UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Directional coupler

cmjones01 · 27th Nov 2017, 4:45 pm

Inspired by Wendy's recent thread, I have a query about the use of a directional coupler.

The attached fragment of circuit diagram is from the early version of the Tektronix 7L12 spectrum analyser plugin. On the middle left is a connection labelled 'Aux output', J35. This is an output of one of the local oscillators (input via the DC block at the left), used to drive a separate tracking generator.

There is no termination shown on J35, and sure enough, inside my 7L12, J35 is present but unterminated. However, later versions of the 7L12 bring J35 out to the front panel, where it's terminated, and shown as such on the circuit diagram.

My question is, what is the effect of this addition or removal of termination on the other outputs of the directional coupler? Presumably adding a terminator has to absorb energy from somewhere, so the level at the coupler's other outputs will reduce? There appear to be no other changes in the later circuit diagram to take account of this, so perhaps the change in level doesn't matter - it's feeding a mixer anyway?

Chris

27th Nov 2017, 4:45 pm	#1
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,676	Directional coupler - terminated and unterminated Inspired by Wendy's recent thread, I have a query about the use of a directional coupler. The attached fragment of circuit diagram is from the early version of the Tektronix 7L12 spectrum analyser plugin. On the middle left is a connection labelled 'Aux output', J35. This is an output of one of the local oscillators (input via the DC block at the left), used to drive a separate tracking generator. There is no termination shown on J35, and sure enough, inside my 7L12, J35 is present but unterminated. However, later versions of the 7L12 bring J35 out to the front panel, where it's terminated, and shown as such on the circuit diagram. My question is, what is the effect of this addition or removal of termination on the other outputs of the directional coupler? Presumably adding a terminator has to absorb energy from somewhere, so the level at the coupler's other outputs will reduce? There appear to be no other changes in the later circuit diagram to take account of this, so perhaps the change in level doesn't matter - it's feeding a mixer anyway? Chris Attached Thumbnails __________________ What's going on in the workshop? http://martin-jones.com/