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Highly symmetric oscillator - RF Cafe Forums
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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
Top
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.
Top
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
Posted 11/12/2012
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