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SWR_Bridge_AD8307.php    6707 Bytes    28-02-2024 15:32:56

A HF/RF/VHF Bridge ... and the AD8307

SWR Measurement • Return Loss Bridge, 1 kHz ... 1 GHz

✈ Notes

It deems that all SWR Bridges out there use inductive components. As we aimed for a broadband thing, we used ohmic resistors to form a wheatstone bridge. We replaced the Current Meter of the Bridge with a Logarithmic Detector - the venerable AD8307. And we use it to measure the Voltage. And yes, this is a scalar measurement. So is the SWR or the VSWR.

It was designed for our Cavity-Team, where a 'Shortwave-Antenna' asked for some affection :-)

✈ Circuit Description

A Wheatstone Bridge ... and the AD8307

For maximum Flexibility, the case has 3 x SMA Connectors to allow for any HF Source, Load or Reference to be connected. R1 and R2 are 49.9 Ω, 1206.

The AD8307 measures the Bridge Voltage. It has a high Input Impedance of approx. 1.1 kΩ. The usually seen Shunt Resistor of approx. 52.3 Ω has been omitted. The lower Cutoff Frequency is determined by the Coupling Capacitors. We use 2 x 470 nF.

In case the DUT has the same Impedance as the REF, the difference has a minimum AC voltage measured by the AD8307. In case the DUT is open or shorted, the Voltage across the Logarithmic Detector is half the Voltage of the HF-Source.

As this is a Scalar Thing (like the SWR or VSWR) the lower the Voltage at the Output, the closer to the REF (= 50 Ω) we are. We used the ANNE-50 from Mini Circuits as the Reference. It can be obtained from Mini Circuits or Mouser. It claims to have a Return Loss better than 46.64 dB @ 596 MHz. And yes, below it is even better :-)

And - as we measure differences, the AD8307 can be used beyond 1 GHz.

✈ Performance

A Wheatstone Bridge ... and the AD8307

The Resistors are 1%, 1206, from the Constructiva Set (Distrelec).

As these curves are not centered around 50 Ω, we measured the used ANNE-50 from MCL. It had 48.6 Ω. Hmmmm - looks like we need something more precise ...

✈ Downloads

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t1 = 6577 d

t2 = 181 ms

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