micrf219a Micrel Semiconductor, micrf219a Datasheet - Page 17

• Manufacturer Part Number: micrf219a
• Description: 300mhz To 450mhz Ask/ook Receiver With Auto-poll, Rssi, And Squelch
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Application Information
Initial Startup
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
MICRF219A. The SHDN pin needs to have 50kΩ
resistor to GND and a coupling capacitor to VDD as
shown in the evaluation board schematic to ensure that
the part starts up in shutdown mode first. Then the micro
controller can bring the SHDN pin voltage down to turn
the part on.
Length of Preamble
When using MICRF219A in auto-polling mode, the
preamble of the corresponding transmitter should be
long enough to guarantee that the MICRF219A becomes
fully awake during the preamble portion of the burst.
This way the entire data portion will be received. A good
rule of thumb to use is:
The factor of 1.2 is to accommodate sleep time variation
due to process shift.
Figure 13 shows an example of insufficient length
preamble. MICRF219A starts checking bits during the
data portion of the burst, so by the time it becomes fully
awake and releases DO, part of the data portion is lost.
In Figure 14, the preamble length is sufficient. The chip
wakes up during the preamble and is ready for the data
portion.
July 2011
undervoltage
Preamble length = 1.2 x sleep time + length of
Figure 13. Preamble Length Too Short
lockout
valid bits sequence
(UVLO)
circuit
inside
17
Antenna and RF Port Connections
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
Figure 14. Sufficient Preamble Length
Frequency (MHz)
433.92
[e-mail]
315
390
418
Z Device (Ω)
23 − j290
14 – j230
17 – j216
12 – j209
or [phone]
M9999-071811-A
MICRF219A