Re: KITS: Noise Bridge announcement

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From: Dave Benson (nn1g@earthlink.net)
Date: Wed Dec 05 2001 - 13:22:33 CST


George-

>>-----Original Message-----
From: George, W5YR <w5yr@att.net>
To: Low Power Amateur Radio Discussion <qrp-l@Lehigh.EDU>
Date: Wednesday, December 05, 2001 7:58 AM
Subject: Re: KITS: Noise Bridge announcement

Thanks, Dave - sounds like an interesting introduction to noise bridges.

I built a similar affair many years ago, mounted in a large case with AB
pot and variable cap, etc. It was a little bit of a chore to calibrate it,
but it was useful for measuring complex Z. Of course, I had to write a
small computer program to translate the dial readings into impedance, etc.

>From your description, the bridge will read from the pot position the
resistive component of the impedance. The depth of the null will be
determined by the cap trimmer and will be at a maximum when the trimmer is
correctly adjusted. In your design, evidently there will be no effort to
evaluate the cap reading or position in terms of reactive component value
or sign. This feature is easy to add although it poses a significant
calibration problem.

<<

For the application I'd mentioned (a trap), the actual settings on the
resistive and capacitive trimmers don't matter- it's sufficient to adjust
them both for a deep null. The information you're looking for is the
frequency at which this null occurs.

You're absolutely right- calibrating a noise bridge to make it onto a piece
of impedance-measuring gear is a somewhat more arduous task. This is where
the phrase "left as an exercise for the reader" leaps to mind. In the
interest of avoiding parts acquisition problems, I've kept it as simple as
possible and hope to see other QRP-L contributors pick it up and run with
it.

>>As you have already figured out, generating the noise is no problem, but
designing that trifilar winding affair is, if reasonable accuracy over a
wide frequency range is required.
<<

No problem indeed, at least over the application range intended for this
bridge. At 1.8 Mhz, the unbalanced output reads 'S9+20dB' and at 30 Mhz,
'S8'. Plenty of signal to work with. As to balance, the trifilar itself is
pretty repeatable- it uses a trio of *solid* color-coded ribbon wire (thanks
to Dave Gauding, NF0R) for his generous contribution of this item) and
layout is well-controlled. To the extent that balance is an issue, we'd be
back to the words "arduous" and "calibration". A spreadsheet would indeed
be a useful calibration tool to make the jump to 'instrument-grade'.

73- Dave


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