The RFB-H
RF bridges are very useful for to measure the impedance of an object. They can of course be centered around almost any impedance, but it is safe to assume that the most common measurements are carried out on 50 or 75 W devices. Still, there are tricks for to extend the range! If you want to measure a 300 W, put a 200 W resistor in parallel and read 120 W!
A source is connected to the 'GEN' port of the bridge and the unknown object is connected to the 'Z' port. Due to the large dynamic range of LogProbe the source level can be anything from -60 to +10 dBm. For deep zeroes it is important that the source is relatively free from harmonics, since the detector is just as happy to detect the harmonics as the fundamental tone! (We have a tunable band pass filter for this purpose).
Turn the R and the C for a minimum signal on LogProbe. The value of the unknown impedance can be read on the dials. Negative C indicates an inductance of the same admittance. R and C in the bridge are in parallel. Assume we read 'a little bit negative C', say -10 pF at 150 MHz:
XC = 1/(2*Õ *f*C) => 1/(2*Õ *150*106*10*10-12) = -j106.1 Ohm
The inductor with the same reactance can be calculated:
XL = 2*Õ *f*L so L = XL/(2*Õ *f) => 106.1/(2*Õ *150*106) = 112.58 nH.
So, the unknown impedance is the reading on the R dial in parallel with 113 nH.
Since '-10 pF' will represent a different inductance for various frequencies it is not possible to calibrate the dial direct in nH, but for at one frequency. Besides, it would go 'backwards' with an infinitely large value near the 0-mark.
The bridges have very little frequency dependence, but it would be a hard call to make a bridge that covers 0.1<MHz<500, or even more, in one configuration. We have therefore decided to make one for as low frequency as possible, working well up to 30 MHz and another one for as high frequency as possible (at least to 500 MHz) working well down to 30 MHz. We call them the RFB-H and RFB-V respective.
We have also chosen to make the R dial 200 W long. The rationale is that 50 W systems are the most likely to be measured. A reading of 50/4=12 can still be resolved with reasonable accuracy, as can 50*4=200, so there is a 4:1 range 'on both sides' of 50 W.
The passive bridges work with the LogProbe when powered from any normal voltage.
The active bridges, with a built-in oscillator, require 12 V supply.
For mail or orders, please contact us at
