Digital Signal Processing Reference
In-Depth Information
frequency and higher harmonics. Because of this, clipping occurred in
the three final sections of the eight-stage variable gain amplifier. The
issue was temporarily resolved by adjusting the internal routing table
of the receiver in order to redirect the signal from an earlier gain stage
directly to the output buffers. However, the sensitivity of the receiver
was seriously degraded due to the problem of clock injection.
1
Charge injection
2
Clipping due to
from clock signal.
in-band spurs.
I
2
2
multiphase window clock
Q
2
2
note: The architecture with window switches behind the mixer is not
further discussed in the remainder of this text.
By positioning the window circuit in front of the downconversion mixer, it
is avoided that switching noise becomes an in-band interferer signal in the
frequency band-of-interest. This is because the passband frequency of the re-
ceived rf pulses is located between 3
.
1 and 10
.
6 GHz, while the main psd
lobe of the 1 ns-wide receive window is located in the frequency band below
500 MHz. As a result, charge injection from the clock signal directly into the
signal band does not corrupt the signal-of-interest, the latter being a passband
signal at the input of the downconversion mixer.
Of course, energy from the sidelobes of the receive window may also corrupt
the received signal, so the clock signal which is driving the receive window
should be low-pass filtered to smooth its rising- and fall slopes. Because the
first notch of the receive window and the lowest rf signal band (3
.
1 GHz)
are separated less than a decade in frequency, this is preferably done using a
higher order filter with a steep rolloff. Removing some of the high-frequency
information from the clock signal results in much smoother transitions from the
on- to off-state of the window switches (Figure 6.2). It follows that the shape
of the receive window is not rectangular any more, but has a rather sine-shaped
profile.
1
The result is a reduced sensitivity near the edges of a receive slot.
1
To be more precise, the shape of the clock signal is also multiplied by the transconductance characteristic
of the cmos switch itself.
