Longitudinal balance measuring bridge circuit

1. A longitudinal balance measuring bridge circuit, which comprises:

a common mode signal injection circuit, the common mode signal injection circuit being coupleable to an item under test (IUT) and to a source of common mode signal and providing the common mode signal to the IUT, the common mode signal injection circuit having balanced outputs;

a common mode filter, the common mode filter being coupled to the balanced outputs of the common mode signal injection circuit and attenuating the common mode signal, the common mode filter having balanced outputs on which are provided a transverse signal resulting from the common mode signal provided to the IUT; and

a signal output balun, the signal output balun being coupled to the balanced outputs of the common mode filter and converting the transverse signal to an unbalanced signal.

2. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the common mode signal injection circuit includes a balanced matching transformer.

3. A longitudinal balance measuring bridge circuit as defined by claim 2 , wherein the transformer of the common mode signal injection circuit is a 4:1 balanced matching transformer.

4. A longitudinal balance measuring bridge circuit as defined by claim 3 , wherein the 4:1 balanced matching transformer is a 200 ohm to 50 ohm transformer.

5. A longitudinal balance measuring bridge circuit as defined by claim 2 , wherein the balanced matching transformer is a Guanella circuit.

6. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the common mode signal injection circuit is a wide band, impedance matching transformer having a center tap input on which is provided the common mode signal, the impedance matching transformer providing the common mode signal to the IUT.

7. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the common mode signal injection circuit includes a balanced to balanced transformer circuit.

8. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the common mode filter includes at least one filter stage, the at least one filter stage having an in-line transformer and a shunt transformer coupled to the in-line transformer and to ground.

9. A longitudinal balance measuring bridge circuit as defined by claim 8 , wherein the in-line transformer includes a wound twisted pair of transmission line of a selected impedance.

10. A longitudinal balance measuring bridge circuit as defined by claim 9 , wherein the wound twisted pair of transmission line of the in-line transformer includes a pair of wires which are wound in the same direction with respect to one another.

11. A longitudinal balance measuring bridge circuit as defined by claim 9 , wherein the shunt transformer includes a wound twisted pair of transmission line of a selected impedance.

12. A longitudinal balance measuring bridge circuit as defined by claim 11 , wherein the wound twisted pair of transmission line of the shunt transformer includes a pair of wires which are wound in opposite directions with respect to one another.

13. A longitudinal balance measuring bridge circuit as defined by claim 8 , wherein the common mode filter provides at least 30 dB (decibels) of common mode attenuation.

14. A longitudinal balance measuring bridge circuit as defined by claim 8 , wherein the at least one filter stage provides about 30 dB (decibels) of common mode attenuation.

15. A longitudinal balance measuring bridge circuit as defined by claim 8 , wherein the common mode filter includes a first filter stage and at least a second filter stage, the first filter state and the at least second filter stage being cascaded in series.

16. A longitudinal balance measuring bridge circuit as defined by claim 15 , wherein each of the first filter stage and the at least second filter stage has an in-line transformer and a shunt transformer coupled to the in-line transformer and to ground.

17. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the signal output balun includes an in-line balanced to unbalanced transformer circuit.

18. A longitudinal balance measuring bridge circuit as defined by claim 1 , wherein the signal output balun includes a transformer, the transformer including a wound twisted pair of transmission line of a selected impedance.

19. A longitudinal balance measuring bridge circuit as defined by claim 18 , wherein the wound twisted pair of transmission line of the transformer of the signal output balun includes a pair of wires which are wound in the same direction with respect to one another.

20. A longitudinal balance measuring bridge circuit as defined by claim 1 , which further comprises a matching resistive circuit coupled in parallel with the balanced outputs of the common mode signal injection circuit, the matching resistive circuit having an impedance selected in order to match the impedance of the bridge circuit to the input resistance of the IUT.

21. A method of measuring the longitudinal conversion loss of an item under test (IUT), which comprises:

injecting a common mode signal into the IUT and generating therefrom a balanced output signal having a common mode signal component and a transverse signal component;

attenuating the common mode signal component;

converting the transverse signal component to an unbalanced signal; and

comparing the unbalanced signal having the transverse signal component with the injected common mode signal to derive therefrom the longitudinal conversion loss of the IUT.