T5760 データシートの表示（PDF） - Atmel Corporation

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T5760

UHF ASK/FSK Receiver

Atmel Corporation 

T5760 / T5761

and exhibits the best possible sensitivity and at the same

time power matching at RF_IN.

RSens can be connected to VS or GND via a µC. The

receiver can be switched from full sensitivity to reduced

sensitivity or vice versa at any time. In polling mode, the

receiver will not wake up if the RF input signal does not

exceed the selected sensitivity. If the receiver is already

active, the data stream at Pin DATA will disappear when

the input signal is lower than defined by the reduced

sensitivity. Instead of the data stream, the pattern accord-

ing to figure 5 is issued at Pin DATA to indicate that the

receiver is still active (see also figure 32).

DATA

t DATA_min

t DATA_L_max

Figure 5. Steady L state limited DATA output pattern

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 the bit

ASK/_FSK in the OPMODE register. Logic ‘L’ sets the

demodulator to FSK, applying ‘H’ to ASK mode.

In ASK mode an automatic threshold control circuit

(ATC) is employed to set the detection 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 competing transmitters.

If the S/N (ratio to suppress in-band noise signals) ex-

ceeds about 10 dB the data signal can be detected

properly, but better values are found for many modulation

schemes of the competing transmitter.

The FSK demodulator is intended to be used for an FSK

deviation of 10 kHz ≤ Df ≤ 100 kHz. In FSK mode the

data signal can be detected if the S/N (ratio to suppress

inband noise signals) exceeds about 2 dB. This value is

valid for all modulation schemes of a disturber signal.

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 pass-

band can be adopted to the characteristics of the data

signal. The data filter consists of a 1st-order highpass and

a 2nd-order lowpass filter

The highpass filter cut-off frequency is defined by an

external capacitor connected to Pin CDEM. The cut-off

frequency of the highpass filter is defined by the follow-

ing formula:

fcu_DF + 2

p

1

30 kW

CDEM

In self-polling mode, the data filter must settle very

rapidly to achieve a low current consumption. 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 requirements according to

the data signal. Recommended values for CDEM are

given in the electrical characteristics.

The cut-off frequency of the lowpass filter is defined by

the selected baud-rate range (BR_Range). The

BR_Range is defined in the OPMODE register (refer to

chapter ‘Configuration of the Receiver’). The BR_Range

must be set in accordance to the used baud-rate.

The T5760/T5761 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 2 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. They should not

be exceeded to maintain full sensitivity of the receiver.

Receiving Characteristics

The RF receiver T5760/T5761 can be operated with and

without a SAW front-end filter. In a typical automotive

application, a SAW filter is used to achieve better selec-

tivity and large signal capability. The receiving frequency

response without a SAW front-end filter is illustrated in

figures 6 and 7. This example relates to ASK mode. FSK

mode exhibit similar behavior. The plots are printed rela-

tively to the maximum sensitivity. If a SAW filter is used,

an insertion loss of about 3 dB must be considered, but the

over all selectivity is much better.

When designing the system in terms of receiving band-

width, the LO deviation must be considered as it also

determines the IF center frequency. The total LO devi-

ation is calculated to be the sum of the deviation of the

crystal and the XTO deviation of the T5760/T5761. Low-

cost crystals are specified to be within ±90 ppm over

tolerance, temperature and aging. The XTO deviation of

the T5760/T5761 is an additional deviation due to the

XTO circuit. This deviation is specified to be ±30 ppm

worst case for a crystal with CM = 7 fF. If a crystal of

±90 ppm is used, the total deviation is ±120 ppm in that

case. Note that the receiving bandwidth and the IF-filter

bandwidth are equivalent in ASK mode but not in FSK

mode.

Rev. A2, 19-Oct-00

5 (32)

Preliminary Information

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