![Fig. 5. (a) The passive mixer electrical model [19]. (b) The variation of the input impedance of the mixer with the LO frequency and (c) the value of CR programmed for each case. The comparison of the calculated and simulated input impedance of the mixer at (d) 500 MHz and (e) 1 GHz.](/figures/fig-5-a-the-passive-mixer-electrical-model-19-b-the-3sktp17s.webp)
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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS–I: REGULAR PAPERS 1
40-nm CMOS Wideband High-IF Receiver Using
a Modified Charge-Sharing Bandpass
Filter to Boost Q-Factor
Filipe Dias Baumgratz , Student Member, IEEE, Sandro Binsfeld Ferreira, Member, IEEE,
Michiel S. J. Steyaert, Fellow, IEEE, Sergio Bampi, Senior Member, IEEE,
and Filip Tavernier, Member, IEEE
Abstract—A 40-nm CMOS wideband high-IF receiver is
presented in this paper. The low-noise transconductance ampli-
fier (LNTA) uses dual noise cancellation in order to improve its
noise figure. The LNTA has also a folded-cascode structure to
increase its output impedance and prepare for a current-mode
passive mixer. This structure is merged into the output stage of
the LNTA, so there is no need for extra transistors. Additionally,
a modified charge-sharing bandpass filter with cross-connected
transconductors to boost Q-factor is proposed and discussed. The
highest voltage gain achieved by the receiver (RX) is 30 dB. The
RX noise figure is 3.3 dB at the maximum gain, while the IIP3
is −2.5 dBm at 1 GHz. The area of the receiver is very competitive
for the wide band considered, merely 0.137 mm
2
.TheRXand
clock generation circuitry drain 25 mA from a 0.9-V supply.
Index Terms—Receiver, wideband, high-IF, super-heterodyne,
low noise transconductance amplifier, LNTA, switched-capacitor
filter, bandpass filter, current mixer, positive feedback, Q-factor,
image attenuation.
I. INTRODUCTION
N
EW wireless applications such as software defined
radios or cognitive radios are demanding multi-band
and multi-standard operation, increasing the requirements for
flexibility and integrability in the radios [1]. Typical solutions
adopt either zero intermediate frequency (IF) or low-IF in the
receiver (RX) architectures since they offer less or no concerns
about image rejection. In such architectures, the channel
selection is easily performed on-chip by a low-pass filter (LPF)
after the mixer [2] which also adjusts the channel bandwidth.
Despite these interesting characteristics, the zero-IF and
low-IF topologies are constrained by 1/ f noise, second-order
Manuscript received August 4, 2017; revised November 8, 2017 and
December 19, 2017; accepted January 8, 2018. This work was supported
in part by the Brazilian National Council for Scientific and Technological
Development (CNPq), in part by CAPES, and in part by FAPERGS. This
paper was recommended by Associate Editor M. Onabajo. (Corresponding
author: Filipe Dias Baumgratz.)
F. D. Baumgratz is with PGMicro, Federal University of Rio Grande do
Sul, Porto Alegre 90040-060, Brazil, and also with KU Leuven, 3000 Leuven,
Belgium (e-mail: fdbaumgratz@inf.ufrgs.br).
S. B. Ferreira is with Unisinos University, 93020-190 S˜ao Leopoldo, Brazil.
M. S. J. Steyaert and F. Tavernier are with KU Leuven, 3000 Leuven,
Belgium.
S. Bampi is with PGMicro, Informatics Institute, Federal University of Rio
Grande do Sul, Porto Alegre 90040-060, Brazil.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TCSI.2018.2792909
nonlinearity, and DC-offset. To achieve high-performance,
the receivers overcome these constraints by extensively using
calibration [3]–[5].
Super-heterodyne RX architectures are not affected by these
common issues due to their high IF [6]–[8]. Nevertheless, they
traditionally require external filters for image rejection and
channel selection. Thus, they have drawn less interest for many
years.
