Spurs, Crosstalk and Self-Interference

Radio-Frequency Integrated Circuits

9 Spurs, Crosstalk and Self-Interference

Spurs, Crosstalk and Self-Interference

\[ L_\mathrm{spur} = 20 \log_{10} \left( \frac{V_\mathrm{spur}}{V_\mathrm{carrier}} \right)\;\text{dBc}. \]

\[ \left|\frac{V_\mathrm{victim}}{V_\mathrm{aggressor}}\right| \approx \omega C_\mathrm{c} \left|Z_\mathrm{victim}\right|, \]

In practical RFICs, unwanted tones and unintended coupling paths often limit the achievable dynamic range (expressed for example as SNR or EVM) or cause performance issues (like out-of-band emissions in TX or RX blocking) that render standards-compliant operation impossible. In modern carrier-aggregation transceivers, the integration of several TX and RX paths together with multiple local oscillators on a single die turns this into a major source of concern (Sadjina et al. 2020).

A spur (short for “spurious tone”) is a discrete spectral line at an unwanted frequency. A common metric is the spur level relative to the wanted carrier, expressed in dBc:

Crosstalk is undesired signal transfer from an aggressor node to a victim node through electric, magnetic, substrate, or supply coupling (Verghese et al. 1995). Coupling mainly happens through parasitic capacitances from high voltage-swing nodes, or inductively from high-current loops. Shared traces (like supply and ground connections) can also lead to ohmic coupling.

A first-order estimate for capacitive crosstalk from aggressor to victim is

showing that coupling tends to worsen at higher frequency and higher victim impedance. In high-performance RFICs, crosstalk and spur levels on the order of \(-80\) dBc or lower are often required to meet system-level specifications.

9.1 Coupling Paths

Coupling Paths

Figure 105: Block diagram of a typical transceiver (TRX) showing the main coupling paths.

Figure 105 shows the main on-chip coupling paths in a typical TRX architecture. The following discussion uses the crosstalk nomenclature introduced in (Sadjina et al. 2020).

Coupling Paths

• TX to RX (“TX leakage”): desensitization, reciprocal mixing, RX-band noise, IM2/IM3 (Aparin et al. 2006)
• RX VCO to LNA/mixer (“RX LO leakage”): IM2 products, LO radiation
• LO to LO: reciprocal mixing (RX), OOB emissions (TX), injection pulling (Razavi 2004)
• TX VCO to PA (“TX LO leakage”): LO leakage, EVM degradation

• TX-to-RX coupling through the RFIC, duplexer and shared front-end elements (“TX leakage”) can cause issues such as gain desensitization, reciprocal mixing, receive-band noise from the TX, and IM2/IM3 distortion in the RX (Aparin et al. 2006).
• RX VCO coupling (“RX LO leakage”) from the RX VCO/PLL into the LNA/mixer can cause increased IM2 products and LO radiation.
• LO-to-LO coupling through the LO distribution network can cause reciprocal mixing due to LO spurs in RX, or OOB emissions on TX. When the two oscillators run at nearby frequencies, injection pulling between the VCOs can also degrade the spectral purity of both LOs (Razavi 2004).
• TX VCO coupling (“TX LO leakage”) from the TX VCO/PLL into the PA can cause increased LO leakage and EVM degradation.

9.2 Synthesizer Spurs

Synthesizer Spurs

• Reference spurs at \(f_\mathrm{LO} \pm k\, f_\mathrm{ref}\): CP mismatch and leakage, coupling; set by loop-filter rejection
• Fractional-\(N\) spurs from the \(\Delta\Sigma\) modulator (Section 7.4.1); reduced by higher order and dithering
• Divider and digital-switching spurs coupling via supply and substrate
• Digital spurs from TDC/DCO nonlinearities in ADPLLs

In addition to the inter-block coupling discussed above, the frequency synthesizer itself is a notorious source of intra-PLL spurs (see also Section 7). The most relevant mechanisms are:

• Reference spurs at integer multiples of the reference frequency, \(f_\mathrm{spur} = f_\mathrm{LO} \pm k\, f_\mathrm{ref}\), \(k \in \mathbb{N}^+\). They are caused by charge-pump mismatch and leakage, finite isolation of the loop-filter node, and supply or substrate coupling at \(f_\mathrm{ref}\). Their amplitude is set by how strongly the loop filter rejects components at \(f_\mathrm{ref}\) relative to the loop bandwidth.
• Fractional-\(N\) spurs generated by the periodicity and quantization noise of the \(\Delta\Sigma\) modulator that controls the divider (see Section 7.4.1). They typically appear as families of tones around \(f_\mathrm{LO}\) at offsets related to the fractional ratio. Higher-order modulators and dithering reduce their level at the cost of increased close-in noise.
• Divider and digital-switching spurs at sub-harmonics of \(f_\mathrm{LO}\) and at digital clock frequencies, coupling into the VCO through the supply and substrate.
• Digital spurs originating from TDC or DCO nonlinearities in all-digital PLLs, or from digital blocks in the vicinity of the VCO.

