srf-2724cs RF Micro Devices, srf-2724cs Datasheet - Page 11

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

The SRF-2724CS is a fully integrated 1.5 Mbps frequency shift keyed (FSK) transceiver that operates in the unlicensed 2.4GHz
ISM frequency band. The device has been optimized for digital cordless telephone applications and includes all the frequency
generation, receive and transmit functions for a raw data rate of 1.5Mbps. This high data rate allows for data spreading, such
as Direct Sequence Spread Spectrum (DSSS) modulation, which improves range. The SRF-2724CS receiver architecture is a
dual conversion Low IF, which has all of the sensitivity and selectivity advantages of a traditional super-heterodyne receiver
without requiring costly, bulky external filters.
adjacent channel signals. An active image reject mixer converts this signal down to a Low IF frequency, where the data is lim-
ited, filtered, and demodulated. This architecture provides all the benefits of direct conversion to baseband while maintaining
the stability and robustness of a traditional super-heterodyne.
A single synthesizer is used for both the receiver and the FSK transmitter. The phase locked loop (PLL) is completely integrated,
including the voltage controlled oscillator (VCO), tuning circuits, and VCO resonator.
In RECEIVE MODE, the SRF-2724CS is a dual conversion Low IF receiver. No external SAW filters are required. The integrated
image reject mixer gives sufficient rejection in this channel. All channel filtering and demodulation is performed using active fil-
ters, which are automatically aligned. A matched bit rate filter and a data slicer follow the demodulator. The sliced data is pro-
vided at the DOUT pin, and the analog data is available at AOUT.
In TRANSMIT MODE, the SRF-2724CS generates a 2.4GHz output using the transmit mixer. An auto-aligned transmit data filter
and modulation compensation circuit results in an adjustment-free transmitter. The VCO is modulated by the transmit data,
which is put through a sigma-delta fractional-N PLL ensuring modulation accuracy. This modulation occurs while the phase
locked loop is closed, thus allowing practically infinite transmit or receive times with excellent frequency accuracy and stability.
A 3dBm FSK-modulated differential signal is output at the TXO/TXOB pins at the 2.4GHz carrier frequency.
The integrated PLL frequency synthesizer includes a fully integrated VCO, prescaler, phase detector and charge pump. The ref-
erence frequency is generated from the incoming signal at the FREF pin, which can be either 6.144MHz or 12.288MHz. The
loop filter is external to allow customers to optimize their loop bandwidth to their system’s lock time and in-band phase noise
requirements. This frequency-agile synthesizer allows the SRF-2724CS to be used in frequency hopped spread spectrum
(FHSS) applications with nominal channel spacing of 2.048MHz. Carrier frequency is programmed via the configuration regis-
ters and 3-wire serial interface. The VCO tank circuit (inductor and varactor) is fully integrated.
Figure 2: SRF-2724CS Internal Block Diagram
Prelim DS090410
The RF mixer down-converts the 2.4GHz RF input signal to the first intermediate frequency (IF), where it is filtered to remove
Receive Input
TXO/TXOB
2.4 GHz
2.4 GHz
Output
RXI
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at ([phone] or [e-mail].
Proposed
2
Transmit
Mixer
Bandgap
Functional Description
Divide
by 2
Downmixers
VTUNE
Quadrature
Quadrature
Divider
1.6 GHz
VCO
Alignment
Filter
Divider
PLL
Modulator
Two-port
PLL Loop
RSSI
Filter
P. D.
t o
F
V
QPO
Test Mux
Registers
Control
Divider
Control
Mode
Ref.
AOUT
DATA
PAON
RXON
XCEN
Transmit Data In
CLK
Receive Data
Frequency
Reference
TPC
EN
FREF
RSSI
DOUT
Out
DIN
Control Bus
Serial
control lines
Parallel
SRF-2724CS
11 of 26