To apply these calculations, consider a transmission system where the signal is OFDM modulated with a BW of 160MHz and each sub-carrier modulated with a QAM256 modulation scheme (this is similar to a WiFi 802.11ac transceiver). In addition, consider the implementation of a Zero-IF demodulation scheme.
In this case, the resulting baseband quadrature demodulated signal at the input of I and Q ADC has, respectively, a channel bandwidth of 80MHz.
Further assuming the following characteristics of the AFE:
- ADC SNR = 62 dB (SNRnyq)
- ADC sampling rate = 160 MSPS (Fs)
- Clock long term jitter = 8 ps-rms (σLTJ)
- OFDM signal peak to average ratio = 12 dB (PAR)
- Signal back-off = 10 dB (IBO)
- ADC signal BW = 80 MHz (BW)
Then, the overall SNR of the AFE is:
- SNRJ = 52.7 dB
- SNRADC = 43.0 dB
- SNRtotal = 42.6 dB
In this example, the EVM requirements to demodulate a QAM256 signal are in the order of -33.8dB, (required SNR is 33.8dB). There is a margin of ~8.8dB to the required SNR, resulting in an acceptable degradation of only 0.6 dB of the total system performance.
Similar SNRtotal can be achieved with an AFE with the following characteristics:
- ADC SNR = 66 dB (SNRnyq)
- Clock long term jitter = 20 ps-rms (σLTJ)
It is therefore possible to trade-off clock jitter with ADC performance to achieve the same goal.