MLX71122_16 Datasheet, PDF (29/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
3.3.2. Mixer 2 (MIX2)
The second mixer is a double-balanced image rejection mixer in Hartley architecture using a complex poly-
phase filter that converts the IF1 to the IF2 signal. The default LO injection type is low side (f_LO2 = f_IF1 - f_IF2),
but also high side injection is possible (f_LO2 = f_IF1 + f_IF2), by setting SSBSEL to LOW (see 4.1.1). As for mixer 1,
the injection side determines the polarity of the output signal. Two gain settings of mixer 2 can be selected
by MIX2GAIN (see 4.1.1), a setting at 9dB (default) and one at -2dB.
SSBSEL
LO2
IF1
90°
0/90°
CROSS
SWITCH
90°/0
polyphase IF2
filter
MIX2GAIN
Fig. 12: Mixer 2 (Image Rejection Mixer)
3.3.3. IF Filter (IFF)
The MLX71122 comprises an internal IF filter with a -3dB bandwidth of 220kHz and a -40dB attenuation
bandwidth of 1.6MHz. The filter contains three capacitively coupled bi-quad stages that represent resonant
tanks close to the filter center frequency of 2MHz. The filter prototype is given in Fig.13. There exists an
approximation formula to calculate the elements Ccx and Ctx when the center frequency, the -3dB
bandwidth, the impedance level R and the inductivity L are given. The Ccx to Ctx ratio scales with the Q-
factor of the bandpass filter. As a consequence of this the shape of the filter doesn’t change in the Bode-
plot if L is increased or decreased. This means the filter should always have about the same bandwidth if it
is tuned to a certain center frequency.
R
CC1
CC2
CC2
CC1
Vout
Vin
L
L
L
R
CT1
CT2
CT1
Fig. 13: IF filter prototype
Each LC-tank is realized as a bi-quad stage using transconductance cells that can be tuned by changing the
bias current. This allows tuning of the center frequency. The bandwidth is related to the center frequency
by the ratio of the coupling and the tank capacitors. Accuracy of the bandwidth relies on matching of the
capacitors. The internal control word in IFFVAL (see 4.1.8) determines the current consumption of the filter
and therefore of the whole receiver. Higher values lead to higher current consumptions. The deviation
from the nominal current consumption can be about ±0.75mA. Four gain settings of the IF filter are
selectable via IFFGAIN in register R0 (see 4.1.1). The default value is 0dB, other options are -14dB, -6dB and
+6dB. It is recommended to leave the value at 0dB.
REVISION 014 – AUGUST, 2016
390 10 71122 01
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