Posts: 1
Hi!
I'm new here and have probably a pretty easy question for you. I'm a physicist which means that I hardly know anything about RF technology.

My problem is that I observe a phenomenon in my experiment (a DC Voltage) that depends on the polarity of my RF-Signal. (no kidding) I'm using a commercial RF-Generator with a 50Ohm connector. I tried decoupling with capacitors, used an RF-transfomer, but the problem stays. Funny enough, if I take another RF generator (both Rohde & Schwarz) the DC Voltage changes polarity.

It's not just the grounding!!! Is has to do something with reflections on the outer conductor or whatever. I mentioned the transformer I used: if I ground one of the connections on the secundary (sample) side it doesn't influence my DC-Voltage. But if I change the polarity on either the primary or the secondary side my DC-Voltage flips.
I learned something about balun-transformers but was wandering how complicate it is to build an absolutely symmetric RF oscillator that would provide me an RF Signal. I would like to transfer the RF over the inner-conducors of two 50Ohm coax-cable to my sample - that would be a symmetric 100Ohm transmission line (?)
All the diagrams I've seen for oscillators sofar did not seem to be completely symmetric. Do you have any suggestions? The frequency should be somewhere between 1-100MHz and the signal about 0.2-1V_eff.
Cheers, Fabian


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Guest
Post subject: Balanced signalPosted: Wed Oct 05, 2005 3:42 pm
The term you're looking for is "balanced".

You can get a balanced signal either by using a balun (which is just a shortened form of balanced-to-unbalanced) transformer, or by using a balanced signal source.

Most people use baluns, and they're both widely available and easy to make (see the book by Jerry Sevick, "Transmission line transformers" for details on how to make your own.

Balanced oscillators are also a possible choice. Googling on "balanced oscillator circuit" should get you a good reference or two. (I'm at work right now and don't have access to my books).

Unfortunately, you're not guaranteed a 100 Ohm differential characteristic impedance by using two 50 Ohm lines. Agilent has application notes on their Network Analyzers which discuss this aspect of things.

Good Luck!


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Guest
Post subject: Posted: Wed Oct 12, 2005 4:21 am
Hi Fabian

Balanced or unbalanced, I am not sure we yet understand what is going on here. A signal from a Rohde & Schwarz (lucky you) generator coming down a coaxial cable does not normally have a DC component to start with, and if it did, a series capacitor would certainly remove it.

The signal can aquire a DC component in lots of ways depending on what it is connected to. Deal first with the "reflections on the outer conductor". These depend on the the value of the termination impedance, ie, what your kit looks like to the arriving cable, the frequency in use, and the length of the cable. If the kit looks like 50 Ohms, and the cable is 50 ohms type, the reflections don't happen. If the mismatch is there, then they do happen, and the reflected signal going back meets the signal arriving, resulting in a standing wave set of RF voltages along the cable as the signals either add together, or cancel each other depending on where you are along the cable. The key thing to know here is that a coaxial cable in this condition also has a standing wave on the OUTSIDE of the braid, and it will be radiating all over the place like an antenna, using you, the cables and kit around, and anything it is connected to as the other half of its dipole.

This condition can mess up all sorts of nearby instrumentation. I have seen a passive multimeter read right across its scale from dc generated internally on rectification diodes. Hums and buzzes everywhere. traces climb on the scope. If there is enough power involved, the RF burn one gets just from reaching to touch a knob can be painful. In this mode, your own body is the counterpoise for the radiated field. Hams call it "shackitus" The cures are..

1. Match up. Provided your kit looks like a much higher impedance than 50 Ohms, (say > 1000 ohms), a resistor across the arriving cable will do.
If not, them make up a matching network to suit. You will get lots of advice here on how, once we know what the downstream kit is.

2. Suppress any current on the cable outer, using small ferrite loading tubes threaded on. A few turns, (say 10 or so) wound into the cable approximately saucer-size, secured with tape can halt a braid current. Passing the cable several times through a ferrite toroid will also do it.
You can even get clip-on ferrites, but be aware that the types that suppress EMI by lossy absorption do not work in the same way as ferrites that genuinely to provide inductance at 100MHz. This trick is called a "choke balun". Look on the ARRL site for cure techniques to TV interference. It comes with picture HowTos.

3. Whatever you do, if the arriving signal is not handled in a way that preserves its waveform symmetry, you are "making dc". This includes overload clipping on downstream amplifiers, samplers, whatever. Perish the thought, but is the waveform symmetrical as it leaves the R & S gen while loaded? Check using a T-piece connector and a scope probe.

4. The phenomenon you describe .. that is.. you get a different dc component when you "change polarity", is normally that the shape of the wave is not the same on the positive side as the negative going side. But here rises the question of exactly how you "change polarity". If you started with a coax, you cannot just connect it the other way about! Here is where the balun transformer comes in. "Unbalanced" transmission line is not the same thing as "unsymmetrical waveform".

5. Finally, be very sure your attempts to measure do not introduce the DC artifact. I have done this myself. I just have to assume you know about differential scope measurements, channel gain balancing, etc.

Good luck with this.


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Graham
Post subject: Posted: Wed Oct 12, 2005 4:27 am

Colonel


Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
Dunno how it happened, but that previous "Guest" was me
My regards
Graham



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