The N-path filter proposed in [6] is an evolution of the origi-
nal concept [9], [10], and it solves the super-heterodyne exter-
nal filter problem by implementing a bandpass filter (BPF)
using a passive switched-capacitor (SC) topology. The low-Q
filter is translated to the radio frequency (RF) input by
the mixer, solving the requirement for large external filters.
Following the introduction of this new approach, other passive
SC BPFs have been reported [11]–[16]. Those BPFs use only
switches and capacitors and are consequently more friendly
to process scaling due to the reduction of the switches ON
resistance. In fact, their performance should improve in smaller
CMOS nodes, and the scalability can be increased by using
MOS capacitors.
Switched-capacitors BPFs allow for the design of fully
integrated receivers with a high-IF [6], [13]–[17], avoiding the
shortcomings presented previously. Like the super-heterodyne
architecture, the high-IF works with IFs of tens or even
hundreds of megahertz. The image and blockers are filtered
along the receiver chain by BPFs in the RF-domain, as shown
in [6], and in the IF-domain, as shown in [6] and [13]–[16].
This paper presents a wideband high-IF receiver designed
in TSMC 40 nm CMOS, using SC BPFs to attenuate the
image and out-of-band interferers. The RX operates from
0.5 GHz to 4 GHz and two main architecture innova-
tions are introduced here. First, the low noise transconduc-
tance amplifier (LNTA) implements a clever utilization of a
folded-cascode structure, which not only increases the output
impedance of the LNTA but also reduces the number of
transistors. Second, a SC BPF, in which the Q-factor is boosted
by cross-connected transconductors at the input, is presented
and discussed. Moreover, since the mixer performs a single-
to-differential conversion, the LNTA input is single-ended and
there is no need for a chunky transformer at the input. This
results in a receiver showing high performance in a very wide
band, low power consumption, and small area.
1549-8328 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
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2 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS–I: REGULAR PAPERS
Fig. 1. The proposed receiver chain chain with the clock generator modules.
This paper is organized as follows. Section II discusses the
receiver architecture. The circuits that compose the receiver
are presented in sections III - VI. Section VII shows the
measurement results. Finally, section VIII summarizes the
main achievements of this paper.
II. H
IGH-IF RECEIVER ARCHITECTURE
The receiver chain using a high-IF (HIF) architecture with
quadrature down-conversion is presented in Fig. 1. This topol-
ogy has been selected due to its low-noise and high-linearity.
Despite the passive blocks such as the mixer and the BPFs,
the noise remains low thanks to the high performance LNTA.
Moreover, the first BPF shapes the input impedance of the
mixer and attenuates the interferers.
As the first block in the receiver chain, the LNTA has the
primary task of mitigating the noise contribution from the
whole receiver chain. In addition, since the following block is a
current mode mixer, the LNTA must work as a transconductor,
having a large output impedance which maximizes the AC
current delivered to the mixer [18]. The LNTA has also
an additional linearity burden; since the following mixers
and filters are passive, i.e. highly linear but gain-less and
noisy, the LNTA must have a higher gain, which reduces its
linearity. In this design the supply voltage is limited to 0.9 V,
which limits the LNTA linearity even further. Consequently,
the LNTA becomes the linearity bottleneck of the receiver.
In [6] the LNTA is a self-biased inverter-based transconduc-
tor. Also, a large on-chip transformer to convert the input from
single-ended to differential has been used, which gives an extra
10 dB of voltage gain. Recently published LNTAs [14], [17]
use noise canceling techniques to improve noise figure. In [17]
a two-stage LNTA is presented; the first stage is a high linear
low noise amplifier since it works with a supply voltage higher
than the nominal (2 V), and the second stage does the voltage
to current conversion. In [14] the LNTA has only one stage
which reduces the power consumption, but requires a cascode
at the output to create a high output impedance which limits
the LNTA linearity due to the 0.9 V supply voltage. Both
circuits create the input match using a common-gate transistor
with their sources biased with external inductors. Our proposed
circuit uses a fully integrated single-ended LNTA, without
the need for external inductors or a chunky transformer.
The choice for a single-ended topology is further discussed
in section III.
