The bridged-T fixed-pad attenuator contains four resistors and is frequency-invariant (Fig. 1a). Adding a floating bridging capacitor across the bridging resistor R1, along with a grounded inductor in series with the shunt resistor R4, will convert the bridged-T into a high-pass network (Fig. 1b). Such a circuit is commonly used to amplitude-equalize frequency-sensitive coaxial cables. A summary of the bridged-T circuit elements is shown in the table.
The bridged-T section is derived from a 3-dB fixed pad attenuator with source and load impedances of 75 Ω. The high-pass cutoff frequency of the bridged-T is 8.85 MHz.
The basic bridged-T equalizer section, which is fixed, exhibits high return losses. Unfortunately, fixed equalizers aren’t always adequate. To realize a variable equalizer while maintaining high return loss, the circuit elements of a balanced bridge can be switched. This was achieved by switching bridging and shunt resistors while keeping the capacitor and inductor fixed.1
In this article, a simple switching method is employed to keep the number of different components to a minimum. Up to five identical bridged-T equalizer sections can be inserted or deleted using five identical doublepole, double-throw toggle switches (Fig. 2). This switching technique has been used for many years with commercially available variable attenuators. At baseband frequencies, supplementary compensation of the switches usually isn’t necessary.
For a 10-MHz baseband frequency range, the total amplitude slope variation will be 0.33 dB per section and 1.65 dB with all five sections inserted. For a 6-MHz baseband, this reduces to 0.16 dB per section or 0.8 dB with all five sections inserted.Using identical sections with toggle switches is still more complex than a fixed equalizer. In some applications, the variable amplitude equalizer can be used for diagnostic purposes prior to replacement with a fixed equalizer.
- Gmitrovic, M.V., et al, “Fixed and Variable Slope CATV Amplitude Equalizers,” Applied Microwave & Wireless, Jan/Feb 1998; pp. 76, 78, 80-82.