Low Band Frequency Synthesis Section 7: 2-9
6881091C63-F
The VCOs, together with Fractional-N synthesizer U1201, generates the required frequencies for
transmit and receive modes. The TRB line (U1201, pin 2) determines which VCO/buffer circuit is to
be enabled. A high level on TRB will turn on the transistors in U1378 to turn on via R1376, applying
the 8.3 volt VSF source to the receiver VCO and first buffer. The second buffer in each string
operates from the 9.3V source and become active when RF is applied to their inputs.
The RF signal at the bases of the second buffers are combined and fed back to the Fractional-N
synthesizer via PRE_IN where it is compared to the reference frequency as described below in
Section 4.3. The Fractional-N IC provides a DC steering voltage VCTRL to adjust and maintain the
VCO at the correct frequency.
With a steering voltage from 2.5 V to 11 V at the appropriate varactor diode (CR1302 for the
RX VCO, or CR1310 for the TX VCO), the full VCO tuning range is obtained. Each VCO uses an
AGC circuit to maintain a constant VCO output level across the frequency band. A diode (CR1306 in
the receive VCO, or CR1314 in the transmit VCO) is configured as a voltage doubler which rectifies
the RF level sampled at the VCO drain and applies a proportional negative DC voltage to the VCO
gate. Increased RF level reduces the VCO gain to compensate.
The VCO output is taken from the source and applied to the first buffer transistor (Q1304 receive,
Q1307 transmit). The first buffer output is further amplified by the second buffer transistor (Q1305
receive, Q1308 transmit) before being applied to the receiver first mixer or transmitter first stage
input.
In TX mode the modulation signal coming from the LVFRAC-N synthesizer IC (MODOUT, U1201 pin
41) is superimposed on the DC steering line voltage by capacitive divider C1215, C1208 and
C1212, causing modulation of the TX VCO using the same varactor as used for frequency control.
4.3 Synthesizer Operation
The complete synthesizer subsystem comprises mainly a low voltage LVFRAC-N synthesizer IC,
Reference Oscillator (crystal oscillator with temperature compensation), charge pump circuitry, loop
filter circuitry, and voltage controlled oscillators and buffers. A sample of the VCO operating signal
PRE_IN is amplified by feedback buffer Q1202, low-pass filtered by L1205, C1222 and C1224, and
fed to U1201 pin 32 (PREIN).
The pre-scaler in the synthesizer (U1201) is basically a dual modulus pre-scaler with selectable
divider ratios. The divider ratio of the pre-scaler is controlled by the loop divider, which in turn
receives its inputs via the serial interface to the µP. The output of the pre-scaler is applied to the loop
divider. The output of the loop divider is connected to the phase detector, which compares the loop
divider’s output signal with the reference signal. The reference signal is generated by dividing down
the signal of the reference oscillator, whose frequency is controlled by Y1201.
The output signal of the phase detector is a pulsed DC signal which is routed to the charge pump.
The charge pump outputs a current at pin 43 of U1201 (IOUT). The loop filter (consisting of
R1205-6, R1208, and C1212-14) transforms this current into a voltage that is applied to varactor
diodes (CR1310 for transmit, CR1302, for receive) and alters the output frequency of the
appropriate VCO. The current can be set to a value fixed in the LVFRAC-N IC or to a value
determined by the currents flowing into BIAS 1 (U1201-40) or BIAS 2 (U1201-39). The currents are
set by the value of R1211 or R1207 respectively. The selection of the three different bias sources is
done by software programming.
To Next Page
Loading...
