TDMA Digital Radio Processor
CMX7161
7.5.5 Signal Control
The CMX7161 offers two signal inputs (I Input, Q Input), and two modulator outputs (Mod 1 Output, Mod 2
Output). The analogue gain/attenuation of each input and output can be set individually.
During transmission the device outputs the filtered symbol stream on both Mod 1 and Mod 2. These can
then be used to drive VCOs in order to create FM modulation. The two outputs can be independently
controlled (gain, offset, polarity).
In receive mode the I and Q inputs accept in-phase and quadrature signals, and may be independently
inverted. (Note that inverting one of the inputs has the same effect as swapping the I and Q signals.)
Use of I/Q receive mode brings with it the problem of I/Q DC offsets. These are DC offsets caused by the
radio receiver, resulting in the signal into the CMX7161 having a DC offset other than VBIAS. This offset
needs to be removed prior to demodulation. Offsets typically remain constant for a particular radio
frequency selected, but may vary if that frequency is changed. Gain within the radio receiver may also
affect the DC offset seen by the CMX7161.
I/Q DC offset effects are a radio issue which is beyond the control of the CMX7161 but the device does
provide DC offset calculation and removal. The host can set DC offset for each input and either lock them
or allow the CMX7161 to track them, in which case the updated offsets can be read back later if required.
7.6
Modulation Format
The CMX7161 uses 4-FSK modulation running at 4800 symbols/s (9600bits/s) with a deviation index of
0.27 for transmission in a 12.5kHz channel bandwidth. The symbol stream is converted to a series of
impulses which pass through a root-raised-cosine (=0.2) pulse shaping filter before frequency modulation
at the transmitter and again after frequency demodulation at the receiver. The bit-to-symbol mapping is
shown below:
Bit Pair
01
Symbol
+3
Deviation
+1944Hz
+648Hz
-648Hz
00
+1
10
-1
11
-3
-1944Hz
7.6.1 Typical Transmit Performance
Using the test system shown in Figure 12 the CMX7161 internal PRBS generator was used to modulate
the RF FM signal generator. Some typical results are shown in Figure 13. The desired deviation was
achieved by adjusting the deviation control on the RF signal generator.
Mod1 or Mod2
RF Signal
Generator
Spectrum Analyser /
Vector Signal Analyser
CMX7161
DC FM
Modulation
Input
Buffer
Amplifier
(if required to drive
RF signal generator
modulation input)
Figure 12 Tx Spectrum and Modulation Measurement Configuration for Two-point Modulation
2013 CML Microsystems Plc
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D/7161_FI-1.0/4
CMLMICRO [ CML MICROCIRCUITS ]
