MLX71122_16 datasheet(29/59 Pages) MELEXIS

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MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 29 of 59

390 10 71122 01

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 (fLO2 = fIF1 - fIF2),

but also high side injection is possible (fLO2 = fIF1 + fIF2), 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.

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.

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.

90°/0

0/90°

IF1

LO2

CROSS

SWITCH

IF2

polyphase

filter

90°

SSBSEL

MIX2GAIN

Vin

CC1

CC2

CT1

CT2

Vout

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