Theory Of Operation - Linx HP-3 series Design Manual

THEORY OF OPERATION

Data
In
MODE
CS0
CS1
CS2
Figure 14: HP-3 Series Transmitter Block Diagram
The TXM-HP3 is a high-performance, multi-channel RF transmitter capable of
transmitting both analog (FM) and digital (FSK) information. FM/FSK modulation
offers significant advantages over AM or OOK modulation methods including
increased noise immunity and the receiver's ability to "capture" in the presence of
multiple signals. This is especially helpful in crowded bands like those in which the
HP-3 operates.
Let's take a brief look at each transmitter section. A precision 12.00MHz Voltage-
Controlled Crystal Oscillator (VCXO) serves as the frequency reference for the
transmitter. Incoming signals are filtered to limit their bandwidth and then used to
directly modulate this reference. Direct reference modulation inside the loop
bandwidth allows a fast startup while allowing a wide modulation bandwidth and
near DC modulation capability. This results in accurate reproduction of analog
and digital content and eliminates the need for code balancing.
The modulated 12.00MHz reference frequency is applied to the Phase-Locked
Loop (PLL). The PLL, combined with a 902-928MHz VCO, forms a stable
frequency synthesizer that can be programmed to oscillate at the desired
transmit frequency. An on-board micro-controller manages the PLL programming
functions and greatly simplifies user interface. The micro-controller reads the
channel-selection lines and programs the on-board synthesizer. This frees the
designer from complex programming requirements and allows for manual or
software channel selection. The micro-controller also monitors the status of the
PLL and indicates when the transmitter is stable and ready to transmit data by
raising the CTS line high.
The PLL locked carrier is amplified and buffered to isolate the VCO from the
antenna and to increase the output power of the transmitter. The output of the
buffer amplifier is connected to a filter network which suppresses harmonic
emissions. Finally, the signal reaches the single-ended antenna port, which is
matched to 50 ohms to support commonly available antennas, such as those
manufactured by Linx.
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28kHz Low Pass
Filter
PLL
4MHz
Int. Osc.
VCO
Modulator
12MHz
Crystal
Band Pass
Amplifier
Filter
RF Out