Digital display for VK-AMPS 2kW directional coupler – part 1

The VK-AMPS 2kW directional coupler is available at modest cost.

The coupler was purchased as an assembled module as pictured above.

It is a Sontheimer coupler and uses AD8307 logarithmic detectors in the FWD and REF channels.

The manufacturer publishes performance information not often published for such things.

From the above data, we can plan a digital display based on conversion of each of the RWD and REF detector output voltages.

As a first step, we can calculate the intercept and slope used to convert those voltages initially to dBm. Taking slope to average 0.0266V/dB and given 5W producing 1.925V out, we can calculate coefficients and write \(P_{dBm}=-35.4+\frac{V}{0.0266}\), and this would result in an output voltage from 0 to 2.7V.

Now the intended ADC will have a full scale deflection (FSD) around 1V, two options are being considered:

  • SAMD21 M0 with Vref=1.0V; and
  • AtMega328P with Vref=1.1V.

Low cost hardware is readily available in the form of Nano Zero (or similar such as above), and Arduino Nano boards.

So, using a 10bit ADC (quite sufficient resolution for this application), we get 1024 steps and it would suit us if the AD3807 had a slope of 10mv/dB so a 100dB range used most of the ADC’s FSD.

The diagram above shows the output stage of the AD8307 is a 2µA/dB current mirror and 12500Ω internal load. If we shunt that load with an external load resistor of appropriate value, we can get sensitivity to 10mV/dB.

Above is an extract from a spreadsheet which calculates the preferred external load resistor is 8200Ω for a slope of 9.9mV/dB.

The spreadsheet goes on to calculate the slope in terms of ADC count C, and it is 0.9861dB/C.

So the initial calculation in firmware is \(P_{dBm}=-35.4+0.09861 C\). These calibration coefficients for each of the FWD and REF detectors will be stored in EEPROM, so firmware does not contain the calibration coefficients in code.

The next step is to build a prototype and measure it to make final calibration adjustments.

Above is a plot of log conformance of the AD8307 from the datasheet. It would seem that the two pink spots are good candidates for a two point calibration, lets choose -48 and -6dBm equivalent at the AD8307 input which equates to 0.6 and 42.6dBm into the coupler.

So let’s say we measure a prototype detector (both FWD and REF must be done), and get actual vs input, we can calculate the corrected calibration coefficients.

Above is an example calculation, the new coefficients would then be stored in EEPROM.

A work in progress..