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Basics of Designing a Digital Radio Receiver (Radio 101)
Brad Brannon, Analog Devices, Inc.
Greensboro, NC
Abstract: This paper introduces the basics of designing a
digital radio receiver. With many new advances in data
converter and radio technology, complex receiver design has
been greatly simplified. This paper attempts to explain how to
calculate sensitivity and selectivity of such a receiver. It is not
by any means an exhaustive exposition, but is instead a primer
on many of the techniques and calculations involved in such
designs.
Many advances in radio design and architecture are now
allowing for rapid changes in the field of radio design. These
changes allow reduction of size, cost, complexity and improve
manufacturing by using digital components to replace un-
reliable and in-accurate analog components. For this to
happen, many advances in semiconductor design and
fabrication were required and have come to fruition over the
last few years. Some of these advances include better
integrated mixers, LNA, improved SAW filters, lower cost
high performance ADCs and programmable digital tuners and
filters. This article summarizes the design issues with and the
interfacing of these devices into complete radio systems.
What is the radio?
Traditionally, a radio has been considered to be the ‘box’ that
connects to the antenna and everything behind that, however,
many system designs are segmented into two separate sub-
systems. The radio and the digital processor. With this
segmentation, the purpose of the radio is to down convert and
filter the desired signal and then digitize the information.
Likewise, the purpose of the digital processor is to take the
digitized data and extract out the desired information.
An important point to understand is that a digital receiver is
not the same thing as digital radio(modulation). In fact, a
digital receiver will do an excellent job at receiving any analog
signal such as AM or FM. Digital receivers can be used to
receive any type of modulation including any analog or digital
modulation standards. Furthermore, since the core of the
digital processor is a digital signal processor (DSP), this
allows many aspects of the entire radio receiver itself be
controlled through software. As such, these DSPs can be
reprogrammed with upgrades or new features based on
customer segmentation, all using the same hardware.
However, this is a complete discussion in itself and not the
focus of this article.
The focus of this article is the radio and how to predict/design
for performance. The following topics will be discussed:
1. Available Noise Power
2. Cascaded Noise Figure
3. Noise Figure and ADCs
4. Conversion Gain and Sensitivity
5. ADC Spurious Signals and Dither
6. Third Order Intercept Point
7. ADC Clock Jitter
8. Phase Noise
9. IP3 in the RF section
Single-Carrier vs. Multi-Carrier
There are two basic types of radios under discussion. The first
is called a single-carrier and the second a multi-carrier
receiver. Their name implies the obvious, however their
function may not be fully clear. The single carrier receiver is a
traditional radio receiver deriving selectivity in the analog
filters of the IF stages. The multi-carrier receiver processes all
signals within the band with a single rf/if analog strip and
derives selectivity within the digital filters that follow the
analog to digital converter. The benefit of such a receiver is
that in applications with multiple receivers tuned to different
frequencies within the same band can achieve smaller system
designs and reduced cost due to eliminated redundant circuits.
A typical application is a cellular/wireless local loop
basestation. Another application might be surveillance
receivers that typically use scanners to monitor multiple
frequencies. This applications allows simultaneous monitoring
of many frequencies without the need for sequential scanning.
LNA
X
Select
Filter and
Gain
X
Select
Filter and
Gain
BPF
DSP
Freq.
Synth.
Freq.
Synth.
ADC
Typical Single-Carrier Receiver
Typical Multi-Carrier Receiver
Benefits of Implementing a Digital Radio Receiver
Before a detailed discussion of designing a digital radio
receiver are discussed, some of the technical benefits need to
be discussed. These include Oversampling, Processing Gain,
Undersampling, Frequency planning/Spur placement. Many of
these provide technical advantages not otherwise achievable
with a traditional radio receiver design.
LNA
ADC
FREQUENC Y
SYNTHESIZER
ANT
BPF
WIDEBAND
CONVERTER
CHANNELS 1 – n
NCO
LPF
LPF
DSP
INCLUDES:
CHANNE L ENC.
CHANNE LDEC.
EQUALIZATION
NETWORK
INTERFACE
AMP
BPF
NCO
LPF
LPF
DSP
INCLUDES:
CHANNE L ENC.
CHANNEL DEC.
EQUALIZATION
NET WOR K
INTERFACE
ATT N
Matr i x