U3741BM Datasheet, PDF (8/32 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

FSK/ASK Demodulator
and Data Filter
The signal coming from the RSSI amplifier is converted into the raw data signal by the
ASK/FSK demodulator. The operating mode of the demodulator is set via pin ASK/FSK.
Logic 'L' sets the demodulator to FSK, Logic 'H' sets it into ASK mode.
In ASK mode an automatic threshold control circuit (ATC) is employed to set the detec-
tion reference voltage to a value where a good signal-to-noise ratio is achieved. This
circuit also implies the effective suppression of any kind of in-band noise signals or com-
peting transmitters. If the S/N ratio exceeds 10 dB, the data signal can be detected
properly.
The FSK demodulator is intended to be used for an FSK deviation of ∆f ≥ 20 kHz. Lower
values may be used but the sensitivity of the receiver is reduced in that condition. The
minimum usable deviation is dependent on the selected baud rate. In FSK mode, only
BR_Range0 and BR_Range1 are available. In FSK mode, the data signal can be
detected if the S/N Ratio exceeds 2 dB.
The output signal of the demodulator is filtered by the data filter before it is fed into the
digital signal processing circuit. The data filter improves the S/N ratio as its bandpass
can be adopted to the characteristics of the data signal. The data filter consists of a
1st-order high-pass and a 1st-order low-pass filter.
The high-pass filter cut-off frequency is defined by an external capacitor connected to
pin CDEM. The cut-off frequency of the high-pass filter is defined by the following for-
mula:
f_{cu_DF} = ₂-----_×------_π-----_×------₃---₀-----^1k---_Ω------_×-----_C-----_D----_E----_M---
In self-polling mode, the data filter must settle very rapidly to achieve a low current con-
sumption. Therefore, CDEM cannot be increased to very high values if self polling is
used. On the other hand, CDEM must be large enough to meet the data filter require-
ments according to the data signal. Recommended values for CDEM are given in the
“Electrical Characteristics” on page 23. The values are slightly different for ASK and
FSK mode.
The cut-off frequency of the low-pass filter is defined by the selected baud rate range
(BR_Range). BR_Range is defined in the OPMODE register (refer to section “Configu-
ration of the Receiver” on page 17). BR_Range must be set in accordance to the used
baud rate.
The U3741BM is designed to operate with data coding where the DC level of the data
signal is 50%. This is valid for Manchester and Bi-phase coding. If other modulation
schemes are used, the DC level should always remain within the range of
VDC_min = 33% and VDC_max = 66%. The sensitivity may be reduced by up to 1.5 dB
in that condition.
Each BR_Range is also defined by a minimum and a maximum edge-to-edge time
(tee_sig). These limits are defined in the “Electrical Characteristics” on page 23. They
should not be exceeded to maintain full sensitivity of the receiver.
8 U3741BM
4662B–RKE–10/04