RFRXD0420 Datasheet, PDF (12/32 Pages) Microchip Technology – UHF ASK/FSK/FM Receiver

rfRXD0420/0920
3.1.7 LNA TUNED CIRCUIT
The LNAOUT (Pin 3) has an open-collector output. It is
pulled up to VDD via a tuned circuit. It is also connected
to 1IFIN (Pin 4) via a series decoupling capacitor. The
1IFIN input impedance is approximately 33 Ω || 1.5 pF.
Important: To ensure LNA stability the VSS pin (Pin 1)
must be connected to a low impedance ground.
As shown in Figure 3-6, components C1 and L1 make
up the tuned circuit and provide collector current via
pull-up. Together with decoupling capacitor C2, they
provided impedance matching between the LNA and
MIXER1. To a lesser extent, C1, L1, and C2 provide
band-pass filtering at the receive frequency (f_rf).
Component values depend on the selected receive
frequency. The challenge is to design the circuit with
the fewest components setting Q as high as possible
as limited by component tolerances. For a majority of
applications it is best to design a wide bandwidth tuned
circuit to account for manufacturing and component
tolerances. The best approach is to design the tuned
circuit using a filter simulation program. Table 3-3 lists
example component values for popular receive
frequencies.
FIGURE 3-6: LNA OUTPUT TO MIXER1
EXAMPLE CIRCUIT.
VDD
C Bypass
C1 L1
C2
3
4
TABLE 3-3: LNA TUNED CIRCUIT EXAMPLE
COMPONENT VALUES
f_rf
C1
L1
C2
315 MHz
7.0 pF 22 nH 6.0 pF
433.92 MHz
3.0 pF 15 nH 6.0 pF
868.3 MHz
2.0 pF 7.6 nH 3.0 pF
915 MHz
2.0 pF 6.8 nH 3.0 pF
These values are for design guidance only.
3.1.8 MIXER1 BIAS
The 1IF+ (Pin 6) and 1IF- (Pin 7) are bias connections
to the MIXER1 balanced collectors. Both pins are
open-collector outputs and are individually pulled up to
VDD by a load resistor. Figure 3-7 shows a MIXER1
bias example circuit.
FIGURE 3-7: MIXER1 BIAS EXAMPLE
CIRCUIT
VDD VDD
R1
470 Ω
6
R2
470 Ω
7
3.1.9 INTERMEDIATE FREQUENCY (IF)
FILTER
The IF filter defines the overall adjacent signal selectiv-
ity of the receiver. For a majority of applications, low-
cost 10.7 MHz ceramic IF filters are used. These are
available in a variety of bandwidths and packages.
IF filter bandwidth selection is a function of:
• modulation (ASK, FSK or FM)
• signal bandwidth
• frequency and temperature tolerances of the
transmitter and receiver components
The typical input and output impedance of ceramic
filters is 330 Ω. 1IFOUT (Pin 9) has an approximately
330 Ω single-ended output impedance and provides a
direct match to the ceramic IF filter. The internal resis-
tance of the 2IFIN (Pin 11) is approximately 2.2 kΩ. In
order to terminate ceramic IF filters a 390 Ω resistor
can be paralleled to the 2IFIN and FBC2 (Pin 13).
Figure 3-8 shows an example circuit schematic using a
10.7 MHz ceramic IF filter.
3.1.10 IF LIMITING AMPLIFIER EXTERNAL
FEEDBACK CAPACITORS
FBC1 (Pin 12) and FBC2 (Pin 13) are connected to
external feedback capacitors. Figure 3-8 shows
component values and connections for these
capacitors.
DS70090A-page 12
Preliminary
 2003 Microchip Technology Inc.