The other well known challenge with the ZIF receiver is the image created by the gain and phase imbalances between the I and Q channels. The image mirrors the signal around DC and falls inside the wanted band as illustrated in the following figure. A 0.2 dB gain and 2-degree phase mismatch creates an image at 39 dBc from the main signal. Typically, an uncalibrated ZIF IQ receiver has 30 to 35 dB of image level. There are many components in the I and Q signal path that exhibit mismatches and contribute to the image level, including:
- Low-pass filter.
- I/Q independent gain stages.
- Digital step attenuator (DSA) for RX AGC function that adjust I and Q separately.
- LO 90-degree phase shifter.
The ZIF receiver design often employs an IQ calibration mechanism to reduce the image level. The process to calculate the IQ imbalance and provide the compensation is often non-trivial and requires additional hardware in the design to support the calibration. The quadrature error correction (QEC) block in ZIF RFIC devices might be designed with a single complex correction tap, suitable for correcting IQ imbalance at a single frequency point or for narrow instantaneous bandwidth (iBW). This tends to be enough for 4G systems with smaller carrier and operating bandwidth. For wider band 5G radios, the IQ mismatch has a larger frequency dependency as the gain and phase mismatch can vary across the band. As a result, the single point correction QEC is not enough. This concept is illustrated in the following figure. For ease of visualization, half of a 400 MHz band is populated with signal. The left plot shows the spectrum of a perfectly balanced I and Q path. The center plot shows the image at –40 dBc level when the I and Q paths differ by 0.1 dB in gain and 1 degree in phase. The plot on the right shows the image after a single point QEC correction. The QEC correction does a very good job around the center of the 200 MHz signal. However, the signal edges do not get much correction at all due to the frequency variation of the gain and phase mismatch. Mismatches caused by temperature variation further complicates the issue.
To reduce the image across the entire band, a more complex QEC filter is required. This leads to an increase in power, as well as computational complexity in the calibration mechanism. In practice, over operating conditions of temperature, signal dynamic range, and RAT types achieving a 50 dBc image level across 400 MHz or more of instantaneous bandwidth is a challenge.