Everything you wanted to know about wideband, low-frequency transformers
Wideband, low-frequency transformers are useful components in various passive circuits, such as the return-loss bridge (Reference 1, Figure 2). Figure 1a shows a conventional transformer. If you connect the transformer windings differently, you can configure a transmission-line transformer (Reference 2, Figure 1b). These transformers use a magnetic core of modest size, and unit cost is reasonable. The basic transformer uses a Fair Rite (www.fair-rite.com) toroid (type 597700601), which has a nominal outer diameter of 0.825 in., a nominal inner diameter of 0.525 in., and a nominal thickness of 0.25 in. The toroid uses number 77 material and has an inductance factor (AL) of 1175. To obtain useful performance at audio frequencies, the transformer uses a 129-turn bifilar winding of number 26 magnet wire. To avoid the use of expensive commercial bifilar wire, you can twist together monofilar red and green windings using a hand drill before winding it on the toroid.
Table 1 gives the measured performance with 50Ω source and load impedances of the conventional transformer in Figure 1a. Figure 1b shows the schematic diagram of a one-to-one transmission-line transformer. Table 2 gives the measured performance with 50Ω source and load impedances. This transformer provides bandwidth enhancement with useful behavior down to dc. You can use the conventional transformer in Figure 1a in a passive return-loss bridge (Figure 2) or for stand-alone dc isolation. Table 3 gives the measured performance of the 50Ω return-loss bridge. At 1 MHz, the return-loss bridge exhibits a forward insertion loss of 12 dB and an open-circuit-to-short-circuit ratio of 0.5 dB. You can use the conventional transformer to isolate a grounded signal from a balanced test piece. We built the circuits for the wideband transformers and return-loss bridge using single-clad vector board and enclosed them in die-cast aluminum boxes with BNC connectors.
Wetherhold, E, "Design and Construction of a 9-kHz Highpass Filter and Assembly of a Return Loss Bridge for Filter and PLISN," Interference Technology Engineers' Manual, pg 220, 1993.
Sevick, J, "Transmission Line Transformers," American Radio Relay League, 1990.