Michael Epp

Analog IQ impairments in Zero-IF radar receivers: Analysis, measurements and digital compensation

We address the Zero-IF or homodyne radio architecture as a pursuable way for small Radar receivers. While Zero-IF is beneficial for integration, several inherent analog impairments place a limit on the achievable dynamic range. The most dominant non-idealities are gain and phase imbalance in the IQ branches, mixer nonlinearity, and DC Offset. In the case of IQ Imbalance, careful receiver design can at best achieve an Image Rejection Ratio (IRR) of 30-40 dB. Also, IQ imbalance tends to be frequency-dependent with increasing bandwidth (BW). It has to be investigated, whether sophisticated digital post-processing is able to deliver a dynamic range sufficient for Pulse-Doppler Radar. After establishing some theoretical background and proposing digital correction methods, we will present hardware measurements of frequency-dependent IQ imbalance made on a Zero-IF receiver with large bandwidth. Despite significant improvements can be achieved using an offline calibration, time-varying drifts due to temperature changes will degrade the achievable IRR. Therefore adaptive circularity-based algorithms should be applied to track those changes. However, Radar Chirp signals at complex baseband (BB) cannot be used directly, as they are not circular. To restore the circularity for estimating the Complementary Autocorrelation Function (CACF), we propose applying a digital band-stop to the operational data beforehand. Highly increased IRR values are technically feasible: Digital Assistance acting jointly with state-of-the-art RF circuit design can pave the way for adequate performance in integrated receiver solutions.

Frequency Response Mismatches in 4-channel Time-Interleaved ADCs: Analysis, Blind Identification, and Correction

This article proposes a novel adaptive architecture for blind identification and compensation of frequency response mismatches in 4-channel time-interleaved analog-to-digital-converters (TI-ADCs). Detailed frequency response mismatch modeling is first carried out elaborating in detail the interleaving mismatch spurs characteristics. Stemming from the established mirror-frequency crosstalk nature of the different mismatch spurs, the interleaving mismatch identification process is then carried out using complex second-order statistics based methods. The developed learning algorithm performs the mismatch identification and learns the mismatch compensation filter parameters in a blind manner for almost the full digital bandwidth of the 4 TI-ADC system. The proposed solutions efficiency and performance are verified and demonstrated using state-of-the-art RF-sampling TI-ADC hardware measurements with GHz range instantaneous bandwidth. In addition to this, the relationship between a four-channel TI-ADC and an I/Q sampling 2-channel TI-ADC is explored and an interesting link between the two is established in this work.

2-channel Time-Interleaved ADC frequency response mismatch correction using adaptive I/Q signal processing

A novel adaptive compensation architecture for the frequency response mismatch of 2-channel Time-Interleaved ADC (TI-ADC) is proposed for developing high-yield self-adaptive systems. The proposed approach overcomes the existing methods in the sense that the TI-ADC mismatch identification can be performed without allocating a region where only the TI-ADC mismatch spurs are present. This is accomplished via mapping the TI-ADC problem into an I/Q mismatch problem which allows deploying complex statistical signal processing. As proof of the concept, the compensation architecture is demonstrated and tested on RF-sampling TI-ADC hardware data.

Analysis, Blind Identification, and Correction of Frequency Response Mismatch in Two-Channel Time-Interleaved ADCs

In this paper, novel blind identification and compensation architectures for the frequency response mismatch of a two-channel time-interleaved analog-to-digital-converter (TI-ADC) are proposed. First, detailed modeling of the frequency response mismatch is carried out, establishing a direct connection and similarity to the well-known in-phase/quadrature mismatch problem. Stemming from this modeling, the proposed blind mismatch identification and compensation architectures are then developed building on complex statistical signal processing. Compared to the existing methods in the literature, the proposed solutions can identify and correct the frequency response mismatch in a fully blind manner for the full digital bandwidth (BW) of the ADC system, and are also applicable in sub-sampling TI-ADC devices and RF sampling. The efficiency of the proposed solutions is verified and demonstrated using comprehensive measurements of actual RF-sampling TI-ADC hardware with gigahertz-scale instantaneous BW.

