Active feedback receiver with integrated tunable RF channel selectivity, distortion cancelling, 48dB stopband rejection and >+12dBm wideband IIP3, occupying <0.06mm² in 65nm CMOS

Abstract

The impedance transformation property of passive mixers enables integrated high-Q channel selection at RF with a programmable center frequency through a clock [1,2]. As such, this technique is suitable for addressing both linearity and flexibility requirements in wideband and cognitive radio applications. However, given the typically low resistance level at the RF side of the receiver chain, the RC product necessary for filtering results in large capacitors, and, consequently, large die area that does not scale with technology. In addition, filter rejection at the RF side is limited by the resistance of the switches of the passive mixer. Thus, large switches are typically needed for moderate rejection values (5Ω switches for 16dB rejection [2]), which translates to higher power consumption in the LO buffers. Furthermore, filtering prior to the LNA [1] or eliminating it altogether [3] improves linearity at the expense of noise and switching harmonics being injected directly at the antenna node. Conversely, an LNA first architecture offers an opposite trade-off. This work demonstrates a highly compact design of a direct conversion receiver with an active feedback frequency translation loop to perform channel selection at the LNA output while simultaneously cancelling its distortion. Figure 9.3.1 (top) shows the proposed architecture. Signals at the antenna are down-converted and amplified. Along the feedback path, the desired signal BW, now centered at DC, is rejected using a HPF, while interferers are up-converted and fed back at the LNA output. With a high loop gain, node 'A' becomes a virtual ground for interferers beyond the corner frequency of the HPF, which effectively creates a channel select filter at node 'A'. Since the feedback loop sinks current, filter rejection at the RF side is not limited by the resistance of the down-conversion mixer switches. As opposed to other feedback-based receivers [4,5], the proposed architecture incorporates the receiver's downconversion path within the loop to provide an IF output instead of having a separate rejection loop after the LNA with an RF output that needs further down-conversion.The channel bandwidth is now determined by the corner frequency of the HPF (ωHPF) divided by 1+TO, where TO is the loop gain ( Fig. 9.3.1-bottom). That is, for a given bandwidth, the capacitance needed for channel selection, and therefore die area, is reduced by the available loop gain. The loop bandwidth (ωDOM) sets an upper limit on the frequency range of interferers that can be suppressed. Such a filtering loop is therefore suitable for implementation in a modern high speed process and its performance is expected to improve with technology scaling.To cancel LNA distortion, the V-to-I conversion circuits of the LNA and feedback amplifier are matched (Fig. 9.3.2). Under this condition, a downconverted inverted replica of input interferers is forced at the output via the feedback action, causing the feedback amplifier to perfectly sink the distortion currents sourced by the LNA. This arr...

Active Feedback Technique for RF Channel Selection in Front-End Receivers

“…As such, the work presented in this paper targets one of the key co-existence problems in radio receiver design. The receiver is shown to result in a highly compact and tunable design that mitigates the performance limitations of integrated RF filters discussed above [9], namely: large capacitance/die area, limited stop-band rejection and the trade-off between noise, linearity and harmonic radiation. Other issues that are equally important in the context of co-existence include spurious responses due to harmonic mixing, phase noise and even-order distortion.…”

“…These include the need for large capacitance/die area, limited stop-band rejection, and a trade-off between noise, linearity and harmonic radiation. In [9], we presented an alternative approach for providing RF channel selectivity via active feedback. The approach overcomes the aforementioned drawbacks of passive mixer filters while retaining their inherent flexibility.…”

Frequency translation loops for RF filtering : theory and design

“…By choosing g m2 > g m1 , the gain of high frequencies can even be made less than unity. This difference in gain between low and high frequencies effectively creates a LPF at the feedback point, where the bandwidth of the LPF is determined by the corner frequency of the HPF and the available loop gain [34]. Thus, a HPF is transformed into a LPF.…”

“…independent of the receiver's down-conversion path) to provide an RF notch and an RF pass-band filter, respectively. In [44], the forward down-conversion path of the receiver is incorporated into the loop to provide RF filtering at the antenna interface, but unlike [34], the loop is in positive feedback configuration. In a different context, [45] uses a frequency translation loop to achieve low noise 50Ω matching at the input of a multi-band receiver.…”

“…The receiver is shown to result in a highly compact and tunable design that mitigates the performance limitations of integrated RF filters discussed above [34], namely: large capacitance/die area, limited stop-band rejection and the trade-off between noise, linearity and harmonic radiation. In the context of co-existence, other issues that have been described in Section 1.2 are equally important.…”