Baseband Calculations for ISM-RF Receivers
Many industrial/scientific/medical (ISM) band radio frequency (RF) receivers use an external Sallen-Key data filter and a data slicer to generate the baseband digital output. This tutorial describes the ISM-RF Baseband Calculator, which can be used to calculate the filter capacitor values and the data slicer RC components, while providing a visual example of the baseband signals.
ISM-RF Baseband Calculator
Baseband System Sheet
More detailed image. (PDF, 850kB)
Figure 1. The Baseband System worksheet.
Slicer Threshold w Peak Detect Sheet
- Filtered Baseband: This trace is the basic analog signal coming out of the limiting amplifiers.
- RC Threshold: This trace is a reduced amplitude, phase-delayed version of the filtered baseband signal.
- Max Peak w/ Decay: This trace uses a selectable tau value to represent a periodically peaking signal.
- Min Peak w/ Growth: Similar to the max peak, this trace uses the same tau to represent a periodically plunging signal.
- Chosen Threshold: This trace uses the RC threshold as its source, or an average of the Max Peak and the RC Threshold, or an average of the max peak and min peak signals.
- Data Output (RC Threshold): This dotted line trace shows the data output if only the RC slicer level is used to generate the threshold level.
- Chosen Data Output: This trace compares the filtered baseband to the chosen threshold to generate the output data signal.
More detailed image. (PDF, 1.1MB)
Figure 2. The Slicer Threshold w Peak Detect sheet.
More detailed image. (PDF, 1.9MB)
Figure 3. The Slicer Threshold w Peak Detect sheet.
Appendix: Example Calculation
- Open the Baseband Calculations workbook.
- Select the Baseband System tab along the bottom of the Excel® workbook.
- Select cell D3 (Data Rate) and enter the value of 3.333. Note the filter knee now shows about 5kHz.
- Enter the values of 1.414 into cell D5 (a), and 1 into cell D6 (b). These values represent a Butterworth configuration of the Sallen-Key filter. Note that the data filter cap value (CDF) now indicates 450pF and the op amp cap (COP+) indicates 225pF. For an actual example, the standard values of 470pF and 220pF are represented in the MAX1471 data sheet.
- Enter the value of 4 in cell D17 (Bit Intervals).
- Enter the value of 0.047 in cell D20 (Data Slicer Cap). Note that the data slicer resistor is showing about 25k?.
- Now, select the Slicer Threshold w Peak Detect sheet. The data rate in cell D2 will show 3.333kHz and the Transient Response of Slicing Threshold graph will show traces that would represent various baseband pins on the ISM receivers. For a MAX1471, the Filtered Baseband line would represent the output from pin 20, DSF+ and the RC Threshold trace would represent the signal on pin 19, DSF-.
- Enter the recommended value of 3.6 into cell D26 and observe the change in time-base of the plot. Then change this value to 2 showing about 6 cycles on the plot.
- Choose to display the output without the peak detectors by entering a 0 in cell D17. Next enter a P and observe the improved performance of the output data trace.
- Finally, turn on the noise by entering a 1 in cell D20. Increase the noise on the analog signal by entering the value 0.2 into cell D21. Observe the impact on Data Output based on the RC threshold.