Fig. 1: Receiver Architecture. An Interstage Filter-Free Mobile Radio Receiver with Integrated TX. Leakage Filtering. Rastislav Vazny1, Werner Schelmbauer1, ...
RMO1A-4
An Interstage Filter-Free Mobile Radio Receiver with Integrated TX Leakage Filtering Rastislav Vazny1, Werner Schelmbauer1, Harald Pretl1, Stefan Herzinger2, and Robert Weigel3 1
Danube Integrated Circuit Engineering GmbH, A-4040 Linz, Austria 2 Infineon Technologies AG, 81677 Munich, Germany 3 Institute for Electronics Engineering Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstr. 9, D-91058 Erlangen, Germany; 81677 Munich, Germany Transceiver IC
Abstract — A multi-standard, multi-band fully-integrated interstage filter-free receiver with integrated auto-centered TX leakage filtering is fabricated in a 65 nm CMOS technology. The measured TX selectivity in UMTS band II is 9.1 dB, the receiver gain at 1.96 GHz is 54.1 dB, and NF is 3.68 dB. The 0.5 dB reference sensitivity degradation caused by TX leakage is reached at a TX power level of 20 dBm/3.84MHz. Index Terms — UMTS receiver, On-chip filtering
UMTS Band I
TX RX
PA
A/D LNA
D F E
Gm 90° 0° A/D
PA LNA
I. INTRODUCTION
90° 0°
from TX VCO
UMTS Band II GSM1900
In recent years, the removal of external interstage SAW (Surface Acoustic Wave) or BAW (Bulk Acoustic Wave) filters together with the external LNA’s (Low Noise Amplifier) has become a mandatory feature for a state-ofthe-art full-duplex capable wireless receiver. Since the duplex filters which are currently available on the market can provide only certain isolation of around 55 dB between the Receiver (RX) and Transmitter (TX) the linearity and noise requirements of the RX has to be increased. The design for such tough requirements results in increased current consumption. An alternative way of implementation is the usage of on-chip filtering which relaxes the RX block requirements. In the past, several attempts have been made to implement on-chip TX leakage filtering with Q-enhanced filters [1], [2]. The technical feasibility of such high-Q integrated filters is questionable since the centering of the filter and also the control of the Q value are very problematic, especially over PVT conditions. In this paper we present an interstage filter-free receiver with automatically centered and fully integrated TX leakage filtering which can be turned off in case of reduced TX output power.
Fig. 1: Receiver Architecture.
allows co-banded operation, i.e. UMTS Band II and GSM1900 operation is possible using one physical input port. The inductively degenerated LNA topology has been chosen due to its good linearity properties with reasonable power consumption [3]. All LNA’s are connected in the current domain, and are sharing one common LC tank. The center frequency of the LC tank is tuned by switching capacitors in parallel to the inductor. Therefore the LC tank can cover a wide tuning range of 800 MHz. The TX leakage signal coming from the power amplifier (PA) is filtered out before reaching the mixer stage by the integrated notch filter, which is centered by the internal TX local oscillator (LO) signal. A. Requirements Analysis shows, that the TX leakage signal at the RX input port, compared with other blocking signals from the 3GPP specification [4], represents the strongest blocking signal for the RX. Assuming a duplex filter with TX to RX selectivity performance of -55 dB and 4.5 dB TX front-end insertion loss, a TX leakage signal PTX of 26.5dBm/3.84 MHz is present at the RX input at TX output power of 24 dBm at the antenna. One of the major drawbacks of the DCR receiver architecture are the tough second order linearity
II. ARCHITECTURE AND IMPLEMENTATION In Fig.1 the proposed multi-standard multi-band direct conversion receiver (DCR) architecture with integrated TX leakage filtering is shown. The receiver architecture
978-1-4244-6241-4/978-1-4244-6242-1/ 978-1-4244-6243-8/10/$26.00 © 2010 IEEE
VDD
21
2010 IEEE Radio Frequency Integrated Circuits Symposium
Mixer load
requirements [5]. For a worse-case sensitivity degradation of 0.5 dB due to TX IM2 noise, the TX IM2 noise should be at least 9 dB below the RX noise level of -104.1 dBm (Thermal noise + RX noise figure). The required RX IIP2 can be calculated as:
Rf R1
L1
VDD C5
Zin
LNA out
C1
- + + -
LNA out_x
R2
IIP2TX = 2(PTX − 3) − N IM 2 + CF IM 2 = 2(-26.5dBm - 3) + 113.1dBm - 7.7dB
Quadrature mixer
Rf
C2
Rf
(1)
R1
= 46.4dBm
- + + -
where CFIM2 of -7.7 dB is the correction factor between two tone IIP2 characterization and real modulated UMTS UL channel [5].
