AN2815| Application Note
Maxim/Dallas > App Notes > AUTOMOTIVE
WIRELESS, RF, AND CABLE
Keywords: ASK, amplitude shift keying, OOK, on-off keying, receiver sensitivity, RSSI, IF bandwidth, SNR, signal to noise ratio
Nov 05, 2003
APPLICATION NOTE 2815
Calculating the Sensitivity of an ASK Receiver
The sensitivity of RFIC Amplitude Shift Keying (ASK) or On-off Keying (OOK) receivers is an important specification for designers of Remote Keyless Entry (RKE) systems, Tire Pressure Monitoring (TPM) systems, Home Automation systems, and other applications. These receivers typically operate at 315MHz or 433MHz, but the results apply to almost any carrier frequency. It is important for the users and designers of RFICs to know the theoretical limit of these receivers so that they can determine if their design improvements are fully successful. This application note describes a step-by-step method to predict the sensitivity of an ASK receiver, given a system noise figure, IF bandwidth, and Baseband bandwidth. The results show that the logarithmic amplitude detection in the Received Signal Strength Indicator (RSSI) amplifier decreases the output Signal to Noise Ratio (SNR) for low input SNRs (threshold effect) and that the sensitivity increases as the square root of the IF to Baseband bandwidth ratio. Most moder
n Amplitude Shift Key (ASK) receivers detect data by passing the modulated RF signal to an amplitude detector either directly or after one or more frequency conversions. The amplitude detector is almost always an RF or IF amplifier with an RSSI (Received Signal Strength Indicator) detector whose output is proportional to the logarithm of the input power of the RF or IF signal. Because the RSSI detector is a nonlinear detector, it changes the Signal to Noise Ratio (SNR) of the signal that goes into it. The key to the ASK sensitivity calculation is the SNROUT vs SNRIN curve of the RSSI detector. Once we know the SNROUT vs. SNRIN relationship, the steps to finding the ASK sensitivity for a given Noise Figure, IF Bandwidth, and Data Rate are given below. 1. Determine the Eb/No needed for a target BER (10-3 in this example) then calculate the SNR from the Eb/ No by using SNR = (Eb/No) * (R/BBW) Where R is the data rate and BBW is the Data Filter bandwidth. 4. Reduce the SNR calculated from the previous step by th
e ratio in dB of the IF (pre-detection) BW to the Data Filter BW. For instance, a 600kHz IF BW and 6kHz Data Filter BW means a 20dB reduction in the SNR. This is the SNR of the signal coming out of the RSSI detector before the Data Filter gets rid of the high frequency noise (assumed to occupy the IF BW). At sensitivity, this ratio is usually negative in dB. 5. Use the RSSI SNROUT vs SNRIN curve to find the SNR at the input to the RF or IF Amplifier and RSSI detector. You actually use the curve "backwards" to find SNRIN given the SNROUT you calculated in Step 2. 6. Use the SNR formula for the front end of a receiver to find the signal level at the receiver input. This is the sensitivity, S. S = (SNRIN) * (kTBIFFS)
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