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RF Circuit Design
Chris Fuller
[email protected]
952-607-8506
11/7/2012
Design Process
Define Requirements
Design Prototype
Design Review
Build
Test
Analyses
Review
Iterate Design Process
Define Requirements
Communication distance
Data Rates including security
Physical space available
Available battery energy
Communication media: air, metal, tissue
Unit Price goal
Available development time
Cost/Availability of components
Interference tolerance/likelihood
Operating Frequency
Many others
Overview of Radio Communications
Basic transceiver components:
Antennas, Amplifiers, Mixers, Filters,
Synthesizer, Baseband Processing
Components
Antennas: Interfaces communication media (air, body, etc.) to
transceiver
PA (Power Amplifier): Boosts modulated transmit signal
LNA (Low-Noise Amplifier): Boosts signal sensed at antenna while
adding little noise to the desired signals.
RF Filters: Passes desired RF modulated signals & blocks undesired
signals.
IF Filters: Blocks undesired signals from received signals.
Synthesizer: Reference RF frequency used to convert from
baseband to RF or from RF to baseband.
– Usually very accurate frequency & low-noise
Mixers: Converts baseband signal into a representation of the
baseband signal at an RF frequency (and vice versa).
– Based on trigonometric identity:
Baseband: source and destination for data.
Why is RF Not Easy? Parasitics
Capacitor model for low
frequency circuits
Minimum Capacitor model for
radio frequency circuits
• Capacitor values and their parasitics change in complex
ways as they age and with varying voltages, temperatures,
humidity, vibration levels, etc.
• Slight changes in capacitor values and parasitics can cause
great changes in circuit performance.
• Other types of component types are similarly affected (e.g.
transistors, inductors, resistors, etc.)
Why is RF Not Easy? Component size ≈ λ
λ/4 Long Circuit Board Traces
with Open and Short
Terminations
Open Circuit becomes a short
& Short Circuit becomes open
Effects of component size ≈ λ
– Circuit layout more important
– Components using circuit
traces (e.g. Wilkinson Power
Divider)
Why is RF Not Easy? Super-Sensitivity
Typical cell phone: sensitive to less than 10-12 Watts!
Example self-generated noise interference:
Factors critical for good sensitivity performance:
– Very low impedance ground
– Isolation/protection from power supply
– Isolation/protection from noisy (e.g. digital) circuits
– Shielding of circuitry from external fields
I=J*E formula integral form
Typical RF Tests
Frequency Accuracy: Operating frequency
Output Power: Actual versus design
Sensitivity: Input signal where receiver begins to
no longer detect the received signal.
Noise Figure: How much noise is added to the
received signal.
Selectivity: Ability to only detect desired signal
over undesired signal.
Dynamic Range: Signal level over which the
output signal is a good replica of the input signal.
– Low sensitivity end of range: Thermal and selfgenerated noise floor and environmental.
– High sensitivity end of range: Non-linearities
(amplifiers, mixer, etc.)
RF Stability
Feedback from:
Step 4: Output increases until:
-Circuit components
-Device destruction
-Circuit board & traces
-Impurities
Step 3: Input and
feedback overlap and
add together maximally
INPUT
-Power supply limits
FEEDBACK
-Uncontrolled oscillation
Step 2: Part of amplified
signal is fed back to input of
the amplification device.
+
AMP
OUTPUT
Step 1: Input signal is amplified
Instability = loss of control
Instability = unpredictable affects
– May prevent other circuits from behaving properly
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Stability tests
Monte Carlo simulation of circuit
– Verify stable vs. production tolerances
Load pull instability tests
– Vary circuit impedances to detect instabilities
Opas sweep tests
– Large and small signal stimulate circuit to verify stable
On-board stability tests
– Measure small signal reflections to verify stability
S-parameter stability tests
– Measure circuit characteristics to verify stable
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Example Single Chip Radio
- Microsemi/Zarlink
Example Single Chip Radio
- Microsemi/Zarlink
Example Single Chip Radio
- Texas Instruments CC1020
Frequencies: 402 to 470 MHz, 804 to 960 MHz
Bandwidths: 12.5 kHz and 25 kHz
Price < $9 (one quantity)
Example Single Chip Radio
- Analog Devices ADF7020-1
Frequencies: 135 to 650 MHz
Maximum data rate: 200 kbps
Price < $6 (one quantity)
Conclusions
Design process for RF products similar to other
products.
Components used in RF design implement relatively
simple functions.
RF design is complex (in part) because of complex
parasitics and wavelength effects.
Radio level tests required to ensure specifications
and regulations being met.
Some examples of highly integrated, low-cost single
chip radios described.
RF DESIGN IS COMPLEX, BUT LESS SO IN RECENT YEARS THANKS TO
LOW-COST SINGLE-CHIP RADIOS.
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