SYN450R/SYN460R
13.1.1. Step 1: Selecting the Operating Mode
13.1.1.1. Fixed-Mode Operation
For applications where the transmit frequency is accurately set (that is, applications where a SAW
or crystal-based transmitter is used) the SYN450R may be configured as a standard
superheterodyne receiver (fixed mode). In fixed-mode operation the RF bandwidth is narrower
making the receiver less susceptible to interfering signals. Fixed mode is selected by connecting
SWEN to ground.
13.1.1.2. Sweep-Mode Operation
When used in conjunction with low-cost L-C transmitters the SYN450R should be configured in
sweep-mode. In sweep-mode, while the topology is still superheterodyne, the LO (local oscillator)
is swept over a range of frequencies at rates greater than the data rate. This technique effectively
increases the RF bandwidth of the SYN450R, allowing the device to operate in applications where
significant transmitter-receiver frequency misalignment may exist. The transmit frequency may
vary up to ±0.5% over initial tolerance, aging, and temperature. In sweep-mode a band
approximately 1.5% around the nominal transmit frequency is captured. The transmitter may drift
up to ±0.5% without the need to retune the receiver and without impacting system performance.
The swept-LO technique does not affect the IF bandwidth, therefore noise performance is not
degraded relative to fixed mode. The IF bandwidth is 430kHz whether the device is operating in
fixed or sweep-mode.
Due to limitations imposed by the LO sweeping process, the upper limit on data rate in sweep
mode is approximately 5.0kbps.
Similar performance is not currently available with crystal-based superheterodyne receivers which
can operate only with SAW- or crystal-based transmitters.
In sweep-mode, a range reduction will occur in installations where there is a strong interferer in
the swept RF band. This is because the process indiscriminately includes all signals within the
sweep range. An SYN450R may be used in place of a superregenerative receiver in most
applications.
13.1.2. Step 2: Selecting the Reference Oscillator
All timing and tuning operations on the SYN450R are derived from the internal Colpitts reference
oscillator. Timing and tuning is controlled through the REFOSC pin in one of three ways:
1) Connect a ceramic resonator
2) Connect a crystal
3) Drive this pin with an external timing signal
The specific reference frequency required is related to the system transmit frequency and to the
operating mode of the receiver as set by the SWEN pin.
*This specification is subject to change without notification.
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