TH71221 Datasheet, PDF (14/44 Pages) Melexis Microelectronic Systems – 27 to 930MHz FSK/FM/ASK Transceiver

TH71221
27 to 930MHz
FSK/FM/ASK Transceiver
3.2.1 LNA
The LNA is based on a cascode topology for low-noise, high gain and good reverse isolation. The open col-
lector output has to be connected to an external resonance circuit which is tuned to the receive frequency.
The gain of the LNA can be changed in order to achieve a high dynamic range. There are two possibilities for
the gain setting which can be selected by the register bit LNACTRL. External control can be done via the pin
GAIN_LNA, internal control is given by the register bit LNAGAIN. In case of external gain control, a hystere-
sis of about 340 mV can be chosen via the register bit LNAHYST. This configuration is useful if an automatic
gain control loop via the RSSI signal is established. In transmit mode the LNA-input is shorted to protect the
amplifier from saturation and damaging.
3.2.2 Mixer
The mixer is a double-balanced mixer which down converts the receive frequency to the IF. The default LO
injection type is low side (f_VCO = f_RX – f_IF). But also high side injection is possible (f_VCO = f_RX + f_IF). In this case,
the data signal´s polarity is inverted due to the mixing process. To avoid this, the transmitted data stream can
be inverted too by setting DTAPOL to ‘1’.
The output impedance of the mixer is about 330Ω in order to match to an external IF filter.
3.2.3 IF Amplifier
After passing the channel select filter which sets the IF bandwidth the signal is limited by means of an high
gain limiting amplifier. The small signal gain is about 80 dB. The RSSI signal is generated within the IF ampli-
fier. The output of the RSSI signal is available at pin RSSI. The voltage at this pin is proportional to the input
power of the receiver in dBm. Using this RSSI output signal the signal strength of different transmitters can
be distinguished.
3.2.4 ASK Demodulator
The receive part of the TH7122 allows for two ASK demodulation configurations:
• standard ASK demodulation or
• ASK demodulation with peak detector.
The default setting is standard ASK demodulation. In this mode SW1 and SW2 are closed and the RSSI
output signal directly feeds the data slicer setup by means of OA1. The data slicer time constant equals to
T = 200kΩ ⋅ C3 ,
(8)
with C3 external to pin INT1. This time constant should be larger than the longest possible bit duration of the
data stream. This is required to properly extract the ASK data’s DC level. The purpose of the DC (or mean)
level at the negative input of OA1 is to set an adaptive comparator threshold to perform the ASK detection.
Alternatively a peak detector can be used to define the ASK detection threshold. In this configuration the
peak detector PKDET is enabled, SW1 is closed and SW2 is open, and the peak detector output is multi-
plexed to pin INT2/PDO. This way the peak detector can feed the data slicer, again constituted by OA1 and a
few external R and C components. The peak detection mode is selectable in programmable user mode.
39010 071221
Rev. 005
Page 14 of 44
Data Sheet
June/07