White Paper
Integrated Multi-band and Multi-standard
Broadcast Receiver IC
by
C. Bunney, G. Sumerling and G. Napoli of SWINDON Silicon Systems
Introduction
The Digital Audio Broadcasting (DAB) system, based on Eureka 147, has been adopted by
many countries in Europe and across the world and is becoming the standard for high quality
digital audio broadcasting. This is being coordinated by WorldDMB. DAB uses Orthogonal
Frequency Division Multiplexing (OFDM) to transport audio and data services [1].
The standard has been updated to include Digital Multimedia Broadcasting (DMB), DAB+ and
European terrestrial digital TV broadcast (DVB-T). DMB and DVB-T includes the broadcasting
of live video content [2]. DAB (DMB) operates worldwide in two frequency bands: Band III
(170 to 240MHz) and L Band (1.452 to 1.492GHz).
Digital Radio Mondiale (DRM) [3] is an COFDM digital broadcasting system. It offers audio
qualities rivaling FM over frequency bands traditionally used for AM transmission: LW, MW
and SW/HF (150KHz – 27MHz). DRM is under going a high profile and rapid launch in
Europe and receiving significant interest around the world.
Legacy AM and FM (88 – 108MHz) systems will be operational for some time.
A multi-band multimode receiver is required to receive all the aforementioned standards. The
chip discussed in this paper is the RF front end receiver which accepts input from the
antenna, and provides a digitized output signal for a digital demodulator. The IC is designed
to be compatible to Software Defined Radio solutions and offers a broad range of control and
performance measurement options. To realise a complete broadcast receiver, the IC only
requires the addition of an off-the-shelf DSP, audio DAC, and a few passive components.
Figure 1 - Block Diagram of the Receiver IC.
Loop
Filter
DAB / DRM Receiver Block Diagram
8-bit
DAC
ML1
regulator
FM
(88-108MHz)
22.4MHz Xtal
3 wire ctrl interface
vco
F1
LOHF_
I
LOHF_Q
Synth.Ref
VCXO
Frequency Synthesiser
ML1
TRIM
CTRL
Calibration Tone
Generator
Control
Interface
DAB Band 3
(174-240MHz)
serial
readback
Serial control
data output
MB1
LOLF_I
LOLF_Q
MB1
I
Q
Analogue
Switching
Matrix
Fsync
10
I/Q Baseband
Programmable biquad
filters
BW: 200Khz - 768Khz
ADC
Serial I/F
Data
DAB L Band
(1452-1492MHz)
Frame sync
(Differential
interface)
data ouput
(Differential
interface)
High Speed CLK
from DSP
module
mixer o/p
select
AGC
detector
(prechannel
filtered)
programmable
divider
F1x2
LO DRM
MW/DRM
520kHz-1.6MHz
LW/DRM
150kHz - 280kHz
Low IF = 200Khz
TRIM
CTRL
45 MHz
xtal filter
(ext)
September 2010, SWINDON
Sampling
VCXO
Crystal Oscilllator
output
(differrential)
24.576MHz Xtal
Frequency Conversion
To minimize the component count while achieving the required performance, a range of
internal frequency conversion methods are used:
a) zero IF approach for L Band and Band III
b) Low IF for FM
c) Double conversion for DRM (and AM).
Separate I/Q mixers are provided for L Band; common I/Q mixers for DAB Band III and FM
and 2 mixers for the double conversion DRM.
All mixer outputs are routed to a programmable baseband filter. In the DRM case the routing
is via an external 45MHz bandpass filter to remove signals at the image frequency. The on
chip filter can be software configured as an I/Q 768KHz low pass for DAB, an I/Q 200KHz
bandpass for FM (centred at 100kHz), or a 20KHz bandpass for DRM (centred at 200KHz).
After filtering the signal is digitized in a 10bit ADC. To minimize the number of output data
pins, data is multiplexed into a single bit stream with associated start bit. Two crystal
maintaining circuits are included for the synthesizer reference and the ADC sampling clock.
Receiver IC functionality and dynamic parameter trimming is controlled by the DSP using
information made available from the IC via the three wire digital control interface. The DSP
monitors both the output of the mixers and the ADC and sets the receiver dynamic range
accordingly by adjustment of the LNA, mixer and filter gains. The DSP also adjusts the DAB
and FM I/Q amplitude and phase balance to maximize the rejection of signals at the image
frequency.
Balanced differential circuitry is used throughout excepting the interfaces to the two crystals
and to the DRM bandpass filter. The use of differential circuitry has enabled the necessary on
chip isolation between the serial digital outputs (2 volt swing) and the various high sensitivity
radio inputs (~ 1µV to 3 µV). The IC power consumption is dependent on the radio mode; in
digital reception modes it is kept low by only powering up during the required transmission
signal burst.
