Why Not Go Directly to Digital
in Cellular Radios, and
Connect the A/D to the Antenna?
Paul C. Davis
(Retired from Bell Labs)
pcdavis@ieee.org OR
P308davis@aol.com
SSCS Technical Meeting
April 26, 2010
Cellular Radio is Like Finding a Pin in a Football Field
PCD 4/2010
Outline
1. Intro. A/D to Antenna and RF “Secrets”
2. Heterodyne Receivers
3. Moving the A / D
4. Heterodyne Transmitters
5. Tracking Transmitter
PCD 4/2010
What's so tough about designing
a portable digital cellular radio?
Receiver requirements
1. Sensitivity, which translates to IC Noise Figure
2. Blocker-immunity, which translates to Amplifier
Linearity
3. Stand-by time, or current draw while waiting for
a call
Transmitter requirements
1. Noise in the receive band while transmitting
2. Spurious Signals at any frequency
3. Talk time, or battery life while actually talking
PCD 4/2010
Legal vs. Market Requirements
Legal requirements:
Make a radio that works, and sounds good
anywhere, but does not interfere with another radio.
All cellular radios are "type approved" for
sensitivity, interference resistance, and spurious
signals.
Market Requirements:
Make it cheap. Make it small. Make it have a long
battery life. Make it first.
PCD 4/2010
Types of Radio Systems
Cellular Radio
Frequency:
900, 1800 MHz and higher bands
Access: FDMA, TDMA (GSM), and CDMA or combination
Cordless Phones
Frequency:
45, 900, 1900, and 2400 MHz bands
Access: FM and Spread Spectrum (frequency hopping)
LAN (Bluetooth; IEEE 802.11x, could be a, b, g, etc.)
Frequency:
2400 MHz and 5.2 - 5.8GHz
Access: Spread Spectrum (frequency hopping SS, direct
sequence SS and OFDM)
PCD 4/2010
Types of Radio Systems (cont.)
Cellular Radio
Sensitivity: < - 102 dBm
Distortion:
IP3 > -10 dBm
Handset RF Peak Power Out:
0.6 to 2W
Cordless Telephone
Sensitivity: < - 85 dBm
Distortion:
IP3 > -25 dBm
Handset RF Peak Power Out:
10 mW to 250 mW
LAN (Bluetooth; IEEE 802.11)
Sensitivity: < -70 dBm; < -80 dBm
Distortion:
IP3 > -16 dBm
"Handset" RF Peak Power Out: 1mW, 100mW; 1W
PCD 4/2010
General Block Diagram of a Cellular Radio
PA
Dup-
Transmitter
lexer
Receiver
Synthesizer
Reference
Oscillator
Voice
Freq.
Interface
A/D and
D/A Circuits
Microprocessor
Digital
Signal
Processor
(DSP)
PCD 4/2010
Typical GSM (Radio) Receiver with Single IF
71 MHz
Local Osc.
Duplexer
LNA
SAW RF/IF
RF Filter Mixer
SAW
IF Filter IF Amp
I
-/4

Baseband
+/4
Q
• Sample GSM Receiver Requirements (Portable Radio)
• Sensitivity:
•
-102 dBm at input of receiver chain must yield a 0.01 BER
•
(Requires a signal-to-noise ratio (S/N) of ~9 dB)
• Interference Rejection
•
0 dBm out-of-band single-tone blocking signals
•
-23 dBm in-band single-tone blocking signals
•
-43 dBm in-band two-tone blocking signals
PCD 4/2010
Signal Levels (dBm)
Example GSM Receiver System Requirements
for Type Approval
0
Two-tone
Blocking
-43 dBm
-40
-80
Blocking
-23 dBm
Wanted
-102 dBm
-120
fo
+1 MHz
+2 MHz
+3 MHz
Frequency
Req. rej. of two-tone sig. >66 dB. NF at LNA input < +8 dB.
PCD 4/2010
Two-tone Harmonic Generation
Blocking Signals
Wanted signal
fa
fb
Blocking Signals
Frequency
Wanted signal
2fa-fb
fa
fb
2fb-fa
Frequency
C(E1cosat+E2cosbt)3 = fundamentals + 3rd harmonics
(Third-order
+ (3CE12E2)/4 [cos(2at+bt)+cos(2at-bt)]
IM products)
+ (3CE1E22)/4 [cos(2bt+at)+cos(2bt-at)]
From R. S. Carson, © 1990 Wiley
IP3 (Third Order
Intercept Point)
+10
Measured or
Simulated
Fundamental,
Slope = 1.
-10
-30
IM3
Meas. or
Sim. IM3
Products
Slope = 3
Amplitude
Output Power (dBm)
Two Tone IP3 for Narrow Band Circuits
Such as RF IC’s
fc
fd
-50
Measurement
Error (Noise)
-70
-80
-60 -40
-20
0
Input Power (dBm)
fa fb
3fa
Frequency
PCD 4/2010
Typical Single IF Receiver With Level
Diagrams, GSM Example
Duplexer
LNA
SAW RF/IF
RF Filter Mixer
71 MHz
Local Osc.
I
SAW
IF Filter IF Amp
-/4

