I was working in designing a proper low power signal conditioning circuit for a MEMS based gas sensor for quite a while. Now as these sensors are supposed to be deployed in remote isolated areas deep within coal mines it is always advantageous to read the value of these sensors remotely or in a wireless manner. Now RF power transmission within coal mines are highly regulated and operates at strict protocols. The conditions that I was to follow was that my unit should not in no way transmit more than 5W in RF power. So I designed my power amplifier stage around 4W max.
Next was choosing the transmission protocol. Now as my system was more or less digital I was initially inclined to go for digital communication scheme. So I choose FSK to transmit serial binary information. Turns out there is a single IC TH72031 from Melexis Microelectronic Integrated system capable of performing the entire FSK operation directly on Digital Data. But further analyzing the scheme although it was very simple presented certain problems. The major being FSK signal with 4W power was not being picked up by the receiver at the supposed distance. Another problem was as FSK IC was generating a variable frequency square wave applying it directly to the antenna produce spurious transmission at frequency bands it is not supposed to transmit. The solution was to use a very costly RF band pass filter or change the transmission scheme.
I choose a good old analog transmission scheme Frequency modulation. The idea was the microcontroller reads the gas sensor value performs internal calculation and calibration and then sends the final digital value via I2C BUS to a 16bit DAC. The then received analog value is properly scaled and transmitted.
Let me share the schematic for this.
The MAX2606 is a FM generator chip. It uses only one inductor L4 to set the nominal frequency around which the modulation will be done. We apply the analog signal from the DAC with proper scaling (can be tweaked during operation) to the TUNE input.
The MAX2606 output is about 50ohm and is applied to the PA stage and subsequently to the antenna. However the antenna choice and design is not yet complete.
I will be updating as the circuit development goes on...