One of the most interesting features of the passive mixer
is its transparency [19]. Combining the mixer with a BPF,
the impedance seen at the input of the mixer is shaped by
the BPF transfer function (TF). Therefore, interferers that
eventually arrive at the input have lower gain than the main
signal, increasing the linearity of the LNTA and, consequently,
of the receiver. Moreover, the passive mixer creates an anti-
aliasing filter when working as a sampler [18] and it performs
the single-ended to differential conversion easily, without
requiring additional circuitry.
The SC filters operate in the discrete-time (DT) domain.
Their advantages over the continuous-time (CT) filters were
extensively discussed in [6] and [13]. Overall, DT filters, such
as the N-path filters [6], [11], [20] and the charge-sharing (CS)
filters [13]–[16], are passive filters based on SC, being more
linear and less power hungry than CT filters. Although the
N-path filter has a higher Q-factor than the CS filter, its
transfer function has several replicas, while the CS filter
transfer function has only one peak between − f
s
/2and f
s
/2.
These replicas can reduce the blocker rejection or fold blockers
on top of the main signal. Hence, CS filters were chosen
for this design. Since the CS BPF has a low Q-factor [13],
the receiver needs two filter stages to attenuate the image
and the out-of-band interferers. The first filter is a first order
BPF which operates at the same sampling frequency ( f
s
)as
the mixer to properly cancel the aliasing. The second BPF,
on the other hand, does not need to use the same f
s
as the
mixer since the aliasing requirement was already met. Thus,
the clock of the second BPF is reduced, which then decimates
the input signal. In addition, the Q-factor of this BPF is
enhanced by increasing the order of the filter and by a circuit
improvement introduced in our proposed filter, as explained
ahead in section V-B.
The GM-cells shown in Fig. 1 are an intermediate stage
between the first and second filtering stages. These transcon-
ductors are needed to drive the passive second filtering stage
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BAUMGRATZ et al.: 40-nm CMOS WIDEBAND HIGH-IF RX USING A MODIFIED CHARGE-SHARING BPF 3
Fig. 2. The proposed LNTA topology.
like the LNTA drives the passive mixer. Being the fourth
block in the chain, the GM-cells main restriction is linearity.
As depicted in the block diagram of Fig. 1, the receiver clock-
ing requires two non-overlapping clocks, two 25% duty-cycle
clocks and one 12.5% duty-cycle clock, with two different
circuits to generate them. Finally, the output buffers are simple
source-followers that isolate the circuit and provide a 50
output match to the measurement equipment.
III. LNTA
The output impedance is one of the most important non-
idealities of transconductors since it limits the AC current
delivered to the load and the effective V-to-I conversion (GM).
Therefore, the output impedance (Z
LNTA,out
)oftheLNTA
(Fig. 2) has to be higher than the input impedance of the
passive mixer (Z
MX,in
). The higher Z
LNTA,out
/Z
MX,in
ratio,
the better GM implementation.
The LNTA has two cascodes and one folded-cascode which
ensure a high output impedance. The cascodes also improve
the load isolation, so the input match and noise canceling
are immune to any load variation. The use of long channel
transistors for M2 and M3 would also increase the output
impedance without the need for cascodes, but it would harm
the input match at frequencies higher than 1 GHz since C
gs
increases proportionally to the gate length.
The folded-cascode that is created by connecting M6 to the
source of M5 is the best solution for the connection between
M6 and the output. Although M6 can be directly connected
to the output, it would reduce the output impedance.
The 50 input match is provided by M1. Since its
transconductance (g
m1
) is boosted by the local feedback
through M2, the LNTA input impedance is Z
LNTA,in
≈
1
/g
m1
(g
m2
/g
m4
+1),whereg
m2
and g
m4
are the transconductances
of M2 and M4, respectively. However, since the intrinsic gain
(g
m
/g
ds
) for a minimum length transistor in this technology is
about 5 V/V, g
ds1
cannot be neglected, and C
gs2,3
are also not
negligible at high frequencies. The input impedance including
those effects is given by
Z
LNTA,in
=
1
g
m1
(g
m2
/g
m4
+ 1)
1 + g
ds1
R
D1
+
g
ds1
1 + g
ds1
R
D1
+ s
C
gs2
+ C
gs3
,
(1)
Thus, Z
LNTA,in
is designed to be higher than 50 to
compensate for the parasitic impedances that appear in parallel
with Z
LNTA,in
and reduce its value.
