Saturday, 19 September 2015

Bridging the Gap

The best things in life are either free or simple. Here's an example of the latter...

I'm a big fan of Return Loss Bridges, enjoying the fact that they give you useful information pretty much for free. They are simple instruments, which teach you while you're using them - rather like dip oscillators and slide rules. 

I've been using my old, home-brewed return loss bridge for a long time...

Whilst it hasn't had a write-up in these pages, it did make a brief appearance back here in the description of my experiments with mag loops.

I've been intending to return to return loss bridges (!) again and could have used the old warrior above. But the form-factor isn't quite right - I wanted something a little easier to incorporate into a different kind of experimental context. So...

First try was a little plug-in module, designed to fit into my beloved solderless breadboards...

As you see, input is applied at the left side (in the orientation of the photograph) and the load is connected at the right side. The output from the detector is output from the male header pins on the right side of the board. 

This 0v1 build worked fine - but did not achieve the sort of Directivity I was aiming for and certainly did not match the Directivity of my trusty old RLB. 

For readers who don't know about these sort of things, Directivity (in this context) is a single figure-of-merit for return loss bridges, describing the ratio of the detector output when the bridge is operated into an open circuit to the output when the bridge is terminated by a reference impedance equal to the characteristic impedance the system is designed for - in my case, 50 Ohms. To be practically useful, the bridge needs a Directivity of greater than 40dB.

Not satisfied with 0v1, I made 0v2, which features a physically smaller transformer and bridge resistors...

These improvements gave the required increase in directivity - to comfortably over 50dB at HF frequencies.

However, it doesn't take good eyesight to spot the bigger difference between the two versions - the later bridge also features a relay, by which the output can automatically be switched from the load to open-circuit conditions, through the application of an active-high logic input. 

For readers who don't know about these sort of things, the return loss bridge can be used to derive Voltage Standing Wave Ratios from return loss measurements in i) loaded and ii) open-circuit conditions. Having the ability automatically to switch between these two loading conditions will obviously allow a machine to make such measurements without the intervention of human hands.

The 0v2 hardware also incorporates an on-board reference impedance (a swell name for another parallel pair of 100 Ohm resistors), which can be used as a load just by putting a jumper between the two header pins on the board. I was tired of fishing around for a pair of external resistors.

Here's the new bridge in its native environment...

Notice that the Bridge PCB is dimensioned such that the rows of input and output pins make best possible use of the "blocks" of contacts on the breadboard. 

Off to the left of the new bridge you can see a little QRPppp linear, providing the power to drive the load. 

It all works well - watch this space (and some other UK radio events in the forthcoming weeks) for developments.

...-.- de m0xpd 

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