Field strength monitor receiver for remote control application
A typical industrial remote control system is a narrow band FM FDMA system. A reverse channel from the controlled machine back to the operator is usually not provided. The transmitter therefore has no feedback whether the frequency operates or has already been occupied by another user and interference is caused. This problem can only be solved by adding a field strength monitor to the transmitter. The cost for this should be significantly lower than for a full duplex system. This article describes the concept of such a field strength monitor which can be easily added to an existing transmitter at lower cost than a conventional superhet receiver. No changes in the transmitter itself are necessary. The concept is based on a zero IF receiver using the transmitter as local oscillator. Modulating the transmitter (local oscillator) with a special waveform overcomes the specific disadvantages of zero IF when no IQ demodulator is available. The paper will discuss the concept and its advantages . It will also describe a demonstration system which has been built.