Page 12
INPUTTING ANALOG SIGNALS
The HP-3 series transmitter is capable of sending a wide range of analog signals
including audio. The ability of the HP-3 to send combinations of audio and data
also opens new areas of opportunity for creative design.
Simple or complex analog signals within the specified analog bandwidth and
input levels may be connected directly to the transmitter’s DATA pin. The
transmitter input is high impedance (200k) and can be directly driven by a wide
variety of sources ranging from a single frequency to complex content such as
voice or music. Analog signals at the data input pin may range from 50 Hz to
28kHz. The Typical Performance Graphs on page 3 of this manual illustrate the
modulation linearity for a variety of simple waveforms.
The HP3 is a single supply device
and as such is not capable of
operating in the negative voltage
range, therefore analog sources
should typically provide a 0V to
3V, but not more than 5V P-P,
maximum waveform and should,
in most cases, be AC-coupled into
the DATA pin to achieve the best
performance. The size of the
coupling capacitor should be large
enough to ensure the passage of
all desired frequencies and, at the
same time, small enough to allow
the start-up time desired. After the AC signal passes into the modulation circuit
it will be automatically adjusted to the optimum DC offset by an internal voltage
divider. Since the modulation voltage applied to the DATA pin determines the
carrier deviation, distortion can occur if the DATA pin is over-driven. The actual
level of the input waveform should be adjusted to achieve optimum in-circuit
results for your application.
The illustration above shows the simplicity of transmitting audio with the HP-3
transmitter. In applications where higher audio quality is required, an external
compandor such as a Phillips SA576, may be employed to increase dynamic
range and reduce noise. The HP-3 is capable of providing audio quality
comparable to a radio or intercom. When true high-fidelity audio is required, the
HP will probably not be the best choice, as it has been optimized for data. A
device designed specifically for high quality audio should be utilized instead.
GND
ANT
CHS 0
CHS 1 SS CLOCK
CHS 2 SS DATA
CTS
POWER DOWN
VCC
GND/MODE
DATA IN
V
CC
Channel
Select
TX1
S1
3-Position
DIP Switch
Audio In From
Headphone Or Speaker Jack
Of Amplifier/Tape Player, etc.
Figure 22: Typical Voice Transmitter
CTS OUTPUT
The Clear-To-Send (CTS) output goes high to indicate the transmitter PLL is
locked and the module is ready to accept data. In a typical application, a micro-
controller will raise the PDN line high (powering-up the transmitter) and begin to
monitor the CTS line. When the line goes high, the micro-controller would start
sending data. It is not necessary to use the CTS output. In applications where
CTS is not used, the user's circuit should wait a minimum of 10mSec after raising
the PDN pin high before transmitting data. If data is being sent redundantly, there
is generally no need to monitor the CTS pin or to wait a fixed time.
