srf-2724cs RF Micro Devices, srf-2724cs Datasheet - Page 13

• Manufacturer Part Number: srf-2724cs
• Description: 2.4ghz Low-if 1.5mbps Fsk Transceiver
• Manufacturer: RF Micro Devices

Receive
The SRF-2724CS uses a double-conversion super-heterodyne receiver with a nominal second IF of 1.024MHz. The signal flow
in RECEIVE mode is from the RF input, through an RF down-conversion mixer and integrated IF filter, image reject quadrature
mixer, integrated Low IF filter, hard limiter, frequency to voltage converter, and data filter to the AOUT pin and data slicer where
the digital NRZ data is available at the DOUT pin. A 20dB step AGC extends the dynamic range of the receiver. The SRF-2724CS
receive chain is a Low IF receiver using advanced integrated radio techniques to eliminate external IF filters and minimize
external RF filter requirements. The precision filtering and demodulation circuits give improved performance over conventional
radio designs using external filters while providing integration comparable to advanced direct conversion radio designs.
Figure 3: RSSI Voltage versus Input Signal Level
Automatic Filter Alignment
When the SRF-2724CS is placed in RECEIVE mode, it automatically tunes all the internal filters using the reference frequency
from the FREF pin. When the chip is powered up (VDD first applied), the tuning information is reset to mid-range. This self-cali-
bration sets:
• Discriminator center frequency
• IF filter center frequency and bandwidth
• Receiver data low-pass filter bandwidth
• Transmit data low-pass filter bandwidth
Transmit Mode
In TRANSMIT mode, the PLL is closed to eliminate frequency drift. A two-port modulator modulates both the VCO and the frac-
tional-N PLL. The VCO is directly modulated with filtered FSK transmit data. The PLL is driven by a sigma-delta modulator, which
ensures that the PLL follows the mean frequency of the modulated VCO.
The transmit modulation filter is automatically tuned during every RECEIVE time, alleviating the need for production alignment.
Asserting RXON enables the SRF-2724CS. The rising edge of XCEN triggers a complete calibration of all the on-chip filters,
which takes up to 256μs, which ensures the modulation filters are aligned to prevent unwanted spurious emissions.
Prelim DS090410
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Proposed
SRF-2724CS
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