CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits
Page 10 of 42
The transmit data is internally filtered by the MICRF505 before modulation and
transmission to reduce its spectral content. The degree of filtering is determined by
the MICRF505’s programmed ‘modulator clock” (f
MOD
_
CLK
) setting, which can be
represented mathematically by the following formula:
Mod_clkS)
(7
XCO
MOD_CLK
2
Refclk_K x
f
f
−
=
where:
•
f
MOD_CLK
= modulator clock frequency
•
f
XCO
= crystal oscillator frequency
•
Refclk_K = b5-0 of control register 0x07
•
Mod_clkS = b6-4 of control register 0x06
For example, an f
MOD
_
CLK
setting
of eight times the bit rate results in Gaussian filtering
with a bandwidth-period (BT) product of 1.0. (Please note that a BT=1.0 represents
the maximum amount of filtering available in the MICRF505; higher f
MOD
_
CLK
settings
result in higher BT values, which correspond to lower amounts of filtering.) A value
of 1.33MHz is used for f
MOD_CLK
in this application. Since this f
MOD_CLK
value
corresponds to eight times the bit rate, the transmit data signal is Gaussian filtered
with a BT=1.0 (maximum filtering).
The frequency deviation must be programmed such that the modulation index is
always greater than or equal to two. The modulation index can be represented as
follows:
Baudrate
)
xf
2
(
Index
Modulation
dev
=
…where f
dev
is the single-sided frequency deviation:
offset
dev
f
Baudrate
f
+
=
…and f
offset
is the total frequency offset between transmitter and receiver.
Assuming a baudrate of 166.6kbps and a f
offset
of 20kHz, the required single-sided
frequency deviation is:
kHz
6
.
186
kHz
20
kHz
6
.
166
f
dev
=
+
=
MICRF505 control registers 0x04 through 0x07 are programmed such that an f
dev
of
186.6kHz is realized. After processing, the data waveform is passed to the internal
VCO to create the actual frequency modulation.
The power amplifier is bypassed by setting bit 4 of Control Register 0x02 to 1 in the
SETUPRF firmware subroutine. This was done for two reasons. First, significant RF
power isn’t required for the expected one-meter transmit distance between units.
(Note: designs with expected transmit distances of greater than one meter can
activate the PA and adjust its gain accordingly.) Second, bypassing the PA caused
