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R2 68 L1 balance J2 27-4C Circuit with balance control added C3 140 pF
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27-5 Commercially available amateur noise bridges
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522 Measurements and adjustment techniques Those applications are not limited to the testing of antennas (which is the main job of the noise bridge) In fact, although the two-way technician (including CB) or amateur radio operator, will measure antennas, tuned circuits, and resonant cavities with the device, consumer electronics technicians will find other applications Figure 27-6 shows the circuit of a noise bridge instrument The bridge consists of four arms The inductive arms (L1b and L1c) form a trifilar wound transformer over a ferrite core with L1a, so a signal applied to L1a is injected into the bridge circuit The measurement consists of a series circuit of a 200- potentiometer and a 120-pF variable capacitor The potentiometer sets the range (0 to 200 ) of the resistive component of measured impedance, while the capacitor sets the reactance component Capacitor C2 in the unknown arm of the bridge is used to balance the measurement capacitor With C2 in the circuit, the bridge is balanced when C is approximately in the center of its range This arrangement accommodates both inductive and capacitive reactances, which appear on either side of the zero point (ie, the midrange capacitance of C) When the bridge is in balance, the settings of R and C reveal the impedance across the unknown terminal A reverse-biased zener diode (zeners normally operate in the reverse-bias mode) produces a large amount of noise because of the avalanche process inherent in zener operation Although this noise is a problem in many other applications, in a noise bridge it is highly desirable: the richer the noise spectrum the better the performance The spec-
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T1: toroid trifilar transformer 10T, no 26 over FT-50-43 core
Antenna J1
27-6 Noise bridge circuit
The RF noise bridge 523 trum is enhanced because of the 1-kHz square-wave modulator that chops the noise signal An amplifier boosts the noise signal to the level needed in the bridge circuit The detector used in the noise bridge is a tunable receiver covering the frequencies of interest The preferable receiver uses an AM demodulator, but both CW (morse code) and SSB receivers will do in a pinch The quality of the receiver depends entirely on the precision with which you need to know the operating frequency of the device under test
Adjusting antennas
Perhaps the most common use for the antenna noise bridge is finding the impedance and resonant points of an HF antenna Connect the receiver terminal of the bridge to the antenna input of the HF receiver through a short length of coaxial cable as shown in Fig 27-7 The length should be as short as possible, and the characteristic impedance should match that of the antenna feedline Next, connect the coaxial feedline from the antenna to the antenna terminals on the bridge You are now ready to test the antenna Set the noise bridge resistance control to the antenna feedline impedance (usually 50 to 75 for most common antennas) Set the reactance control to midrange (zero) Next, tune the receiver to the expected resonant frequency (Fexp) of the antenna Turn the noise bridge on, look for a noise signal of about S9 (will vary on different receivers), and if in the unlikely event that the antenna is resonant on the expected frequency you will find yourself right in the middle of the null Adjust the resistance control R on the bridge for a null, ie, minimum noise as indicated by the S meter Next, adjust the resistance control C for a null Repeat the adjustments of the R and C controls for the deepest possible null, as indicated by the lowest noise output on the S meter (there is some interaction between the two controls)
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