IEEE Solid-States Circuits Magazine - Summer 2022 - 60

When considering RF-DACs, there are many
choices to be made, and though significant work
has been completed in the RF bands, there is
still a lot of investigation that needs to happen.
parasitic capacitances in the circuit
are charged and discharged periodically,
and this can consume significant
energy. Layout design at the
slice level should aim to minimize
parasitic capacitance as they reduce
the overall efficiency.
RF-DAC Summary
As the reader can see, when considering
RF-DACs, there are many
choices to be made, and though significant
work has been completed in
the RF bands, there is still a lot of
investigation that needs to happen.
Though there are some RF-DACs
operating at the mm-wave spectrum
[10]-[12], more work is needed in
this space; additionally, support for
multiband carrier aggregation and
reductions in out-of-band noise and
spurious signals are still needed.
Also, spectral purity is something
that must be worked on, and hence,
designs that more optimally filter
spectral images arising from sampling
are needed. No transceiver is
complete without the RX, and hence,
the counterpart to the RF-DAC, the
RF-ADC, is discussed next.
RF-ADCs
Significant progress has been made
on a wide variety of RF-DACs, while
the counterpart RF-ADCs are less
mature. This is partially because
work on RF TXs is often motivated
by the strong desire to improve
energy efficiency, and the switching
operation of the RF-DAC has that
effect. In RXs, the primary concern
is usually to minimize the overall
system noise floor, which typically
is done using an analog low-noise
amplifier (LNA) and bandpass filter
at the RF front end, obviating the
need for extremely wideband directto-digital
conversion.
However, with the increased usage
of software-defined radios and the
opening of the mm-wave spectrum
for 5G, the RF-ADC is becoming more
attractive due to its versatility as
either a front-end RX or as an intermediate-frequency
RX. When considering
an ADC, the input sampling
operation is usually the first thing
that is considered. Direct sampling
at the RF has become more popular
as CMOS device scaling has increased
the switching speed of the transistor.
It can potentially allow for the reduction
of components (e.g., there is no
RF mixer, and the frequency down
conversion is in the digital domain),
and like RF-DACs, it is inherently
wider band. This is for similar reasons
to the RF-DAC.
Converting directly to the digital
domain means that there are reduced
circuit interfaces that require impedance
transforms. This can also lead
to relaxation in the design of the
frequency synthesizer and intermediate-frequency
filtering. An RF-sampling
ADC schematic is shown
in Figure 10(a). A bandpass channel
selection and an antialiasing filter
precede the ADC, which is followed
by a digital direct down converter
consisting of digital mixers driven
by a numerically controlled oscillator
(NCO). The down-converted signals
are then filtered by a digital decimation
filter. The sampling frequency
does not necessarily need to meet the
Nyquist requirements at the RF center
frequency; it needs to meet only
the Nyquist requirements related
to the bandwidth of the signal with
sufficient margin to be able to filter
Nyquist images.
RF-sampling ADCs can readily
enable multiband operation by
using a single RF-ADC and separate
digital down-conversion chains, as
shown in Figure 11. In this case, the
sampling rate needs to be optimized
only to mitigate any intermodulation
RF Sampling ADC
Blocker
Digital Down Converter
Q
N
Decimation
Filter
I
N
fs/2
Frequency (Hz)
(a)
(b)
FIGURE 10: (a) A block diagram schematic of an RF-sampling ADC and (b) an example power spectral density (PSD) including blocker impairment.
60 SUMMER 2022
IEEE SOLID-STATE CIRCUITS MAGAZINE
fs
NCO
sin
cos
ADC
Filter
LNA
RF Filter
Rx Band
Signal
Gain, Relative Power (dB)

IEEE Solid-States Circuits Magazine - Summer 2022

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