LNA
The LNA design concept for all bands is shown in Figure 3. A wide dynamic range is achieved
by implementing each LNA as a set of four switched amplifiers with fixed gains of 19, 9, -1,
and -11 dB. The linearity is higher in the lower gain amplifiers. The LNA achieved a noise
figure of 2.2 dB at a gain of 19dB, and an IIP3 of +18.5 dBm at a gain of -11dB. The mixers
are straightforward balanced mixers.
There are 6 low noise amplifiers (LNA) to cover L Band, Band III, FM, SW, MW, and LW.
Figure 3 – LNA Design Concept
LNA o/p
Amp 1
Amp 2
RF I/P
Unity gain buffers
Amp 3
Amp 4
Amp 1
Amp 3
Amp 2
Gain
Control
Inputs
September 2010, SWINDON
Amp 4
Filtering
The baseband filter is a set of eight software configurable Sallen-Key biquad sections. The
th
DAB and FM filtering is implemented as I/Q 8 order 0.25dB ripple Chebyshev low pass. The
th
DRM filtering is implemented as a single 8 order 0.25dB ripple Chebyshev bandpass. To
overcome the DC offset problems inherent in Zero and Low IF architectures, the filter has an
offset cancellation loop. This acts as a single pole high pass transfer function on the filter
which completes the FM bandpass filter. By setting the effective high pass to <500Hz, the
effect on the DAB OFDM signal is minimal as it only cancels out a null sub-carrier [4]. A fast
settle mode allows the offset cancellation loop to settle within 1 time frame of the DAB packet.
Figure 4 shows the frequency response of the DAB, FM and DRM filters.
In all operating modes the adjacent channel rejection (ACR) performance is heavily
influenced by the bandwidth accuracy of the filters. The bandwidth is a function of on chip
resistor and capacitor accuracy and varies with process spreads; tuning is achieved using
binary trimmed resistors. This is also controlled via the DSP. To aid easy and accurate
trimming an in band and an out of band calibration frequency are generated on-chip.
Frequency Synthesis
The synthesizer controlled PLL uses a single on chip VCO to generate the local oscillator
(LO) frequencies for all reception modes. The VCO operates from 2.6GHz to 3.4GHz and is
tuned by a combination of switched MIM capacitors and MOS varactors. There are three
programmable counters, Figure 5, which together enable a high loop bandwidth and a high
comparison frequency (~200kHz) whilst maintaining fine control of the channel centre
frequency (DAB 16kHz, FM 10KHz, AM 1KHz). The PLL loop gain is controlled via a
programmable charge pump. The reference crystal oscillator can be adjusted over a 100ppm
range with a resolution of 0.025ppm using by a combination of switched MIM capacitors and
MOS varactors (the ADC reference oscillator is identical). The fine control is required for
synchronization to the broadcast reference in the digital reception modes. Figure 6 illustrates
the phase noise of the DAB L band LO. Figure 7 demonstrates the low close in LO phase
noise required for DRM, -90dBc/Hz has been achieved at 25Hz offset from carrier.
Receiver IC Layout
The IC has been fabricated on a 0.35um SiGe BiCMOS process, Figure 2. The process
features include ~1um thick fourth layer metal, MIM capacitors and bipolar transistors with an
Fmax of 50GHz. The die is packaged in an 8x8mm QFN.
Figure 2 – IC Layout
LNAs and Downconverters
September 2010, SWINDON
Reference
Xstal
Oscillator
Sampling
Xstal
Oscillator
ADC and
Output serialiser
VCO and
Synthesiser
Control logic
Baseband Filter
References
[1] ETSI EN 300 401 v1.3.3 (2001-05), ‘Digital Radio Broadcasting Systems; Digital
Audio Broadcasting (DAB) to mobile, portable and fixed receivers’
[2] ETSI TS 102 428 v1.1.1 (2005-06), ‘Digital Audio Broadcasting (DAB); DMB video
service; User Application Specification
[3] ETSI ES 201980 v2.1.1 (2004-06), ‘Digital Radio Mondiale (DRM); System
Specification’
[4] PCT Patent application:
SWINDON Silicon Systems
The full turnkey mixed signal ASIC solution provider specialising in electronic
systems with a core competency on sensor interfaces and ultra low power,
enabling products that address widespread needs within Automotive, Industrial,
Medical and Consumer applications.
Contact us on +44(0) 1793 649400 or sssl@sssl.co.uk
September 2010, SWINDON