Chinese no-name 1-500MHz Return Loss Bridge

This article is an expose of the internals of a common Chinese no-name 1-500MHz Return Loss Bridge available on eBay for around $50 incl post.

Above is the exterior of the device. Specs are sparse: P<23dBm, Directivity>36dB.

Above is a view of the internals.

Above is a closeup of the connection of the output port via the common mode choke at lower left to the bridge, shield to the lower leg of the bridge and inner conductor (which is not so easy to see) to the upper leg of the bridge. The DUT is attached to the upper coax connector, and the reference load is on the PCB at lower middle.

The choke at upper left is to balance the effect of the lower choke, so both sides of the bridge have nearly identical common mode choke impedance to ground.

It is all pretty untidy soldering, but this is after all, Chinese quality; and it comes a lot cheaper than a brand name Return Loss Bridge.

Above is an example sweep with a Rigol DSA815-TG spectrum analyser with tracking generator.

The magenta line is the reference sweep with an O/C DUT (0dB RL), the yellow line is rx power with a 25+j0Ω load, and the green line is the calculated Return Loss. Observant readers will notice that the calculated Return Loss is clearly half of what it should be, the gap between the magenta and yellow lines is around 10dB rather than the 5dB calculated. In fact the expected Return loss can be calculated to be 9.54dB.

Well, the Rigol DSA815 is Chinese and this is one of the several defects. Chinese quality again… and they charge extra for this VSWR/ReturnLoss feature.