RF2905 Просмотр технического описания (PDF) - RF Micro Devices
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RF2905 Datasheet PDF : 22 Pages
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RF2905
11
faster the switching times. Phase noise of the VCO is
another factor. Phase noise outside of the loop band-
width is due to the noise of the VCO itself rather than
the crystal reference. A design trade-off must be made
here in selecting a PLL loop bandwidth with acceptable
phase noise and switching characteristics and minimal
distortion of the modulation data.
AM/ASK SYSTEMS
The transmitter of the RF2905 has an output power
level adjustment (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 51kΩ resistive load will
produce an RSSI voltage of 0.7V to 2.5V, typically. A
parallel capacitor is suggested to limit the bandwidth
and filter noise. For ASK applications, the 18dB range
of the LVL ADJ does not produce enough voltage
swing in the RSSI for reliable communication. The On-
Off keying (OOK) is suggested to provide reliable com-
munications. To achieve this, both 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 a on/off ratio of >50dB. One
unfortunate consequence of modulating this way is
VCO pulling by the power amp. This results in a spuri-
ous output outside the desired transmit band as the
PLL momentarily loses lock and reacquires. This can
be avoided by pulse shaping the TX data to slow the
change in the VCO load to a pace that the PLL can
track with its given loop bandwidth. The loop bandwidth
can also be increased to allow it to track faster
changes due to load pulling.
For the ASK/OOK receiver demodulator, an external
data slicer is required. The RSSI output is used to pro-
vide both the filtered data and a very low pass filtered
(relative to the data rate) DC reference to a 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 get to 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
received data. Again, a coding scheme such as
Manchester such should be used to improve data
integrity.
APPLICATION AND LAYOUT CONSIDERATIONS
Both the RX IN and TX OUT have a DC bias on them.
Therefore, DC blocking caps are required. If the RF fil-
ter has DC blocking characteristics like a ceramic
dielectric filter, then only 1 DC blocking capacitor would
11-62
be needed to separate the DC of RX IN and TX OUT.
These are RF signals and care should be taken to
route these signals keeping them physically short.
Because of the 50Ω/high impedance nature of these
two signals, they may be connected together into a sig-
nal 50Ω device such as a filter. An external LNA or PA
can be used, if desired, but an external RX/TX switch
may be required.
The VCO is a very sensitive block in this 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-
nators and varactor are very important. The capacitor
and varactor should be closest to the RF2905 pins and
the trace length should be as short as possible. The
inductors can be placed further away and any trace
inductance can be compensated by reducing the value
of the inductors. Printed inductors may also be used
with careful design. For best results, the physical layout
should be as symmetrical as possible. Figure 1 is a
recommended layout pattern for the VCO components.
When using loop bandwidths lower than the 5kHz
shown on the eval 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
100Ω to 200Ω and a 1µF or larger capacitor can be
used.
Loop Voltage
GND
33
32
31
30
29
28
Not to Scale
Representative of Size
Vcc
GND
Figure 1. Recommended VCO Layout
For the interface between the LNA/mixer, the coupling
capacitor should be as close to the RF2905 pins as
possible with the bias inductor being further away.
Once again, the value of the inductor can be changed
to compensate to trace inductance. The output imped-
ance of the LNA is in the order of several kΩ which
makes matching to 50Ω very hard. If image filtering is
desired, a high impedance filter is recommended.
Rev B11 010516
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