Radio Frequency Integrated Circuit Design - Webs

Mixers
so that for half the cycle Q 3 and Q 6 are on and taking all of the current i1
and i 2. For the other half of the LO cycle, Q 3 and Q 6 are off and Q 4 and Q 5
are on. More formally, if v 2 >> 2vT , then (7.12) can be approximated as
where
vo
v 1
R C
= u(v 2)
r e + R E
u(v 2) =� 1 if v 2 is positive
−1 if v 2 is negative
203
(7.14)
(7.15)
This is equivalent to alternately multiplying the signal by 1 and −1. This
can also be expressed as a Fourier series. If v 2 is a sine wave with frequency
� LO, then
u(v 2) = 4
� sin (� 4
LOt) +
3� sin (3� 4
LOt) +
5� sin (5� LOt) (7.16)
7.6.1 Why LO Switching?
+ 4
7� sin (7� LOt) + ...
For small LO amplitude, the amplitude of the output depends on the amplitude
of the LO signal. Thus, gain is larger for larger LO amplitude. For large LO
signals, the upper quad switches and no further increases occur. Thus, at this
point, there is no longer any sensitivity to LO amplitude.
As the LO is tuned over a band of frequencies, for example, to pick out
a channel in the 902- to 928-MHz range, the LO amplitude may vary. If the
amplitude is large enough, the variation does not matter.
For image reject mixers (to be discussed in Section 7.10), matching two
LO signals in amplitude and phase is important. By using a switching modulator
and feeding the LO signal into the switching input, amplitude matching is less
important.
Noise is minimized with large LO amplitude. With large LO, the upper
quad transistors are alternately switched between completely off and fully on.
When off, the transistor contributes no noise, and when fully on, the switching
transistor behaves as a cascode transistor, which, as described in Chapter 6,
does not contribute significantly to noise.