11-119
RF2915
Rev A7 001011
11
ASK/OOK SYSTEMS
The transmitter of the RF2915 has an output power
level adjust (LVL ADJ) that can be used to provide
approximately 18dB of power control for amplitude
modulation. The RSSI output of the receiver section
can be used to recover the modulation. The RSSI out-
put is from a current source, and needs to have a resis-
tor to convert to a voltage. A 51k
Ω resistor load
typically produces an output of 0.7V to 2.5V. A parallel
capacitor is suggested to band limit the signal. For
ASK applications, the 18dB range of the LVL ADJ
does not produce enough voltage swing in the RSSI for
reliable communications. The on/off keying (OOK) is
suggested to provide reliable communications. To
achieve this, the LVL ADJ and TX ENABL need to be
controlled together (please note that LVL ADJ cannot
be left high when TX ENABL is low). This will provide
an on/off ratio of greater than 50 dB. One of the unfor-
tunate consequences of modulating in this manner is
VCO pulling by the PA. This results in a spurious out-
put outside the desired transmit band, as the PLL
momentarily loses lock and reacquires. This may be
avoided by pulse-shaping TX data to slow the change
in the VCO load to apacewhich thePLL cantrack with
its given loop bandwidth. The loop bandwidth may also
be increased to allow it to track faster changes brought
about by load pulling.
For the ASK/OOK receiver demodulator, an external
data slicer is required. The RSSI output is used to pro-
vide both the filter data and a very low pass filter (rela-
tive to the data rate) DC reference to the data slicer.
Because the very low pass filter has a slow time con-
stant, a longer preamble may be required to allow for
the DC reference to acquire a stable state. Here, as in
the case of the FSK transmitter, the data pattern also
affects the DC reference and the reliability of the
receive data. Again, a coding scheme such as
Manchester should be used to improve data integrity.
APPLICATION AND LAYOUT CONSIDERATIONS
Both the RX IN and the TX OUT have a DC bias on
them. Therefore, a DC blocking cap is required. If the
RF filter has DC blocking characteristics (such as a
ceramic dielectric filter), then only one DC blocking cap
would be needed to separate the DC of the RX and TX.
These are RF signals and care should be taken to run
the signal keeping them physically short. Because of
the 50
Ω/high impedance nature of these two signals,
they may be connected together into a single 50
Ω
device (such as a filter). An external LNA or PA may be
used, if desired, but an external RX/TX switch may be
required.
The VCO is a very sensitive block in the system. RF
signals feeding back into the VCO (either radiated or
coupled by traces) may cause the PLL to become
unlocked. The trace(s) for the anode of the tuning var-
actor should also be kept short. The layout of the reso-
nator and varactor are very important. The capacitor
and varactor should be close to the RF2915 pins, and
the trace length should be as short as possible. The
inductors may be placed further away, and reducing
the value of the inductors can compensate any trace
inductance. Printed inductors may also be used with
careful design. For best results, physical layout should
be as symmetrical as possible. Figure 1 is a recom-
mended layout pattern for the VCO components. When
using the loop bandwidth lower than 5kHz shown on
the evaluation board, better filtering of the VCC at the
resonators (and lower VCC noise, as well) will help
reduce phase noise of the VCO. A series resistor of
100
Ω to 200Ω, anda1µF or larger capacitor may be
used.
For the interface between the LNA/mixer, the coupling
capacitor should be as close to the RF2915 pins as
possible, with the bias inductors further away. Once
again, the value of the inductor may be changed to
compensate for trace inductance. The output imped-
ance of the LNA is in the order of several k
Ω,which
makes matching to 50
Ω very difficult. If image filtering
is desired, a high impedance filter is recommended.
Not to Scale
Representative of Size
24
23
Loop
Voltage
VCC
Figure 1. Recommended VCO Layout
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