Radio Frequency Integrated Circuit Design - Webs

216 Radio Frequency Integrated Circuit Design
Figure 7.15 Mixer with nonlinearity in the RF input stage.
where, in this case, there is an extra factor of 2 because the circuit is differential.
From here, the 1-dB compression point can also be computed using the relationships
between IP3 and 1-dB compression developed in Chapter 2.
With nonlinearity in the RF input stage, the currents i 1 and i 2 are
composed of a large number of frequency components. Each of these frequencies
is then mixed with each of the LO harmonics, producing the IF output. Many
of the intermediate frequencies (such as mixing of harmonics of the radio
frequencies with harmonics of the LO) have not been shown.
Finally, if the RF input were perfectly linear, mixing action would proceed
cleanly, with the result as previously shown in Figure 7.14. However, undesirable
frequency components can be generated because of nonlinearity in the output
stage, for example, due to limiting action. In such a case, the two IF tones at
f L − f 1 and f L − f 2 would intermodulate, producing components at f L − (2f 1
− f 2) and f L − (2f 2 − f 1), in addition to harmonics of f L − f 1 and f L − f 2.
This is shown in Figure 7.16.
In some cases, the saturation of the switching quad may be the limiting
factor in the linearity of the mixer. For example, if the bias voltage on the base
of the quadrature switching transistors vquad is 2V and the power supply is 3V,
then the output can swing from about 3V down to about 1.5V, for a total
1.5V peak swing. If driving 50�, this is 13.5 dBm. For larger swings, a tuned
circuit load can be used as shown in Figure 7.17. Then the output is nominally
at VCC with equal swing above and below VCC , for a new swing of 3Vp or
6Vp-p, which translates to 18.5 dBm into 50�. We note that an on-chip tuned
circuit may be difficult to realize for a down-converting mixer, since the output
frequency is low, and therefore the inductance needs to be large.