Noise Bridge...Measuring Receiver Input Impedance.

[pwdrive]

Has anyone had any experience of using a Noise Bridge to measure receiver input impedance?

Lawrence.

[Skywave]

Well, no, I haven't, but I have heard of the technique - but only "heard of": I'm ignorant of the details of the 'why' and the 'how'. Can you add any substance to this matter, Lawrence? I've always promised myself that one day I would build myself a wide-band noise source for measuring the SNR of comms. radios, so this topic of yours - which seems related - is of particular interest to me.


Al.

[Yorkie]

Many years ago I built a noise bridge from a kit - quite a simple thing. It just generates white noise, and you place it between the receiver and antenna, rotate the control until there's a null in the noise, which is quite a sharp null, then read off the impedance on the scale of the bridge. Obviously if the input of the receiver is 50 Ohms, you want the antenna to match that, and by inserting a noise bridge between an ATU and the receiver you can tune it for best match. Nothing like as relevant for receiving as for transmitting of course, but you can't transmit through it, so you do the matching on receive.

When I was active in amateur radio I used to experiment a lot with antennas - I built a G4MH style mini-beam and used to make my own traps for trapped dipoles using double sided PCB as the capacitor in the traps, and winding coils on fibreglass spacers, tuning them with a home-brew grid dip oscillator that I made that went from 1.6MHz to 150MHz with a range of six plug-in coils. I calibrated the dial on my homebrew nixie tube digital frequency counter.

Later I bought an MJF 249 Antenna analyser which you could connect to the antenna feeder and sweep across the bands to see where you got a 1:1 match. Handy for drawing an SWR graph to tune the antenna for best match at the desired portion of the band - CW end or SSB. MFJ was not universally highly regarded, some (libellously) saying that the initials stood for 'Made From Junk'. I always felt that they made neat innovative products, nicely designed and built, and at competitive prices. When I bought the MFJ 249, it was the only analyser on the market and includes a digital frequency counter. I put it to a lot of use in the days when I was active in amateur radio.

The MFJ249 antenna analyser manual is here - I dare say that other models have since overtaken it:

http://download.qrz.ru/pub/hamradio/sche...Manual.pdf

I've attached a few pics which might be of interest. Pics of the noise bridge built into a little ali box 3" x 2" x 1" showing the inside and outside. A pic of an antenna trap ready to be housed in a 2" diam plastic tube with end caps, and the GDO that I made, with the set of coils alongside, and a 7.1MHz test coil in front, to check that the GDO is working and on frequency. (It was from a Practical Wireless design, back in the days when PW was mainly a technical and constructional mag).

All my yesterdays!

Cambridge kits do a kit for a noise bridge, but bizarrely, they include a piece of plain PCB and a pattern for you to etch your own PCB rather than supplying a PCB with the kit.

http://cambridge.eu.pn/2012/kits/kithtmjs/kitanb3.htm

Hope that's on interest.

[Skywave]

Thank you David for your lucid description of what a noise bridge is, what it does and how it can be used: I stand enlightened!

As for the grid dip osc., would that be the one designed by Mr. Carpenter, G3TYJ ? Because if it is - and it sounds and looks like the one - right now, in my 'idle moments', I am in the process of building another one! Another one, you might ask? Yes, another one, because I built one back in the late 1960's, but despite a recent very comprehensive search, I couldn't find it. I have a sneaky suspicion that I 'parted company' with it about 12 years ago. That was a time when my interest in all things 'radio' was at an all-time null, mainly because my working life and out-of-work interests rotated around computers . . . and other things.

I've now come back to The Fold and am enjoying every minute.

Al.

[pwdrive]

Hi Al, been roofing hence David beat me to it, he has described it well, as far as I am aware and can remember you can null R by using the pot and null the reactance (+j -j) using the variable capacitor, both of these controls are incorporated in the noise bridge, once the values are read off then a maths calc. will give the impedance (we now know the maths so should be easy) Loads of designs on the web, zenner plus a couple of transistors and a few passives. If a tuned calibrated null detector was built into it say using a meter then a receiver would not be required as the null indicator, would make a nice instrument at reasonable cost.

Lawrence.

[Skywave]

Prior to reading this thread - and much to my surprise - I'd never heard of a noise bridge. I've heard of, used and actually own, a component measuring bridge, directional couplers and similar things. I've also used a noise generator. Anyway, when I meet something like this, something I know nothing about and then receive some information about it, my appetite becomes wetted for more: specifically how does it work? And why?
With this in mind, I set pencil to paper and attempted some circuit analysis. In doing this, it became apparent that it neatly dove-tails into the j Operator Thread that is running and the analysis is very akin to that applicable to a component measuring bridge: more on that item later.

The approach I have taken is to start at the basic a.c. bridge configuration and develop it into a noise bridge, step-by-step.

The basic bridge cct. is shown in figure 1.
[attachment=5200]

‘DET’ is the detector: could be any a.c.-sensing device that is suitable for the frequency and amplitude of Vs.
The conditions for balance are shown: R2 / R1 = Rx / R3. That equation is easy to prove . . . .

If the current through the ACB branch = I1; current through ACD = I2,
and voltage across R1 = Vac; across R2 = Vcb; across R3 = Vad and across Rx = Vdb,
then: Vac = I1.R1; Vcb = I1.R2; Vad = I2.R3 and Vdb = I2.Rx
If the bridge is in balance, then zero voltage appears across the detector.
Therefore, Vac = Vad and Vcb = Vdb.
So: I1.R1 = I2.R3 and I1.R2 = I2.Rx,
So: I1.R1 / I1.R2 = I2.R3 / I2.Rx,
So: R1 / R2 = R3 / Rx,
i.e.: R2 / R1 = Rx / R3.