R2
90° /2 0°
N3
Vcasc
N4
Rf Tx VCO Tx VCO_x
B. On-chip TX leakage filtering
RFin
N1
N2
RF inx
Basically the TX leakage filtering, as depicted in Fig. 2, relies on the impedance translation properties of a passive mixer. The notch filter (or band-stop) topology has been chosen since the effect on the wanted signal gain of the receiver is less pronounced than in a band-pass architecture. From the equations presented in [6] and after some mathematical operations, the input impedance ZRF, of a passive mixer terminated with a baseband impedance ZBB can be calculated (for the fundamental harmonic) as:
Z RF =
4
π
2
[Z
BB
(ω LO + ω m ) + Z *BB (ω LO − ω m )]
L2
Fig. 2: LNA with integrated TX leakage filtering (biasing not shown).
implementation a class-AB operational amplifier has been chosen. Unfortunately, the mixer doesn’t translate only the impedance from fLO - fTX, but also from the fLO + fTX frequency. To avoid the impedance degradation of the TIA at high frequencies, resistors R1 and R2 have been used. The degradation is caused by the capacitive parasitics of the input transistors of the operational amplifier. By inserting R1 and R2, the open loop gain AOL(ω) is not affected in the low frequency range. In case the phase difference between the TX leakage signal and the LO signal reaches 90°, the notch filter input impedance gets very high and the notch filter doesn't work properly. This issue can be overcome with the use of a quadrature mixer together with two TIA loads as can be seen from Fig. 2. The drawback of such a solution is the doubled power consumption compared to single mixer solution, and the generated high impedance characteristic by the TIA has been halved.
(2)
Note that RSW, the resistance of the mixer switching transistors was neglected. Eqn. (2) shows the translation of the baseband impedance ZBB to the RF domain around the frequency fLO. In case the notch filter is centered at the TX frequency, the LO frequency of the translation mixer should be equal to fLO = fTX. For minimum affect on the RX 1 gain, ZBB at the duplex distance should be ideally infinite ( ZBB(ω=2πfdpx) = ∞). To generate such a baseband impedance characteristic a trans-impedance amplifier (TIA) has been used. The input impedance of a TIA can be written as:
Z in = 1 +
Rf AOL(ω )
(3)
C. Mixer and BB blocks
where Rf is the feedback resistor and AOL(ω) is the open loop gain of the operational amplifier. For a typical AOL(ω) value of 80 dB at ω = 0 and an Rf of 1 kΩ Zin is approximately 0 Ω. At ω = 2πfdpx and AOL(ω) is around 0 dB the resulting Zin is ~ Rf. Therefore with the TIA the desired base-band impedance can be realized. Furthermore with proper control of AOL(ω) of the TIA, the bandwidth of the notch filter can be tuned. For the TIA 1
Ibias
The implemented mixer topology of the receiver consists of a Gm input stage and a passive current mixer [7] as shown in Fig. 3. The passive current mixer has been chosen due to its very good flicker noise performance to fulfill the tough noise requirements in GSM mode, because the signal is directly down-converted to the baseband domain. A TIA is used to convert the current signal to the voltage baseband (BB) signal. The TIA is simultaneously acting as a first channel-select filter. All BB operational amplifiers are implemented as class-AB
In UMTS Band II the duplex distance fdpx = 80MHz.
22
VCO
Mixer LNA’s
Fig. 4 Chip photograph (RX section).
drops about 2.3 dB when the notch filter is switched on. The gain drop in this case is caused by the layout parasitics. Due to the fact, that this gain drop happens in the LNA, the receiver NF increases from 2.9 dB when the filtering is off to 3.68 dB when the TX filtering is powered on. The second and third-order linearity of the RX has been measured by a two tone measurement. The measured halfduplex IIP3 (the first tone is located at 1880 MHz and the second at 1920 MHz) with TX filtering of the receiver is 5 dBm, and is dominated only by the IIP3 performance of the LNA, since the filtering circuit is located at the LNA output. The duplex IIP2 has been measured with tones located at 1879.5 MHz and 1880.5 MHz. The measurement result is shown in Fig. 6. The IIP2 reaches the value of +63.4 dBm, which is a 13.9 dB improvement compared to +49.5 dBm without the TX notch filter. The measurement of reference sensitivity is shown in
Fig. 3 Mixer topology.
amplifiers. The receiver includes also an Analog to Digital Converter (ADC) together with digital front end as presented in [7]. III. MEASUREMENT RESULTS To verify the proposed concepts and theory, an IC was fabricated in a 65 nm standard CMOS technology. The photography of the fabricated IC is shown in Fig. 4. The integrated transmitter is not shown, since it is not a part of this work. The receiver is housed in a wafer-level package. In Fig. 5 the measured receiver gain in UMTS Band II is shown. The measured selectivity at an offset of 80 MHz below the receive frequency is 9.1 dB. The notch filter is automatically centered at the TX frequency due to the impedance translation concept. The measured RX gain
65
56 60
notch on notch off
54
IIP2 (dBm)
Gain (dB)
55
52 50
45
48
40
46 44
50
notch off notch on 1850
1900 1950 Frequency (MHz)
35 -45
2000
Fig. 5. Measured RX gain. Notch filter auto centered at the mid TX frequency of UMTS Band II.
23
-40
Fig. 6. Measured intercept point.
-35 -30 Pin (dBm)
duplex
input
-25
referred
-20
second-order
the TX leakage filtering can be neglected, since the filtering is powered up only for the highest TX output power. All measurement results, together with comparison of other work are summarized in Table I.
-102 notch on notch off
(dBm/3.84MHz)
-103 -104
VII. CONCLUSION -105
A multi-standard, fully-integrated receiver with integrated TX leakage filtering fabricated in 65 nm CMOS technology is presented. The integrated TX leakage filter is automatically centered by design and needs no further tuning or calibration. The integrated filtering allows the removal of the SAW interstage filters, and relaxes the requirements of the duplex filters. Measurements showed, that the present receiver meets the 3GPP specifications with enough margin.
-106 -107
0.5 dB degradation
-108 -35
-30
-25 -20 -15 Tx power(dBm/3.84MHz)
-10
Fig. 7. UMTS Band II reference sensitivity measurement -4 (BER
