Chapter 3 Circuit Implementation
3.5 Mixer design
Mixers perform frequency translation by multiplying two signals (and their harmonics). Up-conversion mixers employed in the transmit path have two
distinctly inputs, called the base-band (BB) port and the local oscillator (LO ) port.
Several up-conversion mixer topologies that can be realized in CMOS IC processes are presented. Since balanced mixer designs are more desirable due to its lower spurious outputs, higher
common-mode noise rejection and higher port-port isolation, only balanced type mixer are discussed in
this section[14].
Fig 3.7 Single-balanced mixer
The single-balanced mixer shown in Fig. 3.7 is the simplest approach that can be implemented in most IC processes. The single balance mixer offers a desired single-ended RF input and it does not require a balun at the input.
However, the drawback of single-balance mixer has low 1db compression point, low port-to-port isolation, low IIP3 and high input impedance.
If higher noise figure can be tolerated such as transmitter, the micro-mixer in Fig. 3.8 offers the best IIP3 due to its third-order harmonic distortion cancellation
mechanism. With proper biasing, the input impedance can be set close to 50 ohms which eliminates external matching network. However, using this topology, it would be difficult to increase the gain or reduce the noise figure. It achieves
reasonable port-to-port isolation and eliminates the balun by using a current mirror.
Because having three transistors in stack, limits the maximum signal swing and result in a lower output 1-dB compression point.
Fig.3.8 Micro-mixer
The Gilbert cell (double-balanced) mixer was invented in the 60’s, but it still remains to be the most popular mixer circuit. A basic mixer circuit is shown in Fig.
3.9. Baseband (BB) signal is applied to the lower terminals of the stack, and LO
signal is connected to the upper ones. Thus RF signal is obtained as an output. It can prove high conversion gain and high port-to-to isolation. The linearity is reasonably good. Typically, the RF filter preceding this mixer is single-ended so a balun is needed to convert the single-ended signal to differential signal. However, balun having low insertion loss is very difficult to implement in IC processes.
Fig. 3.9 Gilbert cell mixer
The distortion in RF signal is dominated by the lower BB differential pair rather than the upper LO differential pairs. This is because the BB signal is not a single tone signal [13]. Thus, harmonic distortions in the lower differential pair will cause intermodulation distortion, and these intermodulated components may appear at the same frequency of a wanted channel. This intermodulation distortion can be suppressed by improving the linearity of the mixer itself. Hence linearity is
an important parameter for the evaluation of a mixer performance, and it is usually indicated by 1 dB compression point and third-order intercept pointer (IP3).
The lower differential pair, called source coupled differential pair, is redrawn in Fig. 3.10. The general
relationship between the input signal Vd and the output current Id of the circuit is described as follows [3][13]
Fig 3.10 Source coupled
⎥⎥ which is called a fundamental in general. The rest higher order term (V
(
βISS /4)
0.5Vdd3
, V
d5,
V
d7, …), on the other hand , are distortion components that are not desirable and are often called harmonics. Eq. (3.11) reveals that the linearity of the sourcecoupled pair increases, if tail current source ISS increases. Because the effect is slight, other linearization techniques have been developed.
A differential pair can be degenerated as shown in Figs 3.11 (a) and (b) [3].
In Fig. 3.11 (a), ISS
flows through the degeneration resistors, thereby consuming
voltage headroom of ISSR
S / 2. The circuit of Fig. 3.11 (b), on the other hand, does not involve this issue but it suffers from a slightly higher noise and offset voltage because the two tail current sources introduce some differential error and noise.Fig. 3.11 Source degeneration applied to a differential pair
Resistive degeneration requires accuracy resistors, which is unavailable in today’s IC technologies. As depicted in Fig.3.13, the resistor can be replaced by a NMOS operating in deep triode region. Recall the I-V relationship of the NMOS in triode region
( )
⎥⎦⎤ neglected and equivalent RON is obtained.D n ox
(
GS t)
However, for large input swings, M3 may experience substantial change in its on-resistance. The circuit of Fig. 3.13(a) can be further modified as Fig. 3.13(b).
As the gate voltage of M1 becomes more positive than the gate voltage of M2 , transistor M3 stays in the triode region because VD3
= V
G3- V
G1 whereas M4 eventually enter the saturation region because its drain voltage rises and its gate and source voltage fall. Thus, the circuit remains relatively linear even if one degeneration device goes into saturation.
Fig. 3.12 Source degeneration with NMOS
The proposed mixer is illustrated in Fig. 3.13. Because quadrature modulation is required in modern digital communications, the mixer can be divided into two parts: in-phase and quadrature phase part. The in-phase and quadrature phase signal is combined by inductor load at output. The output of the mixer is chosen differential topology because it can eliminate even-order term harmonic distortion and have better common-mode rejection. The output of the mixer is ac-coupled and directly connected to the pre-amplifier input. The matching network of the inter-stage between mixer and preamplifier is not
required because the impedances of mixer output and preamplifier input are close to conjugate match.
Fig. 3.14 and Fig. 3.15 show the performance of the proposed mixer. It achieves a -5.2dB conversion gain at 5.5GHz with 37.2 dB third-order harmonic rejection and the flatness is less than 0.5dB between 5 and 6 GHz. The proposed mixer dissipates 12.6mW which draws 7mA from 1.8V power supply.
Fig. 3.13 The proposed mixer
5.2E9 5.4E9 5.6E9 5.8E9
5.0E9 6.0E9