Down-Conversion Mixer Basic

According to Eq.(2-20) and Eq.(2-25), the noise figure at the resonant frequency can be written by the following equation

+ inversely proportional to QL. Therefore, a minimum F exits for a particular QL.

2.6 Down-Conversion Mixer Basic

Mixers perform frequency translation by multiplying two signals and possibly their harmonics. Down-conversion mixers employed in the receive path have two distinctly different inputs, called the RF port and the LO port. The RF port senses the signal to be down-converted and the LO port senses the periodic waveform generated by the local oscillator.

The signal amplified by the LNA is applied to the RF port of the mixer. Thus, this port must exhibit sufficiently low noise and high linearity, the latter because nearby interferers are amplified by the LNA and hence can produce stronger IM products.

Mixers can be categorized into the passive mixers and the active mixers. Passive mixers do not provide any gain, but typically achieve a higher linearity and speed. On the other hand, active mixers can reduce the noise contributed by subsequent stages but take the disadvantage of linearity [3].

2.6.1 Conversion Gain

The gain of mixers must be carefully defined to avoid confusion. The voltage conversion gain of a mixer is defined as the ratio of the rms voltage of the IF signal and rms voltage of the RF signal. Note that these two signals are centered around two different frequencies.

The power conversion gain of a mixer is defined as the IF power delivered to the load divided by the available RF power from the source. If the input impedance of the mixer are both equal to the source impedance, for example, 50 , then the voltage conversion gain and power conversion gain of the mixer are equal when expressed in decibels.

Conjugate matching at the input of the mixer is necessary in the first down-conversion stage of heterodyne receivers that employ image-reject filters. This is because the transfer function of these filters is usually characterized for only one standard termination impedance and may exhibit ripples if other impedance levels are used. The load impedance of the mixer, on the other hand, is typically not equal to 50 because most passive IF filters have an input impedance of 500 to 1000 . In architectures such as homodyne topologies, the load seen by the mixer may be even higher to maximize the voltage gain.

2.6.2 SSB and DSB Noise Figures

The noise figure of mixers is often a source of great confusion. The single sideband noise figure (SSB NF) of the mixer is usually used for that the desired signal spectrum resides on only one side of the LO frequency, a common case in heterodyne systems. In this case, the output signal-to-noise ratio (SNR) is half the input SNR because the input frequency response of the mixer is the same for the signal band and

the image band. The input and output SNRs are equal for the homodyne down-conversion mixer. Therefore, double sideband noise figure (DSB NF) is used for that the input signal spectrum on both sides of LO.

In summary, the SSB NF of a mixer is 3dB higher than the DSB NF if the signal and image bands experience equal gains at the RF port of a mixer. Typical noise figure meters measure the DSB NF and predict the SSB value by simply adding 3dB.

2.6.3 Port-to-Port Isolation

The isolation between each two ports of a mixer is critical. The LO-RF feed -through results in LO leakage to the LNA and eventually the antenna, whereas the RF-LO feed-through allows strong interferers in the RF path to interact with the local oscillator driving the mixer. The LO-IF feed-through is important because if substantial LO signal exists at the IF output even after low-pass filter, then the following stage may be desensitized. Finally, the RF-IF isolation determines what fraction of the signal in the RF path directly appears in the IF, a critical issue with respect to even-order distortion problem in homodyne receivers.

The required isolation levels greatly depend on the environment in which the mixer is utilized. If the isolation provided by the mixer is inadequate, the preceding or following circuits may be modified to remedy the problem.

2.6.4 Single-Balanced and Double-Balanced Gilbert Mixer

The circuit in Fig.2.6 (a) and (b), which is called Gilbert mixer, is the most popular CMOS mixer in recent days. If the mixer accommodates a differential LO signal but a single-ended RF signal, it is called single balanced, an example being the topology shown in Fig.2.6 (a). If a mixer operates with both differential LO and RF

inputs, then it is called double balanced.

Fig.2.6 (a) Single-Balanced Gilbert Mixer (b) Double-Balanced Gilbert Mixer

The single-balanced configuration exhibits less input-referred noise for a given power dissipation then the double-balanced counterpart. But the circuit is more susceptible to noise in the LO signal. The double-balanced mixer generates less even-order distortion, thus relaxing the half-IF issue in heterodyne receivers and lowering the beat components in homodyne architecture. However, since the RF signal processed by the LNA is usually single ended, one of the input terminals of the double-balanced mixer is simply connected to a bias voltage This in turn creates different propagation times for the two signal phases amplified M1 and M2 in Fig.2.6 (b), leading to finite even-order distortion.

The MOS transistors M1 in Fig.2.6 (a) and M1, M2 in Fig.2.6 (b) are the trans-conductance stage of the Gilbert mixer. This stage amplifies the input signal from the RF port and delivers to the next stage. The next stage is called switch stage, which is composed of M2, M3 in Fig.2.6 (a) and M3, M4, M5 and M6 in Fig.2.6 (b).

(a) (b)

These MOS transistors are treated as switches, and mixing the signal from trans-conductance stage to the intermediate frequency.