VU3SQM directional wattmeter build – #4

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

At long last, some PTFE rod arrived to permit assembly of the transformers.

For reasons discussed in an earlier article, the transformers use a larger core than the original VU3SQM. They need to stand above the board, and whilst that compromises the mechanical strength of the assembly, it should have better performance. Continue reading VU3SQM directional wattmeter build – #4

VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

The parts have arrived and construction commenced.

Above, the PCB populated with the SM parts and soldered. It was soldered in an IR reflow oven. Continue reading VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #2

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

This article canvasses the issues of the display.

Intention is a digital based display (though not to exclude an analogue meter or bar graph type displays).

So, the output of the AD8307 needs to be digitised.

Let’s first consider the nature of the AD8307.

It is a log detector, so it provides a ‘DC’ voltage proportional to the log of the input signal, but the ‘DC’ voltage can vary very quickly.

The chart above from the AD8307 datasheet shows that the unfiltered response to a burst of RF has a rise time of well under 1µs. Continue reading VU3SQM directional wattmeter build – #2

VU3SQM directional wattmeter build – #1

VU3SQM offers an interesting directional coupler based on a Sontheimer coupler, and using AD8307 power sensing for a nominally HF coupler. I must say that I am not a fan of Sontheimer couplers… but that is what the board uses.

This article lays out a preliminary design review to assist in selection of appropriate toroids, and ordering of the needed parts.

PCB

Above, the top side of a PCB. Continue reading VU3SQM directional wattmeter build – #1

RFPM2 – calibration files

The RF Power Meter 2 (RFPM2) stores calibration constants in a file located in the (SPIFFS) file system in the microcontroller flash.

The file opened by default when RFPM2 starts is /default.cfg, the following is an example.

{
"name":"dBm",
"hostname":"rfpm201",
"vref":3.3,
"avg":3,
"slope":0.12991,
"intercept":-91.406,
"unit":"dBm",
"lcdfsd":16
}

The parameters above capture the most basic operation of RFPM2 as a power meter directly displaying dBm with bar graph in fixed 2dB increments to 16dBm FSD. These values serve as a basis for some other applications as they capture the basic intercept and slope of the AD8307 module in this instance.

Current probe calibration

Alternative config files can be loaded on the fly from the webserver interface, for example http://192.168.0.86/config?filename=/dBA.cfg will load the dBA config file for a certain current probe. Continue reading RFPM2 – calibration files

RFPM2 – calibration

The RF Power Meter 2 (RFPM2) stores calibration constants in a file located in the (SPIFFS) file system in the microcontroller flash.

The WiFi credentials are stored separately at the default location in the flash.

Calibration constants

The AD8307 outputs a voltage from zero to about 3V for inputs from around -90 to +15dBm. The nominal output has a slope of 25mV/dB and intercept of -84dBm.

A starting point for RFPM2 calibration constants is intercept=-84 and slope=0.129.

Modules such as that used here may have pots to adjust the gain and offset of the output. Adjust the gain so that the maximum output voltage is a little lower than 3.3V (the maximum ADC input), say 2.7V.

Clip 194

The response of the AD8397 has some ripple in the transition between log amp stages. The log cell ranges are 14.3dB, so min error repeats every 14.3dB. My own practice is to calibrate at -62 and -5dBm input as they fall approximately on the mid line of the ripple trend (4 cycles of the error curve). Continue reading RFPM2 – calibration

RFPM2 – current probe

This article describes a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2).

For use with RFPM1 and RFPM2, both of which read to 16dBm max, it is convenient that the scaling factor for the probe is 0dBA/dBm, ie that those meters read dBA directly, implying a current range of -75-16dBA or 0.186mA-6.3A.

The probe comprises a ferrite cored transformer that is clamped or placed over the conductor(s) of interest, and uses a 10t secondary which has a low value resistive load, across which the power meter connects.

Above is a screen shot of a spreadsheet calculation of relevant design values. Continue reading RFPM2 – current probe

RF Power Meter 2 (RFPM2)

The RF Power Meter 2 is a development based on the utility of  RFPM1, but it shares nothing with the RFPM1, save using an AD8307 as the sense module.

Design criteria

The design criteria are:

  • small, portable, battery powered;
  • direct reading dB scale;
  • flexibility for a range of adapters to measure power, current etc;
  • local display including bar graph, time, and dB value;
  • log measurements to a serial port of some kind;
  • offer remote access for recent measurement log.

Design outline

The RFPM2 uses an AD8307 log power detector to obtain a analogue ‘DC’ voltage proportional to the log of the input power. The input port is 50Ω SMA, and accommodates from about -75dBm (the noise floor) to +15dBm.

The analogue output of the AD8307 is digitised on a microcontroller board, a NodeMCU which uses an ESP8266 processor with integral WiFi. The board also contains a CP210x USB to serial adapter for programming, power, and serial logging.

The display is deliberately generic, the units are dBm at the SMA input, but they could be dBA with a suitable current probe, or +xx dBV/m with a field strength sense antenna. Continue reading RF Power Meter 2 (RFPM2)

Should you trust your VSWR meter – linearisation

Should you trust your VSWR meter? asked an interesting question, and Should you trust your VSWR meter – detector linearity discussed a problem apparent in may VSWR meters.

This article illustrates one method of linearisation of the detector response of a practical VSWR meter.

Radio-kits SWR meter

This article contains an analysis of the analogue circuitry of the Radio-kits SWR meter.

The directional coupler at top left contains half wave peak detectors for forward and reflected waves. They are wired to the two compensated op amps at lower right (the connections are not shown on the circuit as the coupler may be remote, follow the terminal designations). Continue reading Should you trust your VSWR meter – linearisation

Should you trust your VSWR meter – detector linearity

Should you trust your VSWR meter? asked an interesting question, and based on experience, including a relevant example, concluded:

The answer is no, like any measurement instrument, prove that it is trustworthy in the intended application.

It went on to ask:

If the VSWR meter is designed to fail, why does it fail?

This article contains an analysis of the analogue circuitry of the IC-7300 directional coupler to explain the likely cause of its poor behaviour.

IC-7300 directional coupler schematic


Above is an extract of the IC-7300 circuit in the area of the directional power coupler used for VSWR measurement. The circuit is a quite conventional Bruene coupler, and its response is similar to several types of directional couplers that produce a DC output voltage from a half wave detector. Continue reading Should you trust your VSWR meter – detector linearity