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PowerControl
The output power from the power amplifier is regulated
by an active feedback loop. The reference value POWLEV
is set via a DIA converter in Bertram and is connected to the
positive input of an op-amp. POWSENSE, the detected
output power level, is connected to the negative input of the
same op-amp. The op-amp produces an error voltage which
controls the base of V3 12. This ultimately varies the
collector voltage of V302 which in turn varies the output
level of the driver stage.
RECEIVER BLOCK
General
The Rx-stage is the first link in the receiver chain. Its
purpose is to select and amplify desired frequencies
(851.0125 MHz - 869.9785 MHz). The selected frequency
is mixed down to 77.25 MHz and amplified approximately
18 dB. This section consists of the Rx front end chip (N2
11) which is comprised of a low noise amplifier, mixer, and
a Rx band SAW filter (Z21 1)
DuplexFilter
The received signal comes in through the antenna and
straight into the duplex filter. The duplexer consists of a 3-
pole Tx section of low pass characteristics with a notch at
the Rx frequency and a 4-pole Rx section filter all in one
package.
The task for the Rx branch is to suppress the transmitter
signal to a level low enough not to overdrive the receiver
front end amplifier, and attenuate external spurious signals.
For example, the 1st image spurious frequency located at 2 x
the 1st IF (154.5 MHz) mixed with 868.9875 MHz is
typically 40 dB down. Another task of the duplexer is to
suppress leakage from the local oscillator.
Front End Chip (N211)
The RF input to N21 I is matched for minimum noise
figure of the RF amplifier. To get the desired performance,
the LO power has to be greater than -6 dBm. Inductor L211
tunes the LO input to provide maximum LO drive to the
mixer. The IF output is matched to 900 ohms in parallel with
-1.7 pf at its resonant frequency of 77.25 MHz.
N211 contains an RF amplifier which is fabricated
using a GaAs process. The input is matched for minimum
noise figure requiring a source reflection coefficient of angle
50 degrees and magnitude of 0.67. The output impedance is
internally matched for 50 ohms allowing simple connection
to the following bandpass filter. Receiver sensitivity is
predominately defined by this stage.
The bandpass filter is needed for spurious response
rejection (suppression of the first image frequency). The
filter also suppresses the local oscillator backwards. The
bandpass filter is a SAW filter with a center frequency of
860.5 MHz and a pass band width of 25 MHz. It is specified
to have a maximum insertion loss in the passband of 3.0 dB
and a maximum ripple of 1.0 dB. The attenuation at 1007
MHz is at least 50 dB to meet the specified suppression of
the first image frequency.
The mixer inside N211 is also built using a GaAs
process. An internal buffer for the LO is included allowing
low LO input power and immunity from variations in LO
input power. The RF input, pm. 11, must see a source
reflection coefficient angle from the BPF of+135 to +270
degrees. Inductor L212 provides a short at the IF fre ue
(77.25 MHz) to improve noise performance. The output is
tuned to 900 ohms in parallel with match the filter in
reactance of approximately - 1.7 pf. R266 along with the
output resistance of N211 and the parallel resistance of L23l
provide the real part of the impedance (900 ohms) that the
IF filter sees. In addition, R266 provides a constant load to
the mixer output to improve IM performance and to swamp
out variations in the PCB and other components.
Buffer and IF/AF chip
The buffer/IF/AF part is the second link in the receiver
chain. It performs the major part of the RF amplification and
all the channel filtering. The 1st IF at 77.25 MHz is
buffered, downconverted to 450 KHz, and FM detected in
Bertram. The IF/AF chip generates a DC voltage, which is
logarithmicly proportional to the strength of the received
signal.
The 77.25 MHz SAW filter (Z231) helps protect the
buffer and IF/AF chip (N201) from signals that could cause
desense and intermodulation problems. Z231 is a band pass
filter with a center frequency of 77.25 MHz and a 3 dB band
width of +/-15 KHz. A buffer is placed between the SAW
and IF/AF chip in order to insure the IF/AF will be in
limiting for low signal levels. If the IF/AF chip is not in
limiting the noise figure will appear to suffer. The IF/AF
circuit is an integrated circuit containing a second mixer, IF
amplifier and limiter, RSSI circuits, FM detection circuit
and muting circuit. Only the mixer, limiter, and RSSI
circuits are used. The rest of the flinctions are performed
external to the chip.
The 1st IF frequency of 77.25 MHz is fed to the buffer
and then to the 2nd mixer where it is downconverted to 450
KHz (2nd IF) in N201. The conversion gain of the mixing
process is 34 dB. The 2nd LO signal at 76.8 MHz for the
mixer comes from Bertram. The reference oscillator (12.8
MHz) is multiplied by 6 to generate the 2nd LO.
AE/LZB 119 1890 R1A
9
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