The noise canceling technique cancels only the noise of the
transistor responsible for the input matching [21]–[23], which
is M1 in this design. Hence, the noise generated by the auxil-
iary amplifier, which is initially M2 in this design, remains a
full contributor to the overall noise figure, and it needs to be
reduced by traditional means, like increasing the g
m
.
In order to further reduce noise figure, the noise of M2 is
fed back to the input and amplified through a second auxiliary
amplifier. Thus, the noise of M2 is partially canceled as
explained further. The local feedback is created by connecting
the drain of M2 to the gate of M1 [24], [25], which also
boosts the g
m
of M1. The second auxiliary amplifier is
created by using current-reuse [26], which saves power. Hence,
M3 is added to the circuit. As a result, this LNTA topology
completely cancels the noise of M1 and partially cancels the
noise of M2, which are the major noise sources of the circuit.
After the noise cancellation, M3 and R
D1
are the dominant
noise sources of the LNTA. The noise contribution of M6 is
reduced by the gain g
m1
R
D1
and can thus be neglected.
The contribution of M1 and M2 to the noise factor are
represented as follows:
F
M1
≈ γ g
m1
R
S
g
m2
+ g
m3
− g
m6
G
m1
R
D1
g
m2
+ g
m3
+ g
m6
G
m1
R
D1
2
, (2)
and
F
M2
≈ γ g
m2
R
S
⎡
⎢
⎢
⎣
g
m1
1 + G
m1
+ G
m1
R
D1
g
m6
−
g
m3
g
m4
g
m2
+ g
m3
+ g
m6
G
m1
R
D1
⎤
⎥
⎥
⎦
2
.
(3)
G
m1
is g
m1
(g
m2
/g
m4
+ 1), γ is the noise coefficient, and R
s
is the signal source resistance, which is intended to be equal
to Z
LNTA,in
.
It can be deduced from (2) that the noise contribution
of M1 is zero if g
m2
+ g
m3
= g
m6
G
m1
R
D1
and from (3)
that the noise contribution of M2 is zero if 1 + G
m1
+
G
m1
R
D1
g
m6
=
g
m3
g
m4
. However, these conditions cannot be
achieved simultaneously. The best choice, therefore, is to fully
cancel the noise of M1 since it is the primary noise source and
only cancel the noise of M2 partially.
The proposed single-ended topology has two advantages
over its differential counterpart: a lower power consumption
and a larger GM while keeping the noise canceling con-
dition. The single-ended topology needs R
D1
andM6to
invert the polarity of both signal and noise. This additional
stage increases the degree of freedom of the design since
it decouples the values of g
m2
, g
m3
,andG
m1
.Inthe
differential version of this LNTA, the noise cancellation
condition would be g
m2
+ g
m3
= G
m1
since neither
M6 nor R
D1
are present. Hence, the values for g
m2
and g
m3
would be limited by G
m1
. The single-ended version, on the
other hand, does not have this limitation. Not being lim-
ited by G
m1
, g
m2
and g
m3
can be set to much higher
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4 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS–I: REGULAR PAPERS
TABLE I
LNTA S
IZING PARAMETERS
values which further reduce their noise and increase the
transconductance.
In the single-ended topology of this LNTA, the main prob-
lem is the linearity since both M6 and the cascode transistors
(M4 and M5) impose limits. The former increases the signal
distortion because M1 and R
D1
have already amplified the
signal. The latter reduces the linearity due to the four stacked
transistors within a supply voltage of 0.9 V. Furthermore,
the small V
DS
across those stacked transistors increases the
value of their distortion terms. In particular, the distortion of
M4 and M5 cannot be neglected [27].
