Lazure LCD SWR & Power Meters

**7 Oct 17 - Update:  While attempting to define the exact formula for the power meter's response, I discovered that it is extremely frequency sensitive.  No matter what I tried, readings at 10M were literally 4 to 6 times those of the same amplitude at 80M.  The problem appears to be high-level RF bypassing the input divider and reaching the Log Amp directly.  This will need more experimentation to determine the exact cause, so if you plan to try this project, proceed with caution.

**I have determined that the Log amp, a AD8310, is picking up RF directly from the input connectors.  This pickup is frequency sensitive and responds better to higher frequencies.  Sometime soon, I plan to redesign the PCB to improve the isolation between the log amp and all other circuitry (especially the input).  More to follow...

-Bill Lazure

I wanted a "Smallish" bench SWR & Power meter with a display unit and a separate pickup head. The separate pickup was to possibly allow me to mount the pickup well away from the display so I could "contain" the RF.  Ultimately in terms of operating features, I simply wanted a reading of SWR and measured power with possibly a bar graph showing either/both.

The Updated objective:

Two separate Meters with the combined functions split. Backlit displays to improve readability in dim or dark lighting. Low current drain.

The original design for this project combined the functions of SWR Meter and Power Meter into one device. While it worked reasonably well, I was never sure what affect, if any, the transformer of the SWR meter had on the power accuracy. Also, the LCD used in the original design wasn't backlit. It was impossible to read in my preferred location under an equipment shelf. Thus was born the idea to separate the two meters.

Alignment

Alignment for the Power Meter is accomplished by setting the values of the upper and lower power ADC reference. Adjust R1 for .625V at pin 4 of the PIC. Similarly, adjust R2 for 2.625V at pin 5 of the PIC. Theory of these adjustments is explained below.

Theory of Operation - Hardware Operation

SWR Meter

Transmitter power is introduced into Jack U1. The load (antenna, tuner, or dummy load) is connected to jack U2. The two transformers, L1 and L2 develop voltages proportional to Forward and Reverse Power. These voltages are rectified by D3 and D4 and filtered by C1 and C2. The resultant DC voltages are fed into the buffer amplifiers (U3) that utilize matching diodes in their feedback loops to compensate for the non-linearity of the rectifier diodes at low signal levels. The Outputs of these amplifiers feed two of the Analog to Digital Converter (ADC) inputs of the PIC.

Power Meter

Transmitter power is fed to U4, a Logarithmic amplifier, via R6 and C5. R6 and R7 comprise a voltage divider that reduces the Transmitter power by 60dBv (voltage rather than power, because the AD8310 reads voltage). U4 provides a resultant DC that corresponds to the voltage on its input. This DC is filtered by C6 and fed to a buffer amplifier.

The Outputs of the amplifier feed an Analog to Digital Converter (ADC) input of the PIC. The DC generated by U3 which feeds U4 is the high- and low-references for the ADC when reading power. These references set the overall accuracy of the power function.

Theory of Operation - Software Operation

-The PIC is the heart of both devices. It performs several functions: Controls the display, takes the ADC readings, performs SWR calculations, converts logarithmic DC to power readings in Watts, and converts both device's readings into a graphical representation for the bargraph.

SWR Meter

When initialized, the PIC initializes the LCD and programs special characters into it for the Bargraph. It then takes an SWR FWD reading and an SWR REV reading. It multiplies the FWD reading by 10 to simplify generating the decimal on the output display, then uses those two readings to compute SWR using the formula:

SWR = Vf+Vr/Vf-Vr, Vf = SWR FWD voltage and Vr = SWR REV voltage.

Once the SWR value is obtained it is written to the LCD using a template that is filled by the calculated digits. Finally, the computed SWR value is sent to the bargraph calculator which divides the count by 16 and completely fills each digit of the LCD by that number. The remainder is used to fill the last block of the bargraph with a number of vertical lines which correspond to the remainder.

Power Meter

8 consecutive power readings are taken from U5 and averaged. This is the method used to reduce "jitter" in the power reading.

Once an averaged reading is obtained, it is inserted into one of 4 different tables based on its magnitude. These tables provide range information (mW, Watts, XX Watts, etc), and each of the three digits used to make the power reading.

The accuracy of the power-reading function is assured by feeding the high and low references of the power ADC from a precision reference. The low ADC reading is set to correspond to 1 ADC count (2V/1024 counts = .00195V) below the voltage that represents .1mW (-10dBm), while the high voltage is set to the highest projected reading obtained from the log amp (2.625V = 1000W).

These three digits, formatted in their appropriate range template, are then sent to the LCD.

Finally, the actual Power ADC count is sent to the bargraph calculator which divides the count by 16 and completely fills each digit of the LCD by that number. The remainder is used to fill the last block of the bargraph with a number of vertical lines which correspond to the remainder.