A blind frequency response mismatch correction algorithm for 4-channel Time-Interleaved ADC

A novel approach for the frequency response mismatch mitigation of a 4-channel Time-Interleaved ADC (TI-ADC) is proposed which enables the interleaving mismatch identification to be performed in a fully blind online manner. This is accomplished via generating an appropriate complex valued signal from the real valued TI-ADC output signal which allows deploying complex statistical signal processing methods for the mismatch identification in a manner similar to the I/Q imbalance correction. As proof of concept, the compensation architecture is implemented, demonstrated and tested using real RF-sampling 4-channel TI-ADC hardware data, evidencing spur reduction below 80 dBFS.

Digital full-band linearization of wideband direct-conversion receiver for radar and communications applications

This paper proposes a fully digital post-processing solution for cancelling nonlinear distortion and mirror-frequency interference in wideband direct-conversion receivers (DCRs). Favorable cost, integrability, and power efficiency have made DCRs a popular choice in communication systems. It is also an emerging trend in radar systems since digital post-processing enables sufficient performance. The proposed method cancels the most essential distortion adaptively during normal receiver operation without any prior information. Improved cancellation performance compared to the state-of-the-art is achieved considering inband and neighboring band distortion induced by the strong received signals. This is verified and demonstrated with extensive simulations and true RF hardware measurements.

A 6-GS/s 9.5-b Single-Core Pipelined Folding-Interpolating ADC With 7.3 ENOB and 52.7-dBc SFDR in the Second Nyquist Band in 0.25-

A pipelined folding-interpolating analog-to-digital converter (ADC) with a distributed quantizer is presented. The mismatch-insensitive analog frontend provides excellent spurious-free dynamic range (SFDR) and signal-to-noise ratio without calibration or digital postprocessing. The algorithm of the digital coder relaxes the requirements on the interface between analog core and digital coder. The single-core ADC achieves an effective resolution of 7.3 b and an SFDR of 52.7 dBc in the second Nyquist band at 6 GS/s with an overall power consumption of 10.2 W.

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Abstract Over the last years ongoing advances in ADC technology have enabled RF signals to be sampled at IF frequencies. Undersampling is nowadays employed in software-defined radio or radar receivers and offers the possibility to relieve requirements in the analog receiver partition. Unfortunately, when moving to higher IF concepts, this becomes demanding for the ADC itself, because of inherent spurious-free dynamic range (SFDR) roll-off that increases with input frequency. This fact often limits the receivers IF placement to Nyquist zone (NZ) 2. In this work the emerging concept of Digital Assistance is pursued to give the receiver access to higher NZs while making no compromise on the SFDR. We will present and discuss post-correction results for two 16-bit high-speed converters from two different vendors at 120 and 125 MSPS, respectively. The proposed system-level post-correction decomposes nonlinearity into a static and a dynamic part. For both ADCs under investigation the degraded SFDR in higher NZs could be improved by up to 15 dB using purely digital linearization technologies, thus increasing the detectability of small signals in the presence of very strong signals or interferers. Near-identical results for both ADCs confirm the general validity of the system-level correction approach.

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This paper proposes a novel fully blind frequency response mismatches correction architecture for a four-channel time interleaved ADC (TI-ADC) using digital signal processing. By generating appropriate complex valued signals from the real valued TI-ADC output, a mapping of the TI-ADC problem into a mirror frequency interference problem is obtained, which allows using in-phase/quadrature (I/Q) mismatch correction techniques, e.g., circularity, for the frequency response mismatch identification. Compared to the existing methods in the literature, the proposed method allows for blind frequency response mismatch identification throughout the whole digital bandwidth and has substantially lower computational complexity while maintaining similar correction performance, hence making it suitable for an on-chip implementation. The compensation architecture is implemented, demonstrated and tested using real RF-sampling four-channel TI-ADC hardware data, achieving a multitone spur reduction below 80 dBFS.