RFRXD0420 Datasheet, PDF (11/32 Pages) Microchip Technology – UHF ASK/FSK/FM Receiver

rfRXD0420/0920
3.1.3 PLL LOOP FILTER
An external PLL loop filter is connected to pin LF
(Pin 29). The loop filter controls the dynamic behavior
of the PLL, primarily lock time and spur levels. Gener-
ally, the PLL lock time is a small fraction of the overall
receiver start-up time (see Electrical Characteristics
Section). The crystal oscillator is the largest contributor
to start-up time. Thus, for the majority of applications,
design loop filter values for a wide loop bandwidth to
suppress noise. Figure 3-4 illustrates an example filter
circuit for a wide frequency range suitable for a majority
of applications.
FIGURE 3-4: PLL LOOP FILTER EXAMPLE
CIRCUIT
29
C2
OPTIONAL
C1
1000 pF
R1
10 kΩ
3.1.4 PRESELECTOR
Receiver performance is heavily influenced by the
preselector (also known as the front-end filter). The
purpose of the preselector is to filter unwanted signals
and noise from entering the receiver.
The most important unwanted signal is the image
frequency (f_rf-image). Pay particular attention to the
image frequency calculated in Figure 3-3 as this will be
the frequency that needs to be filtered out by the
preselector.
The preselector can be designed using a simple LC
filter or a Surface Acoustic Wave (SAW) filter. A simple
LC filter provides a low cost solution but will have the
least effect filtering the image frequency. A SAW filter
can effectively filter the image frequency with a
minimum of 40 dB attenuation.
The SAW filter has the added advantage of filtering
wide-band noise and improving the signal-to-noise
ratio (SNR) of the receiver.
SAW filters require impedance matching. Refer to the
manufacturers' data sheet and application notes for
SAW filter pinouts, specified impedances and recom-
mended matching circuits. Figure 3-5 shows a SAW
filter example circuit.
A secondary purpose of the preselector is to provide
impedance matching between the antenna and LNAIN
(Pin 31).
3.1.5 ANTENNA
Receiver performance and device packaging influence
antenna selection. There are many third-party anten-
nas to choose from. Third-party antennas typically
have an impedance of 50 Ω. The preselector compo-
nents should be chosen to match the impedance of the
antenna to the LNAIN (Pin 31) impedance of
26 Ω || 2 pF.
The designer can chose to use a simple wire antenna.
The length of the wire should be one-quarter the wave-
length (λ) of the receive frequency. For example, the
wavelength of 433.92 MHz is:
λ = c / f_rf where c = 3 x 10⁸ m/s
λ = 3 x 10⁸ m/s / 433.92 x 10⁶ Hz
λ = 0.69 m
therefore
0.25λ = 17.3 cm or 6.8 inches
Finally, the wire antenna should be impedance
matched to the preselector. The typical impedance of a
one-quarter wavelength wire antenna is 36 Ω.
3.1.6 LNA GAIN
For a majority of applications, LNAGAIN can be tied to
Vss (ground) enabling High Gain mode. If the applica-
tion requires short range communications, LNAGAIN
can be tied to VDD (pulled up) enabling Low Gain mode.
More Information on LNAGAIN operation can be found
in the Circuit Description section.
FIGURE 3-5: SAW FILTER EXAMPLE CIRCUIT
Antenna
F1
SAW Filter
L1
L2
2 _Input
Output ⁵
1 _{Input Gnd Output Gnd} 6
C1
Case Gnd
3478
LNAIN
C2
Note: Refer to SAW filter manufacturer’s data sheet for pin outs
and values for impedance matching components.
 2003 Microchip Technology Inc.
Preliminary
DS70090A-page 11