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Adi
Post subject: balanced current supply: noise considerations Posted: Mon Aug 21, 2006 11:54 am

Lieutenant

Joined: Mon Aug 21, 2006 11:52 am
Posts: 4
Hi!

I am working on a project in which I need to characterize the hysteresis properties of superconducting wires. The measurement is done by injecting a current, with risetime of the order of ns, into the wire and by monitoring the voltage across the wire. The latter shows a jump when a transition occurs.

Given the fact that the timescales involved are ns and that the distance to the device will be relatively large (~ 1-3 m), I would need to work with transmission lines. However, using a coaxial cable would be a possible problem, due to ground loops or ground potential fluctuations (the shield of the coax needs to be grounded at the source and also at positions closer to the sample). One solution would be to use a balanced configuration, with an arbitrary waveform generator that delivers from 50 ohm single ended outputs two voltages opposite in polarity along two 50 ohm coaxes. Close to the device, each line is terminated in a 50 ohm resistor. The voltage between the signal points of each resistor is used as a source for the device (current bias through a large resistor). This configuration should eliminate ground related problems both at low and high frequency.

I would like to mention that this method is described in an old book on noise isolation techniques: Henry Ott, "Noise reduction techniques in electronic systems". However, I am not sure whether it should work unmodified at high frequencies.

I would greatly appreciate any suggestions and comments on this problem.

Adi


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nubbage
Post subject: balanced current supply: noise considerationsPosted: Tue Aug 22, 2006 3:26 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Adi
I do not know for sure whether such an approach would reduce noise, but it sounds reasonable.
However, to derive a wideband balanced line from an unbalanced 50 ohm input, this is best achieved with a balun (balanced to unbalanced transformer).

There are many varieties of baluns. One simple type takes a copper 50 ohm coax air-line, and at the end remote from the 50 ohm source, cut 2 parallel slots a quarter wavelength long axially in the outer conductor, 180 degrees apart. For a 1 nS pulse, this would be about 15 cm long. Short the inner to the outer on one side of the two ears formed by the slots. Take the balanced output from the ends of the two ears.

If you use a search engine for "balun" you will find a large number of designs.


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Adi
Post subject: Posted: Tue Aug 22, 2006 7:31 am

Lieutenant

Joined: Mon Aug 21, 2006 11:52 am
Posts: 4
Dear Nubbage,

Thank you for your reply. I looked briefly into "baluns" and I intend to do it more later.
There was one detail which I did not understand: are these transitions frequency independent? Because my signal contains pulses with risetimes of a few ns, so it should have flat transmission from DC to ~1 GHz.

Adrian


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nubbage
Post subject: balanced current supply: noise considerationsPosted: Tue Aug 22, 2006 10:58 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
That point concerned me as soon as I had posted my suggestion.
The Fourier spectrum of the pulse with a rise time of 1 nS for sure extends to say 2GHz.
The majority of baluns are intended for RF applications over say a 20% bandwidth of the centre frequency of the carrier, so many will be unsuitable for your application.

The widest bandwidth from a few tens of kHz to 2GHz would be achievable with a toroid transformer with a single primary winding one side of which is to ground and the other to the source. The secondary comprises a winding that provides the balanced output. Several ferrites are suitable for UHF and low microwave frequencies.

Mini-circuits produce a range of these balun transformers at low cost, but they have power limitations. Alternatively you could wind your own. The design basis is to ensure there is enough inductance in the windings at the lowest significant frequency in the spectrum.

Another approach might be a Class A low gain, high gain/bandwidth product transistor amplifier, dc coupled, where the pulse source drives the base, and the balanced output is taken from the collector and emitter. If the balanced feeder is open circuit that might work, but if short circuited you would again face having no transmission at dc, otherwise you would be short circuiting the collector and emitter.

There are also video amplifiers now available that are configured for driving balanced cables, I think from national Semiconductor Inc. They accept an unbalanced coax input drive.


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Adi
Post subject: Posted: Tue Aug 22, 2006 11:05 am

Lieutenant

Joined: Mon Aug 21, 2006 11:52 am
Posts: 4
Thanks a lot, Nubbage!

I will look into the solutions you propose and let you know.

Adrian


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fred47
Post subject: BalunsPosted: Mon Aug 28, 2006 11:09 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!

In my experience, if you need very broad band performance, only transmission-line-based baluns will do the job. Transformer-based solutions can't reach the extreme bandwidths that transmission line ones do. Of course, the low frequency end of your requirement is set by the pulse repetition rate - which might be very low - you didn't mention what it is.

The classic book on the subject is Transmission-Line Transformers, by Jerry Sevick, now reprinted by Noble Press.

Good Luck!

Fred


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Adi
Post subject: Posted: Tue Sep 12, 2006 1:22 pm

Lieutenant

Joined: Mon Aug 21, 2006 11:52 am
Posts: 4
Hi,

I checked, following the suggestion of Nubbage, for solutions using active components. I did not manage to find them at National Semiconductors, but Linear Technology has some models that do the job. Thanks for the tip Nubbage.

Fred47, thank you for your reply. Using your method is indeed an option. The repetition rate will be larger than 1 MHz. one problem which remains though is that I do need to control also the offset level. This could probably be done by adding a DC signal, also from a differential source, ina way which does nor disturb the high frequency properties of the balun (with inductors and/or resistors), so you get some sort of balanced biasT. The inconvenience is then the more complicated RF design and the addition of a DC signal source.

Adi




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