Audio Frequency Bridges

Apparatus. All electrical resistances were measured with an electrolytic conductivity bridge (Leeds and Northrup model 4666) which was constructed according to specifications set forth by Jones (28) and described by Dike (29). The audio-frequency source was a General Radio Co. type 1311-A audio oscillator used with the frequency regulated at 1000 Hz and the output at about 5 V. The detector circuit consisted of a high-gain low-noise tuned amplifier and null detector (General Radio Co. type 1232-A) and an oscilloscope (Heathkit model O-ll) ... 

Another kind of audio-frequency bridge, which has come to the fore recently and which is very convenient to use, is the transformer-ratio-arm bridge, shown schematically in Fig. 5.10. This bridge uses inductive ratio arms to compare the unknown directly with standard components. The voltage... 

In the past, impedance measurements using reactively substituted Wheatstone bridges at audio frequencies have been the easiest to accomplish. Consequently, great emphasis has been placed historically on electrochemical processes having characteristic impedance spectra in the audio frequency range 20-20,000 Hz, namely, double-layer capacitive and moderately fast reaction kinetic effects at plane parallel electrodes. 

The mathematics and methodology of such measurements are well understood (Hague [1957], Armstrong et al. [1968]). However, considerable use still may be made of passive audio frequency bridge measurements in this age of active circuitry, principally in high-precision applications. Following a brief review of bridge cir-... 

Figure 3.1.1. Audio frequency bridge modified to include working electrode dc potential control.

Figure 3.1.1 shows schematically the famihar representation of an audio frequency bridge adapted for use with an imposed dc potential. The condition of balance for the bridge shown is... 

Low-Frequency Limitations. The null detection system traditionally used with an audio frequency bridge consists of an amplifier, filter, and ac voltmeter. This combination imposes three limitations at low frequencies ... 

Low-Frequency Limitations. The use of input and output transformers results in cell current and voltage, and thus detector signals that decrease with decreasing frequency. This effect becomes apparent only at low audio frequencies and imposes a practical lower limit of the order of 100-200 Hz with commercial bridges. 

Wien bridge oscillator (audio frequency oscillator)... 

The variable condenser connected in parallel with the variable resistance, / 3, serves to balance the capacity effects of the conductance cell. Adjustment of C and are made until the detector indicates zero voltage difference between points 1 and 3. In this condition of bridge balance the resistance Rc may be found from the expression 2 = R jRc- The position of balance may be indicated by a minimum signal on an oscilloscope or by minimum sound in earphones using audio frequency alternating voltage sources. 

Radio-bridge — A bridge circuit similar to the - audio-bridge except the signal source produces frequencies in the radio frequency range (e.g., 50 kHz-50 MHz) and a radio receiver (or even an oscilloscope) is used to determine the condition of null. 

The advantages of this method apply principally at low (audio and subaudio) frequencies. It is important to note that the device shown schematically in Figure 3.1.4 is a bridge only in the sense that external variables are adjusted to produce an output null. 

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