Baseband
+/4
0
Signal Levels (dBm)
-20
Q
Single Blocking Signal
(fo + 3MHz)
-40
-60
56 dB
Two-tone Blocking Signals
(fo + 0.8MHz and fo + 1.6MHz)
-80
Wanted Signal (fo)
-100
PCD 4/2010
Phase Cancellation Scheme
to Remove Sideband Noise
RF LO
+/2
+/4
RF In
0

Cancelled
Noise

-/4
-/2

IF Out
0
Eliminates a filter at the cost of extra current and complexity.
PCD 4/2010
“Direct Down” Receiver (No IF)
Local Oscillator
is RF Carrier
Duplexer
LNA
SAW
RF Filter
I
-/4

Baseband
+/4
Q
Alcatel has sold millions since 1994.
Watch for DC offset problems with high DC gain (80 dB) and
carrier leakage back through the antenna
PCD 4/2010
A / D to Antenna
S/H
A/D
Digital
Output
For 900 MHz GSM System:
Sample Rate :
Jitter of S / H:
A / D Linearity:
Noise Figure at Antenna:
Competitive A/D Power:
> 1800 MHz
< 3 ps
~19 bits
< 10 dB (< 4nVpp / Hz)
~40 mW
PCD 4/2010
A / D After LNA and RF Filter
Duplexer
LNA
SAW
RF Filter
S/H
A/D
Digital
Output
For 900 MHz GSM System:
Sample Rate :
Jitter of S / H:
A / D Linearity:
Noise Figure at Antenna:
Noise Figure at S/H
Competitive A/D Power:
> 200 MHz
< 3 ps
~15 bits
< 10 dB (< 4nVpp / Hz)
< 27 dB ( < 28 nVpp/Hz)
~25 mW
PCD 4/2010
A / D After RF-IF Mixer
Duplexer
LNA
RF to
SAW
IF
RF Filter
S/H
A/D
Digital
Output
For 900 MHz GSM System:
Sample Rate :
> 142 MHz
Jitter of S / H:
< 20 ps
A / D Linearity:
~15 bits
Noise Figure at Antenna:
< 10 dB (< 4nVpp / Hz)
Noise Figure at S/H
< 20 dB ( < 12.5 nVpp/Hz)
Competitive A/D Power:~10 mW
PCD 4/2010
Latest answer, wrong question?
UCLA students have replaced the IF
filter/mixer with a 10 mW A/D, publ. in:
“The Path to the Software-Defined Radio
Receiver”
Asad A. Abidi, Fellow, IEEE
IEEE JOURNAL OF SOLID-STATE
CIRCUITS, VOL. 42, NO. 5, MAY 2007
© IEEE, 2007
PCD 4/2010
Indirect-up Transmitter
I (Baseband)
Low Pass Filter
(In the Signal
Path)
-/4

IF Local Osc.
Freq. Out
= HF +/- IF

+/4
Output
Amplifier
Q (Baseband)
HF Local
Oscillator
Advantages:
Phase shift is easier and uses less current at IF
Disadvantages:
n X IF spurs in the RF band. High powered output
amplifier.
PCD 4/2010
Direct-up Transmitter
I (Baseband)
-  /4

RF Local Osc.
+ /4

Output
Amplifier
Q (Baseband)
Single sideband modulator uses two mixers and phase
shifter at 900 MHz. (Higher currents needed.)
Watch out for interference from power amplifier to VCO.
PCD 4/2010
Direct-up Transmitter (with Offset Oscillator)
I (Baseband)
IF LO.
-/4

High
Frequency
(HF) Osc.
HF + IF
HF - IF =
RF LO
+/4

Output
Amplifier
Q (Baseband)
High Frequency VCO different from modulated output signal.
Filter for alternate side-band in LO path, not in signal path.
PCD 4/2010
Block Diagram of Tracking Up-conversion Loop
loop
filter
1/R
fREF
counters
1/N
RF
PD
VCO
phase
detector charge
pump
down
mixer
modulator
+
90o
I
Q
From G. Irvine et.al., ISSCC '98, c 1998 IEEE
fLO
Summary and Conclusions
1. Heterodyne receivers, used for decades, are still the
most popular for cellular, cordless, and LAN.
2. Direct-down conversion has become a commercial
reality and reduced the need for A/D at the IF stage.
3. Moving the A/D to the antenna would reduce the
number of filters, and allow Software Defined Radio.
However, the performance requirements are
impractical in today's technology, to compete
successfully.
4. The tracking transmitter technique reduces the
receive band noise in the transmitted signal.
However, it can only be used with constant envelope
modulation.
PCD 4/2010
Terms of Use
The slides and notes in this Power Point
presentation were created by Paul C.
Davis, for a SSCS Section Presentation on
April 26, 2010, and intended solely for the
information and personal use of the
audience and web-site users.
PCD 4/2010