EEPC
Newbie level 6
I have a question on the effect of IM2 vs IM3 in I or Q-channel individually, versus at complex baseband:
In a quadrature downconverter, assume each path (I or Q) contains an LNA and mixer which contribute IP3 or IP2. The IM3 from the LNA is at the input to the mixers, so the IM3 along with the signal undergo complex downconversion to baseband, and power is split equally between I and Q. Thus, signal-to-IM3 ratio is the same whether looking at I or Q channel individually, or at complex baseband.
IM2 is only contributed by the mixers. If the phase of IM2 at mixer output in the Q channel is exactly 90 deg out of phase with I channel, then in complex baseband (I+jQ), could the IM2 voltages add in phase? If so, then the power of the IM2 product is 6 dB higher at complex baseband than the power of IM2 in either I or Q individually. Thus, the signal-to-IM2 ratio is 3 dB worse at complex baseband compared to I or Q individually?
Is there some error in the logic above? My question is, when we normally characterize the linearity of a quadrature receiver with two-tone test, we measure the amplitude of IM2 and IM3 at the I or Q output individually, right? So if we say the receiver has an SNR spec of 0 dB, for example, that means we assume 0 dB SNR in both I and Q channels. But if the above is true, then that means SNR at complex baseband might not meet the 0 dB spec if IM2 is significant, even if I or Q path meets the IP2 spec on their own?
Thanks!
In a quadrature downconverter, assume each path (I or Q) contains an LNA and mixer which contribute IP3 or IP2. The IM3 from the LNA is at the input to the mixers, so the IM3 along with the signal undergo complex downconversion to baseband, and power is split equally between I and Q. Thus, signal-to-IM3 ratio is the same whether looking at I or Q channel individually, or at complex baseband.
IM2 is only contributed by the mixers. If the phase of IM2 at mixer output in the Q channel is exactly 90 deg out of phase with I channel, then in complex baseband (I+jQ), could the IM2 voltages add in phase? If so, then the power of the IM2 product is 6 dB higher at complex baseband than the power of IM2 in either I or Q individually. Thus, the signal-to-IM2 ratio is 3 dB worse at complex baseband compared to I or Q individually?
Is there some error in the logic above? My question is, when we normally characterize the linearity of a quadrature receiver with two-tone test, we measure the amplitude of IM2 and IM3 at the I or Q output individually, right? So if we say the receiver has an SNR spec of 0 dB, for example, that means we assume 0 dB SNR in both I and Q channels. But if the above is true, then that means SNR at complex baseband might not meet the 0 dB spec if IM2 is significant, even if I or Q path meets the IP2 spec on their own?
Thanks!
