AN1833 Freescale Semiconductor / Motorola, AN1833 Datasheet - Page 15

• Manufacturer Part Number: AN1833
• Description: A Low-Power Wireless Remote Control Transmitter and Receiver
• Manufacturer: Freescale Semiconductor / Motorola

IF Interstage
Filtering
AN1833
MOTOROLA
to V
control voltage decreases.
The RLC values shown will tune the tank to 10.7 MHz and allow for
approximately 900-kHz AFC tuning bandwidth.
in and hold-in range of the AFC loop. An adjustable coil is used to tune
the quadrature detector. The resistor sets the bandwidth by lowering the
Q of the LCR tank circuit. The following relationship shows how to
calculate the response of the discriminator resonant tank.
for R = 820 ; X
then Q ~ 12.2
thus, the 3-dB bandwidth = 10.7 MHz / 12.2 = 880 kHz
Should the overall transmit/receive system be prone to extreme
frequency drift, the quadrature coil circuit bandwidth may need to be
increased to provide adequate tracking range.
Ceramic bandpass filters are placed between:
They allow improved sensitivity by bandwidth, limiting the noise to the
RSSI detectors, especially the limiter. The filters shown in the schematic
have a 3-dB bandpass response centered at 10.7 MHz which is 280 kHz
wide. Choice of IF filter bandwidth will affect the fidelity of the recovered
baseband signals. As the data rate is increased, the filter bandwidth
should be wider.
The filter bandwidth is affected by its source impedance and output load
impedances which are 330 . The MC13158 mixer output and IF
amplifier input and output impedances are 330
10.7-MHz ceramic filters.
6. Q = R / X
7. BW 3 dB = fo / Q
•
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CC
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Mixer output and the IF amplifier input
IF output and the limiter input (see
(ac ground); thus, the LO frequency increases as the varactor
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L
L
= 2 (10.7)(1.0) = 67.2
Figure
Automatic Frequency Control
Figure 8
2)
to match typical
shows the pull-
Application Note
15