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Last Updated: Wednesday, 05 May 2021 23:02
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I've build a digital Watt-SWR meter as published by PD0LEW on a Google group name "Radiostuff". [1]
It is a fork of the original design made by TF3LJ [3] and is under constant development.
There are several versions of this Wattmeter, using different displays and even a double coupler design.
There are 2 modules in 2 different cases, the coupler board and the CPU board. The connection between both boards is made by 2 coaxial cables, one for the forward (FWD) and the other for the reverse (REV) signal.
I've build the simple OLED display version and single coupler.
Here some informations that can be helpful.
Features :
- Band range 1-50 MHz
- Power handling up to 2kW
- Resolution 0.025 dB
- Simultaneous forward & reverse power reading
- Instantaneous, peak and PEP display
- Power in Watts or dBm
- SWR and SWR power alarm
- Audible and visual alarm
- Single or 2 point calibration procedure
- Screensaver
Coupler
The coupler is a Tandem match, also named Stockton design.
It is the easiest to reproduce design and it gives very good results over the HF bands in terms of directivity.
The transformers are build with a turn ratio of 24. This gives a compromise between coupling factor, power handling and directivity. The coupling factor is -27.6 dB and if well build, the directory is around 30 dB.
In order to reduce the power applied to the logarithmic amplifier, an attenuator is included in the coupler case. The resistance values and power ratings are chosen to handle the maximum power of 2kW and for an attenuation of 9.63 dB.
Around the attenuator, there is also a frequency compensation design that is used to flatten the levels over the used bands.
Here my building:
Logarithmic amplifier
The AD8307 has a linear dynamic range in the HF range of about 70 dB from -60 dBm to +10 dBm. It can be enhanced by using the SLOPE and INTERCEPT adjusments via the INT input (pin 5).
For our usage 70 dB are far enough as we will see below.
An additional 15.76 dB attenuator is added on the CPU board at AD8307 front end.
Around the attenuator, there is also a frequency compensation design that is used to flatten the levels over the used bands.
The coupling factor with the described coupler transformers with 24 turns is -27.6 dB.
The attenuator in the coupler has an attenuation of 9.63 dB. We need to reduce the power to be compatible with the AD8307.
The total attenuation is therefore 27.6 + 9.63 + 15.76 = 52.99 dB, say 53 dB.
From that we can calculate the power range that we can measure and still be within the linear range of the AD8307.
Maximum usable power within the limits of the AD8307 inputs.
(Max AD8307 input = + 10 dBm) + 53 dB = 63 dBm = 2 kW
Minimum readable power that can be measured in :
(Min AD8307 input = -60 dBm) + 53 dB = -7 dBm = 0.2 mW
But the usable minimum reading is not less than 10 mW.
That's more than fair !
AD converter
The Teensy 4.0 used does have 2 internal AD (Analog to Digital) converter with 10 bits resolution, but an external 12 bits converter is used to gain precision : the AD7991.
The AD7991 is a 12 bits ADC that can be used with an external voltage reference, in our case it is a 2.600 V high precision regulator.
With 12 bit we can have a resolution of (2 power 12 ) 4096 steps.
So we have a step resolution of 2.600 V / 4096 = 0.635 mV.
When correctly used, the AD7991 as a slope of 25mV/dB at his output. So, the maximum resolution of the ADC is 0.635/25 = 0.0254 dB per step.
That's also more than needed !
Display
There are 2 software versions, one for an OLED 4 lines display and another for TFT 5" or 7" touch screens.
The OLED version needs an encoder to navigate thru the menus, while the TFT uses the touch screen.
Both displays have their advantages. The OLED is smaller, consumes a lot less. The TFT has a nicer display, is more readable and uses a touch screen.
Software
The softwares are provided in a HEX format that doesn't need to be compiled, but the sources are also available.
Compiling can be a bit tricky due to the libraries that have to be installed. The Teensy platform has to be added to the Arduino IDE.
The non standard used libraries are :
Wire.h for the Teensy
NewhavenOLED.h
Building
The complete Wattmeter is build on 3 PCBs. One for the coupler, another for the CPU board and finally one optional for the display connection.
The Gerber files are available on PD0LEWs repository or sets of PCBs can be bought from him or other hams.
The coupler PCB has been designed to be mounted in a HAMMOND 1590BS.
The IN and OUT connectors can be SO239 or N sockets. The FWD and REF signal connectors can be of SMA or BNC type.
Appendix
[1] Radiostuff Google group
[2] PD0LEWs Web page
[3] TF3LJs Web page
The orinality of this description is that the tone generation is made by the use of a DDS and not a sine wave generator made with transistors or a special circuit. It also does not use any special and expensive component but only junk box components, or at least cheap ones !