Therefore, the gain of M6 is kept below one to minimize
distortion. The g
m
/I
D
of M1 and M2 have been selected in
such a way that their third-order distortion terms cancel each
other. The former is biased in strong inversion, and the latter in
moderate inversion [28]. Additionally, M4 and M5 are biased
like in [27] to achieve an optimal trade off between linearity
and noise.
The DC voltage at the output node is kept constant by the
DC-control block. In spite of any variation on the V
DS
of the
cascode, the DC output voltage remains constant at VDD/2,
which maintains these transistors in the selected operation
point.
Table I summarizes the sizing of the transistors, and Fig. 3
shows post-layout simulation results. The LNTA was simu-
lated using a 50 load that corresponds to the minimum
input impedance of the mixer. The simulated NF is below
2.3 dB within the entire band, and it has a minimum value
of 1.8 dB. The gain varies from 17 to 13.5 dB at 0.2 and
4 GHz respectively. Also, the input reflection coefficient (S11)
remains below −10 dB over the entire considered RF band.
Moreover, the LNTA has shown a small variation in corners
as presented in Fig. 3. The worst case NF remains below
3 dB at 125
◦
C which is the worst case corner for noise.
Meanwhile, the voltage gain drops only 2 dB in the SS corner.
Due to parasitic capacitances, the output impedance of the
LNTA reduces at higher frequencies. Consequently, the ratio
Z
LNTA,out
/Z
MX,in
is reduced, which hampers the driving of
the mixer, and compromises the overall RX gain.
IV. F
IRST FILTERING STAGE
The combination of a passive mixer and a charge-
sharing (CS) SC filter is beneficial since the mixer, if properly
designed, cancels the aliasing created by the SC architecture.
However, the aliasing cancellation happens only if the peaks
from the BPF filter are aligned with the nulls from the mixer;
thus their f
s
have to be equal or f
BPF
s
=
f
MX
s
2n
for n ∈ N
∗
,
i.e. using clock decimation for the BPF. The decimation is
avoided here since it would increase the noise figure due to
noise folding. As a result, the first BPF works at a high f
s
,
the same as the mixer, narrowing down the topology of choice
Fig. 3. LNTA simulation results for temperature corners 27
◦
, 125
◦
,and−40
◦
and process corners typical-typical (TT), fast-fast (FF), fast-slow (FS),
slow-fast (SF), and slow-slow (SS).
Fig. 4. Transfer Function of the mixer and BPF when f
s
= 4GHz.
to the 1st order BPF with 4 phases (BPF 4/4) [13], which
works well at high frequencies. Since the mixer has to match
the number of phases of the BPF, this design uses a 4-phase
passive mixer. Also, the 25 %-duty-cycle non-overlapping
clock drives the mixer, preventing I-Q crosstalk [18]. In addi-
tion, the mixer sampling frequency is four times the
LO frequency.
A. Mixer
The anti-aliasing filter is created only if the mixer, which
is presented in Fig 6, works as a sampler when its transfer
function is a sinc function [18],
H( f ) =
sin(π fT
s
)
π fT
s
, (4)
where T
s
is the sampling period and f the frequency in Hz.
Fig. 4 shows the TF of the anti-aliasing filter (dotted black
line). When combined with the BPF TF, the only remaining
peak is at the central frequency of the BPF while all the DT
replicas are attenuated.
A consequence of the mixer transparency is the
up-conversion of the baseband impedance, in this case an IF
impedance. Thus, its input impedance is a function not only
of the resistance of the switches, but also of the IF impedance,
which is a BPF filter in this design. According to [19],
the input impedance of a passive mixer is also affected by
the harmonics of the clock driving the switches. A resistor in
parallel with the IF impedance adds this effect to the electrical
model as can be seen in Fig. 5a. Moreover, the resistance
of only one switch has to be taken into consideration since
the switches are not closed at the same time due to the non-
overlapping clocks.
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BAUMGRATZ et al.: 40-nm CMOS WIDEBAND HIGH-IF RX USING A MODIFIED CHARGE-SHARING BPF 5
Fig. 5. (a) The passive mixer electrical model [19]. (b) The variation of the
input impedance of the mixer with the LO frequency and (c) the value of C
R
programmed for each case. The comparison of the calculated and simulated
input impedance of the mixer at (d) 500 MHz and (e) 1 GHz.
