LO Harmonics Research Articles

A New Analog-based LO Harmonic Rejection Technique with Tunable Notch Frequency

An effective technique for mixer LO harmonic rejection in a SAW-less wideband receiver front-end is proposed. The proposed technique provides a tunable notch that can be placed at any frequency like mixer LO harmonics, to avoid the aliasing in baseband after mixing. An analog LC notch is used in a cascode transconductor, and it can reject one of the 3rd or 5th harmonics. This notch frequency is tunable using the bond wire inductors or fixed Gyrator-C active inductor, and a capacitor array, without any significant Power/Area overhead on overall system. Since the accurate value of inductance is not clear, a calibration circuit is proposed to tune this LC notch. This tuning phase runs in forground, and consumes very low additional power. This technique is used in a wideband receiver front-end. Post-layout simulation in 130nm CMOS results in an average 3rd harmonic rejection ratio of 36 dB. The overall circuit consums 34mW power and a has a noise figure of 3.4dB at 1GHz frequency.

Matching for Concurrent Harmonic Sensing in an ${M}$ -Phase Mixer-First Receiver

This brief proposes the use of passive mixer-first receiver topology to sense signals at higher LO harmonics. The advantages of such a receiver include sensing of multiple bands concurrently and reduced tuning range requirements in the frequency synthesizer. The single and joint harmonic matching performance of a zero-IF ${M}$ -phase mixer-first receiver is analyzed. It is shown that minimum possible return loss for joint matching occurs when the geometric mean of input impedances at the highest and lowest sensing bands equals the antenna impedance. The noise figure (NF) when sensing higher order LO harmonics is shown to result in only modest degradation, with the loss becoming even less with increasing number of LO phases. As an example, a 12-phase harmonicsensing mixer first receiver is simulated and its performance at first, third, and fifth harmonics is evaluated in terms of input matching and NF.

A Passive Mixer-First Receiver With Digitally Controlled and Widely Tunable RF Interface

A software-defined radio (SDR) receiver with baseband programmable RF bandpass filter (BPF) and impedance match is presented. The passive mixer-first architecture used here allows the impedance characteristics of the receiver's baseband circuits to be translated to the RF port of the receiver. Tuning the resistance at the baseband port allows for a real impedance match to the antenna. The addition of complex between I and Q paths allows for matching to the imaginary component of the antenna impedance. By implementing both real and imaginary components with resistors in feedback around low noise baseband amplifiers, noise figure is also kept low. Tunable sampling capacitors on the baseband side of the passive mixer translate to tunable-Q filters on the RF port which allow for very good out-of-band linearity. Furthermore, the concept of in-band and out-of-band must be redefined as the impedance match and BPF center frequency move with the LO frequency, such that matching and filtering track the receive frequency. Additionally, 8-phase mixing is shown to provide significant benefits such as impedance matching range, rejection of blockers at LO harmonics, and lower noise figure (NF). Measurements from the receiver implemented in 65 nm CMOS show 70 dB of gain, NF as low as 3 dB, and 25 dBm out-of-band IIP3. Furthermore, tunable impedance matching shows that S11 <;- 30 dB can be achieved at any receive frequency from 0.1-1.3 GHz.

A Balanced Even Harmonic Quadrature Mixer Using Anti Parallel Diode Pairs

An even harmonic quadrature mixer (EH-QMIX) with a balanced configuration is proposed for a direct conversion receiver. The unit even harmonic mixer (EHMIX) used for I/Q paths consists of two anti parallel diode pairs (APDPs) and a pair of diplexers. When the second harmonic of LO (2LO) from the LO section is applied to the LO port as a spurious component, a conventional single-ended EHMIX using APDP converts the 2LO leakage from the LO section into the baseband and the d.c. offset and the self-detected LO noise arise at the baseband degrade the sensitivity. This proposed balanced EHMIX configuration can cancel out the 2LO leakage in itself. Therefore, the d.c. offset and the LO noise are significantly suppressed and the degradation of the sensitivity can be avoided. The suppression characteristic of the d.c. offset and the LO noise are verified by the simulation and the measurements. By using this balanced configuration, the fabricated EH-QMIX achieves wider frequency band characteristic than that of the single-ended EH-QMIX, and it shows 20% relative bandwidth at L-band.

Spurious mixing of off-channel signals in a wireless receiver and the choice of IF

Circuit nonlinearity and LO harmonics can cause large interferers to translate on to the same intermediate frequency as the desired channel. The mechanisms responsible for spurious mixing, which are distinct from intermodulation distortion, are analyzed and catalogued. The analysis leads to an optimal choice of IF for a fully integrated 900 MHz GSM receiver that resists all blockers.

A diode mixer with harmonic-distortion suppression

It is shown that terminating the second and third LO harmonics and associated mixing frequencies in a short circuit results in substantial reduction in a diode mixer's harmonic distortion. It was found experimentally that the second-harmonic distortion level dropped a maximum of 28.7 dBm, and the third-harmonic distortion level also dropped a maximum of 16 dBm. The practical possibility of improving IM distortion levels is established. This technique should prove valuable in the lowering of IM distortion levels of diode mixers. >