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Signal Chain Basics #152: Concerned about DC offset and sideband suppression in your zero IF receiver? Consider complex IF mode

There has been a significant increase in integration of high-performance radio transceivers suitable for applications such as cellular base stations and radars. These transceivers use two different device architectures: zero intermediate frequency (zero IF) and radio-frequency (RF) sampling. Zero IF transceivers are currently the lowest-power solution for signal bandwidths less than 200 MHz. RF sampling allows bandwidths as wide as 1200 MHz, as well as multiband operation. Reference [1] contains a review of different device architectures and details about RF sampling.

One concern many engineers have with zero IF transceivers are the impairments due to mismatch in quadrature modulation. In this article, I’ll introduce complex intermediate frequency (IF) mode, which provides a method to significantly improve the in-band spurious-free dynamic range (SFDR) of a zero IF receiver.

Figure 1 shows the zero IF receiver architecture. A low-noise amplifier first amplifies the RF signal at the antenna before a filter suppresses large unwanted signals at frequencies outside the band of interest. A mixer with a local oscillator that is split into two carriers 90 degrees out of phase down-converts the RF signal to a complex analog baseband signal. A baseband filter further suppresses out-of-band energy and a dual-channel analog-to-digital converter samples the baseband signal.

Zero IF receiver
Figure 1. A Zero IF receiver uses a low-noise amplifier to boost signal amplitude before filter suppresses unwanted frequencies.

The advantage of the zero IF architecture is the ease in filtering at baseband (only low-pass filtering is required) and at RF; plus, lower-power data converters only need to sample low-frequency signals. The disadvantage is that the analog mismatches in the complex path creates sideband images and local oscillator feedthrough in-band. Figure 2 shows the frequency spectrum at baseband for the receiver with impairments, including the blocker image caused by complex mismatch and DC offset.

receiver complex mismatch
Figure 2. Frequency spectrum with impairments for a zero IF receiver where blocker image is caused by complex mismatch and DC offset.

Zero IF transceivers such as the AFE7799 from Texas Instruments have blind correction algorithms (meaning that they do not require any training signal) to correct for DC offset and the sideband image. This prevents large blocker signals from degrading the sensitivity of wanted signals. The DC offset is corrected to ~-85 dBFS and the sideband image to 75-80 dBc. For most communication systems, this is sufficient to meet the blocker sensitivity requirements with margin.

For systems requiring higher rejection, for example in a cellular base station Global System for Mobile Communication (GSM) receiver, it is possible to use a device such as the AFE7799 in complex IF mode where the wanted band is offset from the local oscillator, as shown in Figure 3.

frequency spectrum impairments
Figure 3. Frequency spectrum with impairments for complex IF mode. The desired band is offset from the LO.

With the band offset to one side of the local oscillator, any in-band blocker image falls outside of the band. The DC offset is also outside the band. An out-of-band blocker, as shown in blue in Fig. 3, is first suppressed by the RF band filter. The sideband suppression of the analog front-end complex demodulation then further rejects the sideband image.

Reaching (for example) a rejection of 100 dB for the out-of-band blocker image requires only 25 dB of RF filtering because the analog front end provides 75 dB of sideband suppression.

Figure 4 shows an example of using complex IF mode to improve the spurious-free dynamic range (SFDR) for Third Generation Partnership Project (3GPP) Band 3 uplink, which covers 1710 MHz-1785 MHz. Band 3 is a GSM band, which requires a very low spurious level (less than -90 dBFS) due to the large in-band blocker requirement. The local oscillator is set to 1695 MHz, so Band 3 covers 15 MHz to 90 MHz in the baseband output spectrum (the green part of the spectrum). A tone simulating an in-band blocker is placed at 1715 MHz or +20 MHz at baseband. The spurs from the quadrature imbalance, DC offset and the sideband image both fall outside the band. The largest nonlinear distortion spur, HD3, also falls at -60 MHz (all odd-order harmonics fall in the opposite sideband). The in-band spurs are all less than -93 dBFS, meeting the requirement with margin.

complex IF receiver
Figure 4. Output spectrum for Band 3 in complex IF mode shows several characteristics.

Finally, because the receiver only needs the 75 MHz spectrum of the wanted band, the AFE7799 contains a final digital mixer and decimation stage that reduces the sample rate to 122.88 MSPS and centers the band in the output spectrum. This lowers the interface rate, allowing all four channels to share one serializer-deserializer transmitter.

For systems requiring the highest SFDR, complex IF mode provides a method to significantly improve the SFDR of a zero IF receiver.

Stay tuned for the next Signal Chain Basics article, with advice on working with data converters, amplifiers, interface or other analog design challenges.

Reference

  1. RF Sampling for Multiband Radios.” Texas Instruments application report SBAA328, April 2019.

Robert Keller is a systems manager in TI’s Wireless Infrastructure group.

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1 comment on “Signal Chain Basics #152: Concerned about DC offset and sideband suppression in your zero IF receiver? Consider complex IF mode

  1. vancouverseo
    November 25, 2019

    Nice post

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