DS2722-F-03 MICRO-LINEAR [Micro Linear Corporation], DS2722-F-03 Datasheet - Page 16

• Manufacturer Part Number: DS2722-F-03
• Description: 900MHz Low-IF 1.5Mbps FSK Transceiver Final Datasheet
• Manufacturer: MICRO-LINEAR [Micro Linear Corporation]

Receive Signal Strength Indication (RSSI)
RSSI is an indication of field strength. It is typically used to control transmit power to conserve battery life. It may also
be used to determine if a given channel is occupied (see Figure 3).
Automatic Filter Alignment
When the ML2722 is placed in RECEIVE mode, it automatically tunes all the internal filters using the reference
frequency from the REF pin. This self-calibration sets:
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§
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TRANSMIT MODE
In TRANSMIT mode, the VCO is directly modulated with filtered FSK transmit data. The ML2722 transmitter is a 2-FSK
transmitter using a directly modulated open loop VCO. The ML2722 design supports transmit time slot lengths up to
10ms, and the time required to set up the transmitter for a new time slot (TXCAL mode) is 62.5µs.This type of
transmitter is simple, low power, and well suited to a time-time slotted system. The transmitter uses the stored VCO
tuning voltage on the PLL loop filter to set the VCO frequency for the duration of the transmitter time slot. The
modulation is introduced through a second VCO tuning port. This modulation port has a much lower tuning sensitivity
than the main tuning port in order to produce the ±460KHz FSK deviation. Compensation circuits stabilize the
modulation deviation over the VCO tuning range, and internal logic manages the correct transition from TXCAL to
TRANSMIT mode.
The operating cycle of the transmitter starts with the falling edge of RXON. In the first 62.5µs after the falling edge of
RXON, a calibration mode (TXCAL) is triggered. The data filter input is zeroed and the PLL locks the VCO frequency to
the desired RF channel center frequency. A CW signal at the selected RF channel frequency comes out of the Transmit
RF output. When the internally generated PLLEN control line changes state, the transmitter starts its transition to
TRANSMIT mode. The PLL charge pump is disabled, leaving the PLL loop filter to hold the correct tuning voltage for
this channel. The data formatter injects an NRZ bipolar data waveform into the Transmit data filter. The Transmit data
filter bandlimits this waveform, and feeds it to the modulation compensation circuits. These scale the modulation voltage
(depending on the VCO tuning voltage) and drive the VCO tuning port with the scaled, filtered modulation. The voltage
on the modulation port swings above and below its central value to produce 2-FSK modulation on the VCO. The
modulation filtering is sufficient to meet the FCC occupied bandwidth and out-of-band emissions requirements, and
does not introduce significant ISI (Inter Symbol Interference).
The transmit modulation filter is automatically tuned during the first part of every RX and TX time (RXCAL&TXCAL) to
remove the need for production alignment. When the chip is first powered up, the tuning information is reset to mid-
range. The ML2722 should then be enabled with RXON high. The rising edge on XCEN will trigger a complete
calibration of all the on chip filters, which takes 320µs. This ensures the modulation filters are aligned to prevent
unwanted spurious emissions.
DS2722-F-05
Discriminator center frequency
IF filter center frequency and bandwidth
Receiver data low-pass filter bandwidth
Transmit data low pass filter bandwidth
3.00
2.50
2.00
1.50
1.00
0.50
0.00
-100 -90 -80 -70 -60 -50 -40 -30 -20 -10
FINAL DATASHEET
Figure 3. Typical RSSI Response
Receive RF Level (dBm)
RSSI voltage vs RF Level
DECEMBER 2003
ML2722
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