MICRF104BM Micrel Inc, MICRF104BM Datasheet - Page 8

• Manufacturer Part Number: MICRF104BM
• Description: IC ASK TRANSMITTER UHF 14SOIC
• Manufacturer: Micrel Inc

Specifications of MICRF104BM

• Frequency: 300MHz ~ 470MHz
• Applications: RKE
• Modulation Or Protocol: ASK, OOK
• Data Rate - Maximum: 20 kbps
• Power - Output: -2dBm
• Current - Transmitting: 12mA
• Data Interface: PCB, Surface Mount
• Antenna Connector: PCB, Surface Mount
• Voltage - Supply: 1.8 V ~ 4 V
• Operating Temperature: 0°C ~ 85°C
• Package / Case: 14-SOIC (0.154", 3.90mm Width)
• Peak Reflow Compatible (260 C): No
• Supply Voltage Max: 4V
• Transceiver Type: RF Transmitter
• Data Rate Max: 20Kbps
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• No. Of Drivers: 1
• Supply Current: 35mA
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Features: -
• Memory Size: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant, Contains lead / RoHS non-compliant

MICRF104
Applications Information
Design Process
The MICRF104 transmitter design process is as follows:
Reference Oscillator Selection
An external reference oscillator is required to set the transmit
frequency. The transmit frequency will be 32 times the
reference oscillator frequency.
Crystals or a signal generator can be used. Correct reference
oscillator selection is critical to ensure operation. Crystals
must be selected with an ESR of 20 Ohms or less. If a signal
generator is used, the input amplitude must be greater than
200 mV
Antenna Considerations
The MICRF104 is designed specifically to drive a loop an-
tenna. It has a differential output designed to drive an induc-
tive load. The output stage of the MICRF104 includes a
varactor that is automatically tuned to the inductance of the
antenna to ensure resonance at the transmit frequency.
A high-Q loop antenna should be accurately designed to set
the center frequency of the resonant circuit at the desired
transmit frequency. Any deviation from the desired frequency
will reduce the transmitted power. The loop itself is an
inductive element. The inductance of a typical PCB-trace
antenna is determined by the size of the loop, the width of the
antenna traces, PCB thickness and location of the ground
plane. The tolerance of the inductance is set by the manufac-
turing tolerances and will vary depending how the PCB is
manufactured.
In the simplest implementation a single capacitor in parallel
with the antenna will provide the desired resonant circuit.
The resonant frequency is determined by the equation:
MICRF104
1). Set the transmit frequency by providing the
2). Ensure antenna resonance at the transmit
3). Set PC pin for desired transmit power.
f
f
correct reference oscillator frequency
frequency by:
a. Either, matching antenna inductance to the
b. Or, matching capacitance with the antenna
TX
P-P
center of the tuning range of the internal
varactor.
inductance by adding an external capacitor (in
series with, or in parallel with, the internal
varactor)
4
32
and less than 500 mV
2
1
CL
f
REFOSC
C
Figure 3.
L
ANTENNA
P-P
.
8
The tolerance in the antenna inductance combined with the
tolerance of the capacitor in parallel with it will result in
significant differences in resonant frequency from one trans-
mitter to another. Many conventional loop antenna transmit-
ters use a variable capacitor for manual tuning of the resonant
circuit in production. Manual tuning increases cost and re-
duces reliability.
A capacitor correctly tuned during manufacture may drift over
time and temperature. A change in capacitance will alter the
resonant frequency and reduce radiated power. In addition,
a hand close to the antenna will alter the resonant properties
of the antenna and de-tune it.
The MICRF104 features automatic tuning. The MICRF104
automatically tunes itself to the antenna, eradicating the need
for manual tuning in production. It also dynamically adapts to
changes in impedance in operation and compensates for the
hand-effect.
Automatic Antenna Tuning
The output stage of the MICRF104 consists of a variable
capacitor (varactor) with a nominal value of 6.5pF tunable
over a range from 5pF to 8pF. The MICRF104 monitors the
phase of the signal on the output of the power amplifier and
automatically tunes the resonant circuit by setting the varactor
value at the correct capacitance to achieve resonance.
In the simplest implementation, the inductance of the loop
antenna should be chosen such that the nominal value is
resonant at 6.5pF, the nominal mid-range value of the
MICRF104 output stage varactor.
Using the equation:
value determined by the above equation, a capacitor can be
added in parallel or series with the antenna. In this case, the
varactor internal to the MICRF104 acts to trim the total
capacitance value.
Starting with the inductance of the antenna the capacitance
value required to achieve resonance can be calculated.
For example a 315MHz transmitter with a 45.1nH inductance
antenna will require no capacitor in parallel with the antenna,
only the internal varactor that will be tuned to 5.66pF, which
is very close to mid range and can be determined using the
equation:
Where:
If the inductance of the antenna cannot be set at the nominal
f = 315Mhz
L
C
4
4
2 2
1
2 2
1
f C
f L
C
VARACTOR
Figure 4.
C
EXTERNAL
L
ANTENNA
November 8, 2001
Micrel