"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Wireless Communication: Signal Conditioning for IF Sampling
Selection of ADCs/DACs
The mean squared quantization noise power is is P qn = q2 s
12R where q s is the quantization
step size and R is the ADC input resistance, typically 600 Ω to 1000 Ω.
Communication ADCs similar to the THS1052 and THS1265 typically have a full scale
range (FSR) of 1 Vp–p to 2 Vp–p. Generally, wireless systems are based on a 50-Ω input/
output termination, therefore, the ADC input is made to look like 50 Ω. Based on this assumption,
the quantization noise power for a 12-bit, 65 MSPS ADC (THS1265) is
–73.04 dBm.
For a noise-limited receiver, the receiver noise power can be computed as the thermal
noise power in the given receiver bandwidth plus the receiver noise figure NF[3].
For 200 kHz BW (GSM channel), temperature 25C, and 4 dB to 6 dB NF, the receiver
noise power is –115 dBm. Therefore, to boost the receiver noise to the quantization noise
power level requires a gain of 42 dB.
In Figure 13–1, the GSM–900 signal is at –104 dBm (GSM–900 spec for smallest possible
signal at which the raw bit-error rate must meet or exceed 1%) and, therefore, the signalto-noise
ratio (SNR) at baseband or at the converter and due to the thermal noise component
is given by SNRthermal = Eb = –104 dBm +115 dBm = 9 dB.
N0 In order for the raw BER to be 1% in a GSM system, testing and standard curves [4] indicate,
that a baseband SNR (derived from the sum of both thermal noise and ADC noise)
of 9 dB is needed for this performance node.
The process gain G p is defined as:
Gp fs
52 106
BW 200 103 2.6 102 24.15 dB
(13–1)
where GSM channel BW = 200 kHz and ƒ s = 52 MHz (the ADC sampling frequency).
The converter noise at baseband should be much better than the radio noise ( = thermal
noise + process gain). Furthermore, the thermal noise alone brings the system only to the
reference bit-error rate (BER).
Therefore the converter noise (at baseband) = SNR adc + process gain G p.
The ADC SNR adc should be 20 dB to 40 dB above the SNR thermal of the thermal noise
component (+9 dBm). In this example, ADC SNR adc is selected to be 37 dB better than
the SNR thermal of the thermal noise component (9 dB).
In other words, if the converter SNR adc is desired to be 37 dB better than the thermal noise
component (SNR thermal), the baseband converter is chosen to be 9 + 37 dB = 46 dB.
The total noise (N sum) = thermal noise (N t) + converter noise (N conv).
13-7