Sensitivity and Dynamic Range

Sensitivity and dynamic range are two main performance parameters in a receiver system. Sensitivity defines the minimum input signal level that must be detected and demodulated by the receiver with acceptable quality, and the dynamic range defines the entire range of input signal level from the sensitivity threshold up to the maximum tolerable strength.

In addition to the thermal noise most concerned in the conventional formula, various noise-like impairments are taken into account to observe the system performance across the dynamic range. To evaluate the level of noise floor, several dominant noise-like components are summed and given by

) where PN,thermal represents the thermal noise of the circuit blocks, PN,IMD represents the intermodulation distortion components due to the circuit nonlinearity, and PN,PN

denotes the contribution from LO phase noise, and PN,IMRR counts the impairment of quadrature inaccuracy. As normalized to the signal power, the formula can be

total SNR SNR SNR SNR

SNR

In the case of dynamic range evaluation, only the desired signal is taken into account, i.e., no other interferers are received. For a given receiver system, therefore, SNRPN

and SNRIMRR are almost constant across the dynamic range, while SNRthermal and SNRIMD are highly dependent on the strength of input signal.









P

SNR_thermal _sig   _sig 105.2 . (3.6)

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



IMD N sig

IMD P P IIP P

SNR   _, 2 3 . (3.7) Here, the effect of intermodulation distortion considers only the third-order nonlinear component, which dominates the others in the evaluation of dynamic range.

Table 3.6 shows the spreadsheets containing the gain settings for different input level. The corresponding NF and IIP3 are also listed for the dynamic range evaluation.

By substituting the data in this table into (3.6) and (3.7), SNRthermal and SNRIMD at various input power level can be calculated. Assume that the system has a 41dB of SNRPN and a 37dB of SNRIMRR, i.e., the in-band phase noise should be less than 0.5 degree rms and the I/Q accuracy should be better than -37dBc. Based on these performance parameters, the overall SNR across the dynamic range can be calculated from (3.5) and plotted in Fig. 3.9. As can be seen, the thermal noise dominates the noise floor when the signal level is at the threshould of sensitivity. As soon as the signal level becomes larger, the noise contribution from phase noise and I/Q impairments can dominate the thermal noise and limit the maximum achievable SNR.

As the signal level exceeds -10dBm, the contribution from IMD3 significantly affects the received signal quality.

-100 -80 -60 -40 -20 0

0 10 20 30 40 50 60

SNR (dB)

RF input level (dBm)

SNR^thermal SNR^PN SNR^IMRR SNR^IMD SNRtotal

Fig. 3.9 SNR versus RF Input power level in sensitivity test.

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LNA Mixer Filter PGA

-100 -80 -60 -40 -20 0

Pin=-96.6dBm Pin=-32dBm Pin=-28dBm Pin=0dBm PN@-96.6dBm PN@-32dBm PN@-28dBm PN@0dBm

Signal and Noise Level (dBm)

Fig. 3.10 Signal and noise levels along the receiver chain in DR test.

LNA Mixer Filter PGA

0 20 40 60 80 100

Pin=-96.6dBm Pin=-32dBm Pin=-28dBm Pin=0dBm

SNR (dB)

Fig. 3.11 SNR degradation along the receiver chain in DR test.

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Table 3.6 Gain settings versus input level in dynamic range test

Fig. 3.10 illustrates the signal and noise levels along the receiver chain in the dynamic range (DR) test, where four levels of input power are shown as the examples.

Here, only the thermal noise component is shown. By taking the other impairments into account, their corresponding SNR plots are also in Fig. 3.11. As the input signal level is at -96.6dBm, a target sensitivity level, the receiver chain provides 91.5dB of gain and amplifies the signal level to -5.1dBm along with a noise level of -10.04dBm.

The output SNR is 4.94dB, well above the required minimum SNR of 4.6dB for QPSK demodulation. This result conforms to a cascaded NF of 3.65dB as pointed out

Front-end ABB Cascaded Cascaded

Pin(dBm) Gain NF Gain NF NF (dB) IIP3 (dBm)

-97 to -95 36 3.5 56 to 54 25 3.65 -13

-94 to -89 36 3.5 53 to 48 25 3.65 -13

-88 to -83 36 3.5 47 to 42 25 3.65 -13

-82 to -77 36 3.5 41 to 36 25 3.65 -13

-76 to -71 36 3.5 35 to 30 25 3.80 -13

-70 to -65 36 3.5 29 to 24 28 4.21 -13

-64 to -59 36 3.5 23 to 18 32 5.82 -13

-58 to -53 36 3.5 17 to 12 38 9.32 -13

-52 to -47 36 3.5 11 to 6 44 14.4 -13

-46 to -41 36 3.5 5 to 0 50 14.4 -13

-40 to -39 34 4.3 1 to 0 50 16.3 -11.5

-38 to -37 32 4.8 1 to 0 50 18.2 -10

-36 to -35 30 5.5 1 to 0 50 20.2 -8

-34 to -33 28 6.4 1 to 0 50 22.1 -6

-32 to -31 26 7.5 1 to 0 50 24.1 -4

-30 to -29 24 9.0 1 to 0 50 26.1 -2

-28 to -27 21 10 2 to 1 50 29.1 5

-26 to -21 21 10 0 to -5 50 35.0 5

-20 to -15 15 14 0 to -5 50 35.0 7

-14 to -11 0 36 9 to 6 44 44.6 10

-10 to -5 0 36 5 to 0 50 50.2 10

-4 to 0 0 36 -1 to -5 56 56.0 10

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earlier. As the input level is up to -32dBm or -28dBm, the output SNR is bounded at 35dB due to the constraints of phase noise and IMRR.

It is noted that in the case of our targeted receiver the LNA stage is assumed to be bypassed and combined into the mixer, implying that the LNA contributes no impact to the system in this performance evaluation. As the input signal has a level up to 0dBm, the RF front-end is set at a minimum gain of 0dB and the analog baseband has a gain attenuation of -5dB. The output signal level is -5dBm with a SNR of 19.8dB, while over 50dB if only AWGN is considered as shown in Fig. 3.10. As mentioned earlier, the SNR degrades much in the mixer stage due to the effect of third-order nonlinearity.