micrf004 Micrel Semiconductor, micrf004 Datasheet - Page 7

• Manufacturer Part Number: micrf004
• Description: Micrf004/micrf044 Qwikradio? Low-power Vhf Receiver
• Manufacturer: Micrel Semiconductor

Functional Diagram
Functional Description
Refer to “MICRF004 Block Diagram”. Identified in the block
diagram are the four sections of the IC: UHF Downconverter,
OOK Demodulator, Reference and Control, and Wakeup.
Also shown in the figure are two capacitors (C
one timing component (CR), usually a ceramic resonator.
With the exception of a supply decoupling capacitor, these
are the only external components needed by the MICRF004
to assemble a complete UHF receiver. Four control inputs are
shown in the block diagram: SEL0, SEL1, SWEN, and SHUT.
Using these logic inputs, the user can control the operating
mode and selectable features of the IC. These inputs are
CMOS compatible, and are pulled-up on the IC.
Sweep-Mode Enable
Logic-input SWEN selects either fixed-mode or sweep-mode
operation. When SWEN is low, the IC is in fixed mode, and
functions as a conventional superheterodyne receiver. When
SWEN is high, the IC is in sweep mode.
Fixed-Mode Operation
For applications where the transmit frequency must be accu-
rately set (that is, applications where a SAW transmitter is
used for its mechanical stability), the MICRF004 may be
configured as a standard superheterodyne receiver (fixed
mode). Fixed-mode operation receives a narrower band-
width making it less susceptable to competing signals. Fixed
mode is selected by connecting SWEN to ground which
forces the on-chip LO frequency to a fixed value. In fixed
mode a crystal (higher frequency tolerance) must be used
instead of a ceramic resonator (lower frequency tolerance).
Data rates beyond 10kb/s are possible in fixed mode.
February 9, 2000
MICRF004/RF044
Resonator
Ceramic
REFOSC
C
AGC
CR
SWEN
CAGC
SHUT
SEL0
SEL1
VDD
ANT
VSS
MICRF004
RF
Amp
Programmable
Synthesizer
Reference
f
Oscillator
RX
Control
f
LO
Logic
f
IF
IF
Amp
Reference and Control
TH
Band-Pass Filter
, C
500kHz
5th Order
AGC
) and
VHF Downconverter
IF
Amp
7
Sweep-Mode Operation
In sweep mode, while the topology is still superheterodyne,
the LO (local oscillator) is deterministically swept over a
range of frequencies at rates greater than the data rate. When
coupled with a peak-detecting demodulator, this technique
effectively increases the RF bandwidth of the MICRF004,
allowing the device to operate in applications where signifi-
cant transmitter-receiver frequency misalignment may exist.
The swept-LO technique does not affect the IF bandwidth,
therefore noise performance is not degraded relative to fixed
mode. The IF bandwidth is 500kHz whether the device is
operating in fixed or sweep mode.
Due to limitations imposed by the LO sweeping process, the
upper limit on data rate in sweep mode is approximately
2.5kb/s.
Examples of sweep-mode operation include applications
utilizing low-cost LC-based transmitters, where the transmit
frequency may vary up to 0.5% over initial tolerance, aging,
and temperature. In sweep mode, the LO frequency is varied
in a defined fashion which results in downconversion of all
signals in a band approximately 1.5% around the nominal
transmit frequency. The transmitter may drift up to 0.5%
without the need to retune the receiver and without impacting
system performance. Similar performance is not currently
available with crystal-based superheterodyne receivers which
can operate only with SAW- or crystal-based transmitters.
In sweep mode only, a range reduction will occur in installa-
tions where there is an undesired competing signal of suffi-
cient strength within of 2% to 3% around the transmit fre-
quency. This is because the process indiscriminately in-
Detector
Control
AGC
Peak
Low-Pass Filter
Programmable
2nd Order
Resettable
Counter
Switched-
Capacitor
Wakeup
Resistor
R
OOK Demodulator
SC
Compa-
rator
MICRF004/RF044
WAKEB
DO
CTH
C
TH
Micrel