Fig. 5a shows the electrical model proposed in [19], where
R
sw
is the resistance of the switches, R
sh
models the effect of
the clock harmonics on the input impedance, Z
IF
represents
the IF impedance, i.e. the BPF 4/4 input impedance, and ζ
is the impedance up-conversion constant. Hence, the input
impedance of the passive mixer is [19]
Z
MX,in
= R
sw
+ R
sh
||ζ Z
IF
, (5)
where ζ ≈ 0.203 and R
sh
≈ 4.3(R
LNTA,out
+ R
sw
) [19];
R
LNTA,out
is the real part of the output impedance of
the LNTA. Eq. (5) shows that Z
IF
and R
sh
have a considerable
effect on the input impedance of the mixer. Based on (5),
the input impedance of the mixer can be predicted as shown
in Fig. 5b. Since the input impedance of the BPF 4/4 decreases
with the LO frequency increase, the input impedance of the
mixer also drops, thereby reducing the overall gain. The
gain drop can be compensated by changing the value of the
capacitor C
R
(Fig. 5c) which controls the input impedance of
the BPF 4/4 and enable a flat gain response. The values pre-
dicted with (5) are reasonably accurate as observed in Fig. 5d
and Fig. 5e.
The mixer and BPF 4/4 will properly work as long as
Z
LNTA,out
Z
MX,in
, so that the Z
MX,in
is dominant.
As discussed in [18], the gain and the null depth of the
mixer are reduced if the previous condition is not fulfilled.
Henceforth, the aliasing cancellation, which is generated by
these nulls, will be limited.
There are two main reasons to reduce as much as pos-
sible Z
MX,in
: it increases the bandwidth of the LNTA, and
Z
MX,in
has to accommodate for any reduction on Z
LNTA,out
Fig. 6. Schematic of the passive mixer and the BPF 4/4.
that happens at high frequencies due to parasitic capacitances.
Otherwise, the mixer and the BPF 4/4 will not work properly at
these frequencies. Nevertheless, Z
MX,in
ends up being limited
by the CMOS technology since it is directly dependent on the
resistance of the mixer and BPF 4/4 switches.
The noise factor of the mixer and the BPF 4/4 can also be
calculated from the model in Fig. 5a and is given by
F
MX&BPF
= 1 +
v
2
n,sw
v
2
n,R
LNT A,out
+
v
2
n,sh
v
2
n,R
LNT A,out
R
LNTA,out
+ R
sw
R
sh
2
+
v
2
n,IF
v
2
n,R
LNT A,out
R
LNTA,out
+ R
sw
ζ Z
IF
2
(6)
where
v
2
n,sw
is the noise voltage of the mixer switches, v
2
n,sh
is the noise voltage of R
sh
,andv
2
n,IF
is the up-converted
noise from the IF stage, i.e. ζ
v
2
n,BPF4/4
. Eq. (6) shows
the BPF 4/4 as the main contributor to this noise factor.
Since increasing ζ Z
IF
affects the input impedance of the
mixer, the noise factor can only be effectively reduced by
reducing
v
2
n,IF
. In [19] the analysis has been done considering
the interface with a 50 antenna, which is not the case in
this design. Thus the model has to be adjusted to the LNTA
interface. For simplicity, hereafter, only the real part of the
output impedance of the LNTA will be considered.
B. First-Order Bandpass Filter
The main advantage of the BPF 4/4 is its high-frequency
operation; and, its main disadvantage is the low Q-factor
(ideally 0.5). The low Q-factor of the BPF 4/4 limits linearity
improvement on the LNTA. Fig. 6 shows the BPF 4/4, where
C
H
is the history capacitor, that stores the charge until
sampling and C
R
is the rotating capacitor, that shares the
charges with other branches.
The complex Z-domain transfer function of the
BPF 4/4 is [13]
H
4/4
(z) =
V
out
q
in
=
k
(1 − αz
−1
) − j (1 − α)z
−1
, (7)