9.3 Spur and Crosstalk Mitigation

Once these spurs appear on the LO, they translate directly into the receive band through reciprocal mixing of a strong blocker, or into the transmit spectrum through the upconversion mixer. If these spurs are inside the channel bandwidth, they cause EVM degradation and may violate spectral mask requirements.

References

Aparin, Vladimir, Gary J. Ballantyne, Charles J. Persico, and Alberto Cicalini. 2006. “An Integrated LMS Adaptive Filter of TX Leakage for CDMA Receiver Front Ends.” IEEE Journal of Solid-State Circuits 41 (5): 1171–82. https://doi.org/10.1109/JSSC.2006.872738.

El-Nozahi, Mohamed, Ahmed Amer, Joselyn Torres, Kamran Entesari, and Edgar Sanchez-Sinencio. 2010. “High PSR Low Drop-Out Regulator with Feed-Forward Ripple Cancellation Technique.” IEEE Journal of Solid-State Circuits 45 (3): 565–77. https://doi.org/10.1109/JSSC.2009.2039685.

Gebhard, Andreas, Oliver Lang, Michael Lunglmayr, et al. 2019. “A Robust Nonlinear RLS Type Adaptive Filter for Second-Order-Intermodulation Distortion Cancellation in FDD LTE and 5G Direct Conversion Transceivers.” IEEE Transactions on Microwave Theory and Techniques 67 (5): 1946–61. https://doi.org/10.1109/tmtt.2019.2896513.

Poon, Andrew, Andrew Chang, Hirad Samavati, and S. Simon Wong. 2009. “Reduction of Inductive Crosstalk Using Quadrupole Inductors.” IEEE Journal of Solid-State Circuits 44 (6): 1756–64. https://doi.org/10.1109/jssc.2009.2020525.

Razavi, Behzad. 2004. “A Study of Injection Locking and Pulling in Oscillators.” IEEE Journal of Solid-State Circuits 39 (9): 1415–24. https://doi.org/10.1109/JSSC.2004.831608.

Sadjina, Silvester, Christian Motz, Thomas Paireder, Mario Huemer, and Harald Pretl. 2020. “A Survey of Self-Interference in LTE-Advanced and 5G New Radio Wireless Transceivers.” IEEE Transactions on Microwave Theory and Techniques 68 (3): 1118–31. https://doi.org/10.1109/TMTT.2019.2951166.

Verghese, Nishath K., Timothy J. Schmerbeck, and David J. Allstot. 1995. Simulation Techniques and Solutions for Mixed-Signal Coupling in Integrated Circuits. Kluwer Academic Publishers.

Yue, C. P., and S. S. Wong. 1998. “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RF ICs.” IEEE Journal of Solid-State Circuits 33 (5): 743–52. https://doi.org/10.1109/4.668989.

Mitigation of spurs and crosstalk is a complex topic that requires careful design of the entire system, including the architecture, floorplan, circuit design, layout, and packaging. Some common techniques are summarized here. In addition, inductor-to-inductor coupling has to be considered and may require quadrupole inductors (Poon et al. 2009) or patterned ground shields (Yue and Wong 1998).

Substrate and supply coupling deserve special attention because they couple every block to every other block through a common low-impedance node (Verghese et al. 1995). Effective measures include placing sensitive analog and RF blocks in dedicated deep-n-well or triple-well pockets, surrounding aggressors and victims with substrate guard rings, separating supply domains for digital, analog, RF, and LO blocks, and using on-chip and on-package decoupling so that switching currents do not modulate the supply seen by the VCO or LNA. A careful floorplan that keeps high-swing TX nodes, LO distribution, and digital clocks physically far from sensitive RX inputs and VCO inductors is usually more effective than any post-layout fix.

If supply decoupling is insufficient, series or shunt supply regulators and additional low-pass filters for supply and reference lines can help to reduce supply-bound coupling (El-Nozahi et al. 2010). However, these measures increase the power consumption and area of the RFIC, and may not be feasible in all applications.

Where possible, mitigation should also be performed by digital signal processing of the RX or TX baseband signals, for example digital predistortion, digital IM2 cancellation (Gebhard et al. 2019), or digital self-interference cancellation (Sadjina et al. 2020). However, these techniques require additional hardware and power, and are not always feasible.

References