It is an excellent design that by its nature can be readily reproduced. The article has a very thorough and complete adjustment and calibration procedure. I have not seen this article reproduced anywhere else, which is a real pity. No exotic or hard to get components are required.
My implementation of the Impedance bridge complete with test impedances
Double sided copper clad board would just about have to be a most usefull thing since ... well it is the sliced bread of ham radio!
The little veroboard contains a zener diode noise source and a three transistor broad band amplifier. I have brought out the noise power through a seperate jack, it makes an excellent tool for testing radios. Cost of this little project , nil. The calibration is described in the original article. You must make the "reference" impedance, it is just a few meters of coax but must be done right in order to get meaningfull reactive impedance measurements. On the main pcb you can see the one turn coil to balance the stray inductance of the variable resistor and a capacitive trimmer to balance the stray capacitance in the measurement arm.
Using a noise bridge for doing usefull stuff
quickly evaluate surplus and unidentified toroid cores for possible RF use using a noise bridge.
The one thing that I have found this noise bridge most usefull for is to quickly evaluate unknown ferrite toroids and are available for nothing inside junked equipment or as umarked grab bags in surplus stores. Most ferrite toriods thus obtained are designed for common mode RF suppression, a transformer or inductor made from these will still transform and "induct" but only below 1Mhz, they are absolutely useless for anything above 1Mhz .
A toroid for ham use should still have reasoneable Q at 10Mhz. Wind about 10 turns of any wire through the toroid and measure its equivalent resistance. Quickly try to get a null on the reactive arm and then null the bridge on the resistive reading. The resistive reading is the same as would be experienced by either your resonant circuit or transformer. It is interesting to sweep higher in frequency to watch the equivalent resistance go higher and higher as the core loss increases. No air core inductor has a Q as bad as a ferrite toroid. The only reason that they feature so much in the ham radio literature is because they are compact, I would even consider ferrite toroid inductors to not be all that reproducible either. (Tightly specified cores like the Amidon range are excellent but expensive or unobtainable here in Oz)
I have seen the beans being spilled in an Indian amatuer radio journal. They wound toroidal coils on a washer made from plastic sheet. Such a toroid has all the advantages of self sheilding, and absolutely no core loss, inherently higher Q for tuned circuits and they are completely reproducible , unlike the inductors wound on proprietery ferrite toroids. Furthermore the inductance may be readily calculated from formulea having only geometrical input parameters. However , such an air core wound torioidal inductor would not perform as well in broadband "balun" transformer service.
Another profoundly effective test to see if your junk toroid can be used as a broadband transmission line transformer, wind a quick bifilar winding of about 7 turns and terminate the balanced port with a 200 ohm resistor. You should measure 50 ohms real impedance with the reactive component near zero. If the toroid is not suitable the impedance transformation does not occur and you get strange non integer ratios or nonsence readings or signficant reactive impedance.
The real suprise is to examine a couple of ferrite beads. Some are amazing, just threaded over a piece of wire ( one turn coil) comes up at 30Mhz as 40 ohms resistive! and you wonder what force of nature can turn a one inch piece of wire into a high resistance!
With this instrument you can quickly go through your junk box and characterise your junk toroids.
My junk toroids seem to fall into these classes.
| class 1 | Useless above 1Mhz | only good for rf supression, useless for anything else | green,jet black |
| class 2 | Useless above 4 to 5 Mhz | good for rf suppression, some use as transmission line TX , switchmode PSU inductors | yellow, white,black |
| class 3 | "High" Q at 10Mhz | good for transmission line TX, some use possible as tuned circuits (Q and L must be tested at the desired operation frequency) | red, yellow |
The Q of the tuned circuit can be directly measured on the radio by tuning for the 3dB down points.
If the half power points are close to each other then that inductor would be usefull as a tuned element. If the peak is broad then the Q is low, the core could still be usefull as a transmission line transformer.
I have found that, at least in my junk box, the class3 toroids are rare. You can find these on old computer motherboard where they are used as step down buck regulators, they make good rf inductors up to 20Mhz. Sometimes they are used by mistake for RF supression , if which case they are not very effective! Discarded computer PSUs are another possible source.
Sometimes the surplus grab bag toroids are colored. Bare black cores are nearly allways rf supression and useless above 1Mhz. Red cores seem to be ok for RF, Green cores are for RF supression and useless. Yellow cores are ambiguous, they seem to be either good for RF or just for suppression service. Allways check a mystery core before use with the noise bridge to avoid frustration. Its a pity that core manufactures never agreed to an industry standard core color code.
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