There are several good articles talking about how to specify ADC and AGC requirements:
1.How to Specify an ADC for a Digital Communications Receiver
2.AN APPROACH FOR SPECIFYING THE ADC AND AGC Requirements for UWB
Here we talk about how to specify from ACI (Adjacent Channel Interference) point of view.
Wireless standard specifies expected receiver performance under co-channel and ACI interferences.
Taking Bluetooth as an example, the standard specifies
1. When wanted signal is 10dB over the reference sensitivity level, receiver should be able to tolerate an ACI interface of 2Mhz offset and 30dB stronger than the wanted signal with BER less than 0.1%.
2. When wanted signal is 3dB over the reference sensitivity level, receiver should be able to tolerate an ACI interface 3Mhz offset and 40dB stronger than the wanted signal with BER less than 0.1%.
Let’s see how the wanted signal is affected by ACI. In our example design, IF is assumed to be 6MHz before ADC. So the wanted signal is centered at 6MHz and thus 3MHz ACI locates at 6MHz + 3MHz =9MHz. ADC sample rate is designed to be high enough to correctly sample both wanted signal and 3MHz offset ACI into digital domain without alias. It is designed in such a way to lower the cost (area and power) of radio chain since it is not expensive to achieve narrow band filter in analog domain to “cleanly” filter out ACI at 1MHz, 2MHz, and 3MHz. So ACI filtering is done at two stages, one is done at analog domain before ADC and the other is done at digital domain.
After ADC, at digital domain, this composite signal is then mixed with 6MHz LO to down-convert desired signal to DC. You may be surprised to know that 3MHz offset ACI (locate at 9MHz) also goes to DC. The reason is in real silicon LO is never clean as a single tone and suffers phase noise. So the spectrum normally takes a bell shape. Phase noise component at 3MHz offset behaves as a weak 9MHz LO and it down-converts 9MHz ACI to DC. Note although LO phase noise at 3MHz offset is pretty weak, ACI at 3MHz offset can be much stronger than the desired signal so the interference down-converted to DC can still be very strong compared to desired signal at DC if not properly designed. What’s worse is since the interference is at DC, the same frequency location as the desired signal, nothing you can do to remove it anymore.
So we need to make sure ACI is weak enough to get wanted signal detected. Let’s say LO phase noise at 3MHz offset is -120dBc/Hz, signal bandwidth is 1MHz (still using Bluetooth as an example), and the required SNR to correctly detect the wanted signal is 18dB. Then the maximum interference level is
= -PN -BW -SNR
=120 -10log10(1e6) -18dB
Above calculation shows the maximum interference that receiver can tolerate is 42dB higher than the wanted signal. Otherwise receiver will get less than 18dB SNR composite signal which is required to detect wanted signal with acceptable BER.
Then we can look at below diagram for ADC dynamic range analysis.
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