Now we require Vs to be such that it applies a potential across points A and B.
This can be achieved by the use of a transformer with two secondary windings, suitably phased, as shown in figure 2.
[attachment=5201]

It is usual for the four windings to be quad-filar wound.
In this figure, R3 has been replaced by the variables Vc and Vr; Rx has been replaced by the unknown impedance Zx.
R1 and R2 become the secondary windings, which will have equal reactances and resistances, and thus impedances, Z1 and Z2 respectively.
Consequently the equation for balance now becomes:
Z2 / Z1 = Zx / Z3.
But Z1 = Z2, thus Zx = Z3

Figure 3 is the same arrangement as figure 2, but simply drawn in a different manner.
[attachment=5197]

However, there is a measurement limitation on this configuration for determining Zx.
If Zx is a capacitive reactance, then Zx = R – jX.
Now Z3 = R3 – jX3, so for balance:
Zx = R – jX = R3 – jX3 = Z3
Thus: R = R3 and X = X3 separately.
Hence, Zx can be determined: Zx = R3 – jX3

Note that if Zx was an inductive reactance, we would require:
R + jX = R3 – jX3, which is clearly an impossibility, since that requires X = -X3:
i.e. a capacitive reactance equal to an inductive reactance.
Hence, that circuit configuration cannot be used to determine the nature of an inductive reactance at Zx.

I found this article on Wikipedia:
http://en.wikipedia.org/wiki/Antenna_analyzer

In the Theory of Operation section, the first sentence makes no sense, neither grammatically nor technically. Nevertheless, the rest of the article, despite its brevity, is of some value.

I also found an article by ZS1JHG:
http://john-shortwavelistenersite-zs1jhg...ridge.html

In the article, his first submitted cct. is shown in figure 4.
[attachment=5198]

The reasoning for balance condition is similar to the above, but note the inclusion of C2.
The inclusion of C2 enables inductive reactance at Zx to be determined as well as capacitive reactance.
At balance:
Z1(R ± jX – jXc2) = (R1 – jXc1).Z2,
so: R ± jX – jXc2 = (R1 – jXc1).(Z2 / Z1)
Since: Z1 = Z2,
Then: R ± jX – jXc2 = R1 – jXc1.
This is only true if:
R = R1 and ± jX – jXc2 = – jXc1,
So: ± jX = j(Xc2 – jXc1),
i.e.: ±X = Xc2 – Xc1.

From that, we can see the following . . .
If: Xc2 > Xc1, X is > 0, i.e. X is inductive: Z = R + jX
If: Xc2 < Xc1, X is < 0, i.e. X is capacitive: Z = R - jX
If Xc2 = Xc1, X = 0, i.e. Z is resistive only: Z = R ± j.0

Hence, Z = = R + jX, is thus fully determined at balance:
R = R1; X = Xc2 ~ Xc1.

However, C1 and (ideally) C2 are variable-value components,
and since X is inversely proportional to C {Xc = 1/wC, numerically},
if: C2 < C1, Z is inductive;
if C2 > C1, Z is capacitive;
if C2 = C1, Z is resistive and resistive only.

The article by ZS1JHG continues with another configuration as shown in figure 5.
[attachment=5199]

Here, the ‘variables’ are a parallel arrangement, as are Z and C2.
It can be shown that that arrangement gives a result for Z that is qualitatively the same as for the series-connected variables: the bridge will handle capacitive and inductive impedances at Z. However, the resultant equations for R and X (where Z = R ± jX) are much more complicated, so that if the values of R and X need to be known, (the mathematical calculations are not trivial), as opposed to simply balancing the bridge, the series configuration is to be preferred.
(Such an arrangement is met in the Marconi TF 868B component bridge, which I will be discussing in another thread.)

And that little lot will do for now! I hope you found it useful and not too difficult to follow: I'll do my best to answer any questions arising.

Finally, there will be almost certainly some errors in the above: language / grammar and technical / mathematical. If you do spot any, please bring them to my attention.
Thank you.

Al. / June 13, 2012 //

[pwdrive]

A very comprehensive explaination Al, it must have took ages to type that lot in, well done.

Lawrence.

[Skywave]

Thank you Lawrence.

As I said, when I meet something that is totally new to me - such as this is - my first reaction is to discover more about it. To that end, what I have learnt here and on the 'Net went a considerable way to meet that desire. But me being me, I wanted to dig in deeper. That drive probably comes from my main interest in electronics which is analogue R.F. Now I keep various note-books for writing some things down for future reference, but on this occasion I chose to store that analysis on my computer: as a consequence, it then became easy to copy-and-paste it into a post here. The only slight grumble I have is that my corresponding MS Word document contains a lot of subscripts: things like R1, with the "1" as a small number: those subscripts were lost when the text appeared here. Another reason why I felt that it was a useful post was simply because it well-illustrated the value of the use of the j Operator. Now we all know that one can successfully repair all manner of things electronic whilst remaining quite ignorant of that mathematical device and that is just fine, but there are occasions when it is useful - such as designing a specialist piece of test kit, or modifying one already in existence.

One day, I may just design and build a noise bridge for myself: then those notes will come in handy and be readily available.

Al.

[pwdrive]

That would make a good project Al as it can be a versatile peice of kit.

Lawrence.