micrf220 Micrel Semiconductor, micrf220 Datasheet - Page 14

• Manufacturer Part Number: micrf220
• Description: 300mhz To 450mhz, 3.3v Ask/ook Receiver With Rssi And Squelch
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Supply Voltage Ramping
When supply voltage is initially applied, it should rise
monotonically from 0V to 3.3V to ensure proper startup
of the crystal oscillator and the PLL. It should not have
multiple bounces across 2.6V, which is the threshold of
the
MICRF220.
Antenna and RF Port Connections
Figure 8 shows the schematic of the MICRF220
Evaluation Board. Figures 9 thru 11 depict PCB images.
This evaluation board is a good starting point for the
prototyping of most applications. The evaluation board
offers two options of injecting the RF input signal:
through a PCB antenna or through a 50Ω SMA
connector. The SMA connection allows for conductive
testing, or an external antenna.
Low-Noise Amplifier Input Matching
Capacitor C3 and inductor L2 form the “L” shape input
matching network to the SMA connector. The capacitor
cancels out the inductive portion of the net impedance
after the shunt inductor, and provides additional
attenuation for low-frequency outside band noise. The
inductor is chosen to over resonate the net capacitance
at the pin, leaving a net-positive reactance and
increasing the real part of the impedance. It also
provides additional ESD protection for the antenna pin.
The input impedance of the device is listed in Table 6 to
aid calculation of matching values. Note that the net
impedance at the pin is easily affected by component
pads parasitic due to the high input impedance of the
device. The numbers in Table 6 does NOT include trace
and component pad parasitic capacitance, which total
about 0.75pF on the evaluation board.
The matching components to the PCB antenna (L3 and
C9) were empirically derived for best over-the-air
reception range.
Table 6. Input Impedance for the Most Used Frequencies
August 2010
undervoltage
Frequency (MHz)
433.92
315
390
418
lockout
(UVLO)
Z Device (Ω)
23 − j290
14 – j230
17 – j216
12 – j209
circuit
inside
14
Crystal Selection
The crystal resonator provides a reference clock for all
the device internal circuits. Crystal tolerance needs to
be chosen such that the down-converted signal is
always inside the IF bandwidth of MICRF220. From this
consideration, the tolerance should be ±50ppm on both
the transmitter and the MICRF220 side. ESR should be
less than 300Ω, and the temperature range of the crystal
should match the range required by the application.
With the Abracon crystal listed in the Bill of Materials, a
typical MICRF220 crystal oscillator still starts up at
105ºC with additional 400Ω series resistance.
The oscillator of the MICRF220 is a Pierce-type
oscillator. Good care must be taken when laying out the
printed circuit board. Avoid long traces and place the
ground plane on the top layer close to the REFOSC pins
RO1 and RO2. When care is not taken in the layout, and
the crystals used are not verified, the oscillator may not
start or takes longer to start. Time-to-good-data will be
longer as well.
PCB Considerations and Layout
Figures 9 thru 11 illustrate the MICRF220 Evaluation
Board
downloadable from the Micrel website and contain the
remaining layers needed to fabricate this board. When
copying or making one’s own boards, make the traces
as short as possible. Long traces alter the matching
network and the values suggested are no longer valid.
Suggested matching values may vary due to PCB
variations. A PCB trace 100 mils (2.5mm) long has about
1.1nH inductance. Optimization should always be done
with exhaustive range tests. Make sure the individual
ground connection has a dedicated via rather then
sharing a few of ground points by a single via. Sharing
ground via will increase the ground path inductance.
Ground plane should be solid and with no sudden
interruptions. Avoid using ground plane on top layer next
to the matching elements. It normally adds additional
stray capacitance which changes the matching. Do not
use Phenolic materials as they are conductive above
200MHz. Typically, FR4 or better materials are
recommended. The RF path should be as straight as
possible to avoid loops and unnecessary turns.
Separate ground and V
switching power circuits (such microcontroller…etc).
Known sources of noise should be laid out as far as
possible from the RF circuits. Avoid unnecessary wide
traces which would add more distribution capacitance
(between top trace to bottom GND plane) and alter the
RF parameters.
layout.
The
Gerber
DD
lines from other digital or
files
M9999-082610-A
provided
MICRF220
are