Electrical Impedance

Going digital 153

E

1

D

E

2

Z

1

Z

2

(a) 2T DDS ratio bridge

Z

1

Z

2

D

D

E

1

E

2

E

3

(b) 4T DDS ratio bridge

Figure 5.5

Voltage ratio bridges implemented with DDS generators. (a) 2T voltage ratio

bridge implemented with a two-phase DDS generator. (b) 4T voltage ratio

bridge implemented with a three-phase generator.

Adapted from Helbach and

Schollmeyer (1987).

An example of a digitally assisted ratio bridge is shown in Fig. 5.7, for

the comparison of like impedances Z

1

and Z

2

. The bridge is energized with

voltage E

a

by phase 1; the inductive voltage divider has a fixed and known

ratio k, which can be altered by the small voltage E

b

derived from generator

phase 2 (for example, with ...