315MHz Low-Power, +3V Superheterodyne
Figure 2. Generating Data Slicer Threshold
Figure 3. Using PDOUT for Faster Startup
peak detector to dynamically follow peak changes of
the data filter output voltage. For faster receiver startup,
the circuit shown in Figure 3 can be used.
The MAX1470 can be configured to receive ASK modu-
lated data with carrier frequency ranging from 250MHz
to 500MHz. Only a small number of components need
to be changed to retune the RF section to the desired
Table 2 shows a list of changed components and their
values for a 433.92MHz RF; all other components
The integrated image rejection of the MAX1470 is
specifically designed to function with a 315MHz input
frequency by attenuating any signal at 293.6MHz. The
benefit of the on-chip image rejection is that an external
SAW filter is not needed, reducing cost and the inser-
tion loss associated with SAW filters. The image rejec-
tion cannot be retuned for different RF input
frequencies and therefore is degraded. The image
rejection at 433.92MHz is typically 39dB.
A properly designed PC board is an essential part
of any RF/microwave circuit. On high-frequency inputs
and outputs, use controlled-impedance lines and
keep them as short as possible to minimize losses and
radiation. At high frequencies, trace lengths that are
approximately 1/20 the wavelength or longer become
antennas. For example, a 2in trace at 315MHz can act
as an antenna.
Keeping the traces short also reduces parasitic induc-
tance. Generally, 1in of a PC board trace adds about
20nH of parasitic inductance. The parasitic inductance
can have a dramatic effect on the effective inductance.
For example, a 0.5in trace connecting a 100nH induc-
tor adds an extra 10nH of inductance or 10%.
To reduce the parasitic inductance, use wider traces
and a solid ground or power plane below the signal
traces. Using a solid ground plane can reduce the par-
asitic inductance from approximately 20nH/in to 7nH/in.
Also, use low-inductance connections to ground on all
GND pins, and place decoupling capacitors close to all
Table 2. Changed Component Values for
VALUE FOR 433MHz RF
Note: These values are affected by PC board layout.
TRANSISTOR COUNT: 1835