Reciprocal Mixing
Understanding Reciprocal Mixing
Reciprocal mixing is one of the most important and often overlooked receiver degradation mechanisms. In a multi-signal environment, a strong off-channel signal mixing with the LO phase noise skirt produces noise at the IF frequency that adds to the receiver noise floor.
The Mechanism
When the LO phase noise at offset delta_f from the carrier has a level of L(delta_f) dBc/Hz, and a signal at delta_f from the desired channel has power P_blocker, then the noise power at the IF is: N_recip = P_blocker + L(delta_f) + 10 log10(BW). If this exceeds the thermal noise floor, sensitivity is degraded.
Design Impact
- LO phase noise requirement: Working backward from the allowable noise floor degradation determines the LO phase noise specification.
- Blocker specification: Mobile standards (3GPP, IEEE) specify blocking levels that implicitly define LO phase noise requirements.
N_recip = P_blocker + L(delta_f) + 10log10(BW)
Example:
P_blocker = -30 dBm at 1 MHz offset
L(1 MHz) = -140 dBc/Hz
BW = 200 kHz = 53 dBHz
N_recip = -30 + (-140) + 53 = -117 dBm
If thermal noise floor = -121 dBm,
total noise = -115.8 dBm (5.2 dB degradation)
Frequently Asked Questions
What is reciprocal mixing?
Reciprocal mixing occurs when LO phase noise mixes with a strong off-channel signal, creating noise at the IF frequency. This raises the receiver noise floor and degrades sensitivity, even without overloading any component.
How does LO phase noise affect sensitivity?
LO phase noise at offset delta_f mixes with any strong signal at delta_f from the desired channel, producing noise in the IF band. The noise power equals the blocker power plus the phase noise level plus 10log(BW). This can exceed the thermal noise floor.
How do you mitigate reciprocal mixing?
Use a lower phase noise LO (synthesizer with better VCO or lower N in the PLL). Add pre-selection filtering to attenuate strong off-channel signals before the mixer. Use a wider IF bandwidth only when necessary (narrower BW reduces noise power).