TDA5235
Functional Description
The bandpass filter follows the subsequent formula:
fcenter
=
fcorner, low × fcorner, high
Therefore asymmetric corner frequencies can be observed. The use of AFC results in
more symmetry.
A multi-stage bandpass limiter at a center frequency of 274 kHz completes the receiver
chain. The -3dB corner frequencies of the bandpass limiter are typically at 75 kHz and
at 520 kHz.
An RSSI generator delivers a DC signal proportional to the applied input power and is
also used as an ASK demodulator. Via a programmable anti-aliasing filter this signal is
converted to the digital domain by means of a 10-bit ADC.
The limiter output signal is connected to a digital FSK demodulator.
The immunity against strong interference frequencies (so called blockers) is determined
by the available filter bandwidth, the filter order and the 3rd order intercept point of the
front end stages. For Single-Channel applications with moderate requirements to the
selectivity the performance of the on-chip 3rd order bandpass polyphase filter might be
sufficient. In this case no external filters are necessary and a single down conversion
architecture can be used, which converts the input signal frequency directly to the 2nd IF
frequency of 274 kHz.
RX
I-Mix
FSK Data
I-
LP
harm sup
digital
FSK Demod
Mix2
RX
Input
LNA
RX
ASK Data
ASK /
RSSI
ADC
LP
alias sup
Q-
Mix2
RSSI Generator
Q-Mix
IF
Divider
: N
Attenuation
adjust
Channel Filter
Bandwidth select
1st LO
2
nd LO
Figure 4
Single Down Conversion (SDC, no external filters required)
For Multi-Configuration applications requiring different RF channels or systems which
demand higher selectivity the double down conversion scheme together with one or two
external CER filters can be selected. The order of such ceramic filters is in a range of 3,
so the selectivity is further improved and a better channel separation is guaranteed.
Data Sheet
20
V1.0, 2010-02-19
INFINEON [ Infineon ]
