In recent years, much has been said in the industry about the importance of the amplitude (size) of the guided wave radar (GWR) transmit pulse. While the size of the transmitted radar pulse is certainly important, it is a fact that pulse amplitude alone will not always yield accurate level measurement under all process conditions. A far more important parameter in reliable level measurement in difficult applications is the signal-to-noise ratio (SNR), which essentially describes the difference between the desired signal and the unwanted noise.
If the amplitude of the noise approaches that of the level signal, loss of accuracy or linearity is the first observed effect due to distortion of the level signal as it passes through and interacts with the noise. Worse yet, if SNR is bad enough, the adverse signal interaction can actually result in a loss of the level signal.
While it would be desirable to eliminate all the unwanted impedance discontinuities, it is simply not possible. The good news is that today’s most advanced GWR solutions address this critical design issue.
Transmit Pulse Amplitude and Signal-to-Noise Ratio
Many GWR manufacturers talk about transmit pulse amplitude; however, beyond the point at which the transmitter signal amplitude is sufficient to detect a given dielectric, amplitude plays virtually no role in the detected SNR. In fact, if too many sources of unwanted reflections are present, a larger transmit pulse amplitude will simply elevate the noise at the same rate it elevates the level signal. The resulting change to the SNR is zero – the larger pulse provides no benefit in and of itself to SNR.
The only role of a larger transmit pulse is to assure that noise in the system does not become dominant in the overall SNR in low signal return cases (such as long probes under low dielectric conditions). Too small of a transmit signal would result in too small of a received signal in these cases, requiring excessive signal amplification in the level transmitter.
Transmit pulse amplitudes in advanced GWR transmitters are typically several hundred millivolts. This is more than adequate to assure that the overall system performance will be determined by the careful design of the RF signal path – and not by the internal noise of the transmitter. As a result, SNR is the key component of performance that goes beyond simply raising the transmit signal amplitude. For this reason, some advanced GWR transmitters feature an innovative front-end circuit and other components to optimize the SNR.
Diode Switched Front End Design
The new Eclipse® Model 706 uses a new design concept called the diode switched front end, which enhances front-end performance. The design of this advanced GWR transmitter features front-end circuitry innovations that enhance the transmitted pulse amplitude, improve the received signal strength and, most importantly, increase the SNR.
- The diode switched circuit operates by only connecting the transmit pulse generator to the probe circuit during the brief (one nanosecond) time that the transmit pulse is active. The ultra-fast microwave diode then turns off, and is effectively out of the circuit. When the diode switch turns off, it effectively decouples the transmit pulse generator from the probe. The result is complete isolation of the receiver circuit from the transmit pulse generator. This maximizes receiver sensitivity by directing all received energy toward the receiver itself.
- The diode switched technique also deals with unwanted reflections by designing the electronics with a high quality, broadband 50 ohm impedance match at the signal origin (electronics). This ensures that any unwanted signals will be absorbed by the electronics, and not re-reflect back down the probe.