Radio Frequency Integrated Circuit Design - Webs

36 Radio Frequency Integrated Circuit Design
Example 2.7 Determining Dynamic Range
In Example 2.4 we determined the sensitivity of a receiver system. Figure 2.14
shows this receiver again with the linearity of the mixer and LNA specified.
Determine the dynamic range of this receiver.
Solution
The overall receiver has a gain of 19 dB. The minimum detectable signal from
Example 2.4 is −106 dBm or −87 dBm at the output. The IIP3 of the LNA
referred to the input is −5 dBm + 4 =−1 dBm. The IIP3 of the mixer referred
to the input is 0 − 13 + 4 =−9 dBm. Therefore, the mixer dominates the IIP3
for the receiver. The 1-dB compression point will be 9.6 dB lower than this,
or −18.6 dBm. Thus, the dynamic range of the system will be −18.6 + 106 =
87.4 dB.
Example 2.8 Effect of Bandwidth on Dynamic Range
The data transfer rate of the previous receiver can be greatly improved if we
use a bandwidth of 80 MHz rather than 200 kHz. What does this do to the
dynamic range of the receiver?
Solution
This system is the same as the last one except that now the bandwidth is
80 MHz. Thus, the noise floor is now
Noise floor =−174 dBm + 10 log10 (80 × 10 6 ) =−95 dBm
Assuming that the same signal-to-noise ratio is required:
Sensitivity =−95 dBm + 7dB+ 8dB=−80 dBm
Thus, the dynamic range is now −15.6 + 80 = 64.4 dB. In order to get
this back to the value in the previous system, we would need to increase the
linearity of the receiver by 25.3 dB. As we will see in future chapters, this would
be no easy task.
Figure 2.14 Circuit for system example.