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MAX7060
280MHz to 450MHz Programmable
ASK/FSK Transmitter
ASK Amplitude Shaping
At data rates higher than 30kbps Manchester (60kbps
NRZ), it may be necessary to shape the ASK transmitter
pulses to reduce the occupied bandwidth of the trans-
mitted signal to comply with government regulations
(FCC in the U.S., ETSI in Europe). There is no shaping
of the FSK modulation. The MAX7060 has two forms of
amplitude shaping: digital and analog.
Digital Amplitude Shaping
The digital shaping feature allows the user to choose a
linear stairstep function to increase and decrease the
power when the PA is turned on and off for an ASK bit
interval. There are three registers that control the digital
amplitude shaping settings. The first setting is the final
power of the PA when the pulse reaches its maximum
(PApwr register). The second setting is the amplitude
change, in decibels, for each step, which is the vertical
axis of the stairstep (PAstep register). The third setting
is the time interval of each step, which is the horizontal
axis of the stairstep (Tstep register). The final power set-
ting (decimal 0 to 30 in increments of 1dB) is entered in
the PApwr register. The amplitude step (decimal 0 to 30
in increments of 1dB) is entered in the PAstep register.
The time interval (decimal 0 to 60/f XTAL in increments of
4/f XTAL ) is entered in the Tstep register. For example, to
shape an 80kbps NRZ data stream (12.5 F s bit interval),
the user might choose a maximum power level of 0x1E
(30dec), an amplitude step of 5dB, and a time interval
of 0.5 F s assuming a crystal frequency of 16MHz. This
would produce an ASK pulse that ramps up in 3 F s, lev-
els off for 9.5 F s, and ramps down in 3 F s. Because the
amplitude steps are in decibels, the shape of the pulse
rise and fall is exponential on a linear display (an oscil-
loscope, for instance). Because most ASK receivers use
a logarithmic amplitude detector, the demodulated pulse
12.5 μ s
DIN
PApwr =
0x1E (30dec)
0
PAstep = 0x05 (5dB)
PAstep = 0x05 (5dB)
has a linear ramp shape. The digital shaping is disabled
when the Tstep register is 0x00.
If no shaping of any kind is used (digital or analog), the
PA pullup inductor must be connected to PAVOUT and
the analog shaping bits in the Conf0 register must also
be set to either anshp[1:0] = 00 or to anshp[1:0] = 11, as
described in the ASK Envelope Shaping section.
Analog Amplitude Shaping
To use the analog shaping feature, the user must con-
nect the bias inductor to the ROUT pin instead of directly
to the PAVOUT pin. This places a MOS resistor between
PAVOUT and ROUT, which slows down the application of
the PAVOUT voltage to the drain of the PA FET when the
PA is turned on. There are two settings in the anshp[1:0]
bits in the Conf0 register for the rate at which the pulse
ramps up: anshp[1:0] = 10 is approximately 1.5 F s and
anshp[1:0] = 01 is approximately 3 F s. The anshp[1:0] =
11 setting turns analog shaping off and removes the 4 F s
pulse extension, while the anshp[1:0] = 00 setting opens
the connection between PAVOUT and ROUT.
Tuning Capacitor Settings
The internal variable shunt capacitor, which can be used
to match the PA to the antenna with changing transmitter
frequency, is controlled by setting the 5-bit cap vari-
able in the registers. This allows for 32 levels of shunt
capacitance control. Since the control of these 5 bits is
independent of the other settings, any capacitance value
can be chosen at any frequency, making it possible to
maintain maximum transmitter efficiency while moving
rapidly from one frequency to another. The internal tun-
ing capacitor adds 0 to 7.75pF to the PA output in 0.25pF
steps. The PA output capacitance at the minimum cap
setting is approximately 4.5pF.
12.5 μ s
Tstep = 0x2 (0.5 μ s)
Tstep = 0x2 (0.5 μ s)
Figure 7. Digital Amplitude Shaping Timing Diagram
Maxim Integrated
19
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