Chinese no-name 1-1000MHz Return Loss Bridge

This article is an expose of the internals of a common Chinese no-name 1-1000MHz Return Loss Bridge available on eBay for around $65 incl post.

Above is a view of the internals.

Above is a closeup of the connection of the output port via the common mode choke at upper left to the bridge, shield to the upper leg of the bridge and inner conductor (which is not so easy to see) to the lower leg of the bridge. The coax might look like solid copper, but it is miniature semi-rigid coax. The DUT is attached to the lower coax connector, and the reference load is on the PCB at upper middle.

The choke at lower left is to balance the effect of the upper 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 with a 50Ω termination.

The DSA815-TG had a defect that halves the display Return Loss, so the actual Return Loss is double that displayed, 36-44dB over 1-1000MHz in this case.