Direct conversion receiver designs enable multi-standard/multi-band operation

Modern direct conversion provides a compelling solution for field programmable radio designs and offers a cost benefit and potential performance advantage over traditional receiver solutions. In addition, direct conversion architectures offer more freedom in addressing multiple bands of operation using a single hardware solution. This promises to be a more cost effective solution and is now enabling high performance multi-standard/multi-band radio designs. This article discusses the performance and merits of a direct conversion receiver signal chain in the context of 3G and 4G wireless cellular applications.

A High Performance Direct Conversion Signal Chain Lineup
A direct conversion receiver directly demodulates an RF modulated carrier to baseband frequencies, where the signal can be directly detected and the conveyed information recovered. The direct conversion architecture was originally developed in 1932 as a replacement to superheterodyne receivers. The reduced component count that resulted by eliminating intermediate frequency (IF) stages provided an attractive solution.

By eliminating any intermediate frequency stages and directly converting the signal to effectively a zero-IF frequency, the image problems associated with superheterodyne architectures could be ignored. However, other challenges associated with direct conversion, including LO leakage, dc offsets, and distortion performance, have made practical implementations difficult. Recent advances in integrated RF circuit technology now allow the traditional Direct Conversion (Homodyne) architecture to be applied for broadband high performance receiver implementations.

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A wideband direct conversion receiver is presented in Figure 1. Some of the more critical component specifications are highlighted within the signal chain. The receiver signal path begins at the antenna port connection into a duplexer. Duplexers are often used in frequency domain duplex (FDD) systems such as W-CDMA and some versions of WiMax. The duplex filter network ensures that the transmitter does not generate too much unwanted energy outside of the licensed frequency band, while helping to reject any unwanted out-of-band signals from overdriving the input of the receiver.

Typically, several low-noise amplifier stages will follow with additional band selective filtering and padding/matching networks designed to optimize performance over the frequency range of interest. The LNA stages used for demonstration purposes offer very good broadband performance and enhanced narrowband performance with the addition of external tuning networks. In applications where the receiver needs to address a very broad range of frequency bands, it may be necessary to utilize a switch matrix to configure different antenna networks and LNA stages that have been optimized for specific frequency bands.

After the low noise front-end, the desired carrier frequency is down converted to baseband using an IQ demodulator. A local oscillator (LO) is applied to the I and Q mixers at the same carrier frequency as the wanted signal. This generates sum and difference frequencies at the baseband I/Q output ports, where low pass filters heavily reject the summation frequency and allow only the difference frequency to pass. For a zero-IF scenario, the difference frequency reveals the baseband envelope of the wanted signal. It is often advantageous to scale the magnitude of the filtered baseband I/Q signal with variable gain amplification. The VGA allows the I/Q signal levels to be adjusted to an optimum level for analog-to-digital conversion. In general, additional filtering may be applied before the analog-to-digital converters (ADCs) to ensure that high frequency noise and potential leakage or interfering tones do not alias back into the desired signal analysis bandwidth.

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Receiver Dynamic Range
The receiver uses high performance RF integrated circuits, which offer broad frequency coverage and high instantaneous dynamic range. Instantaneous dynamic range is a critical specification for any receiver that needs to operate in a multi-carrier environment where wanted signals may have neighboring interfering signals of significantly greater power levels. Two-tone SFDR can provide system designers with a more accurate prediction of nonlinear behavior. It is common practice to test a receiver's resilience under large signal blocking conditions using single tone and two-tone interfering signals. By studying the nonlinear behavior of the receiver under two-tone excitation, it is possible to calculate various intercept points that help to quantify and model the distortion performance and overall dynamic range capability of the receiver.

Figure 2 depicts the I+jQ output spectrum of the receiver when presented with two large CW interfering tones that are in close proximity to the intended wanted signal frequency. Under this test case, the input signals were applied at "30 dBm input levels. This represents a pessimistic blocking scenario that is much more severe than any standard specified blocking test conditions as required in 3G and 4G cellular systems. When sampling a signal near or at baseband frequencies, harmonic distortions from 2nd, 3rd, 4th, and even 5th and 7th order nonlinearities may limit performance under large signal input conditions. In particular, the nonlinear behavior of the I/Q demodulator needs to be sufficiently adequate to ensure that intermodulation terms generated from both the wanted and unwanted signals do not corrupt the desired signals of interest.

Rather than focusing primarily on third order intercept point (IP3)—a common distortion metric that is a focal point in most narrowband IF sampling receiver designs—it is also important to focus on distortion terms due to even and odd order nonlinearities. Such nonlinearities are often quantified using metrics of IP2, IP4, and IP5. In general it is important to review all spurious signals arriving within the analysis bandwidth of the receiver under worst case input conditions to ensure robust operation. Under such severe blocking conditions, intermodulation products due to high order nonlinearities may fall in band and desensitize the receiver. The more critical nonlinear terms are labeled in Figure 2. Note how odd-order terms fall close to the fundamental input tones. This helps to explain how close-in interfering signals may generate intermodulation products that fall in the band of the desired signal. The difference frequency of the interfering tones (f2"f1), a result of the finite 2nd order nonlinearities of the receiver, may also fall within the desired signal band when using direct conversion architectures.

ADIsimRF™, a free on-line signal chain calculator from Analog Devices, Inc., was used to model the dynamic noise and distortion characteristics of the receiver under various test conditions. The nonlinear intercept performance was modeled and measured out to seventh order nonlinear terms and compared against the cascaded intercepts predicted using ADIsimRF. By reviewing the nonlinear behavior of the individual components and the overall cascaded results, the receiver lineup could be better optimized for the highest level of instantaneous dynamic range performance. Using this approach resulted in a highly sensitive receiver with <2 db="" nf="" and="" less="" than="" 1="" db="" of="" desensitization="" when="" presented="" with="" single="" and="" two="" tone="" interfering="" levels="" as="" described="" in="" the="" w-cdma="" specifications="" (etsi="" en="" 302="" 217-2-2="" v1.2.3="" (2007-09)).="">

LO Leakage and DC Offset Desensitization
Any leakage of the local oscillator emanating back towards the RF input port may reflect back into the receiver and self mix with the LO. The self mixing results in a squaring of the LO waveform which generates a second harmonic, usually at very high frequency and heavily attenuated by baseband filtering, and a dc offset which will fall in band for a direct conversion receiver. Note the DC term in Figure 2.