Return Loss Bridge 131

The Type 131 return loss bridge is specifically designed for television equipment testing, offering precise, repeatable return loss measurements with straightforward calibration and compensation for cable effects, making it a robust tool for broadcast and RF engineers.

Developed by Michael Cox Electronics, it is a precision Wheatstone-type measuring bridge designed for evaluating the return loss of television equipment over a frequency band, in 75-ohm systems. Its primary function is to specify sending and terminating impedances in terms of return loss, a method increasingly preferred for television signal integrity testing.

Circuit Design and Operation

• The bridge is based on a modified Wheatstone configuration, tailored for single-ended source and detector use.
• The source input is compatible with standard television test signal generators (e.g., pulse and bar, multi-burst, frequency sweep).
• The two ratio arms of the bridge consist of high-precision 75-ohm resistors. A third 75-ohm resistor completes a delta section, forming a 6 dB loss splitter pad. This configuration ensures both the reference and unknown impedances are presented with a 75-ohm source, compensating for cable losses when short cable lengths are used on both arms.
• The detector connects across the reference and unknown connectors. Since most oscilloscopes and waveform monitors are unbalanced and lack sufficient gain, a high-gain wideband differential amplifier (integrated circuit) is included to amplify the error signal.

Balancing and Calibration Features

• The bridge includes an adjustment for stray capacitance, allowing balance across the operating frequency band (up to 10 MHz).
• For measurements where return loss is less than 28 dB (large error signals), a switchable -10 dB pad attenuates the source, preventing overload of the differential amplifier.
• A calibration feature allows a 2% reduction in impedance to be switched into one side of the bridge, simulating a 40 dB return loss for oscilloscope calibration.

Measurement Method

• To measure return loss, the amplitude of the error signal at the frequency of interest is noted on the oscilloscope.
• The unknown impedance is then disconnected, and an attenuator is inserted in the source feed until the oscilloscope shows the same deflection as before.
• The attenuator setting (in dB) directly indicates the return loss of the unknown impedance.

Power Specifications &  Price

Available in both battery (£45 ) and mains-powered (£50) versions. 

Here you find 3 models, 2 with images and 0 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
All listed radios etc. from Cox Electronics Ltd., Michael; Middlesex