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used as an input, taking the pad loss in consideration for evaluation, when using external VCO's or
generators, providing the on board VCO is disabled.
A couple of power supply regulators have been added to improve the transmit and receive switching
characteristics of the HFA3783. DC current transients which occurs when the device is switched
from transmit to receive cause VCO spurs to appear.
2.6 PLL
The HFA3783 includes a classical architecture Phase Lock Loop circuit with a three wire serial
control interface to be used with an external VCO. It consists of a programmable "R" counter used to
divide down the frequency of a very stable reference signal up to 50MHz to a phase comparator. A
couple of counters ("A" and "B") with a front end prescaler ("P or P+1"), with dual modulus control,
divides down the frequency of an external VCO signal to the same phase comparator. The
comparator controls a charge pump circuit and an external loop filter closes the loop for VCO control.
The VCO frequency dividing chain works with a dual modulus control as follows: At the beginning of
a count cycle, and if the A counter is programmed with a value greater than zero, the prescaler is set
to a division ratio of (P+1) where P can take programmable values of 16 or 32.
Notice that the prescaler output signal is always fed simultaneously to both A and B counters. Upon
filling counter A, the prescaler division ratio becomes P and the B counter continues on its own with
A standby. This processis known as "pulse swallowing". The expression B-A (counts) is the
remainder of counts carried out by the B counter after A is full. Both A and B counters are reset at
the end of the counting cycle when B fills up. As a result, the total count or division ratio used for the
VCO signal is A*(P+1) + (B-A)*P which simplifies to [P*B+A]. (A and B counters are referred as the
"N" counter).
The Charge Pump (current source/sink) has 4 programmable current settings. This variation allows
the user to change the reference frequency for different objectives without changing the loop filter
components. The user can program the charge pump sign based on the direction of increase or
decrease of the VCO frequency. The most often used VCO's in the market have positive KVCO's
where the VCO frequency increases with an increase in control voltage. In this case, the charge
pump current shall "source" current (to the main capacitor of the loop filter) when the VCO frequency
becomes less than the desired frequency of operation.
2.7 Direct Sequence Spread Spectrum Baseband Processor(HFA3861)
The HFA3861B has on-board A/D's and D/A for analog I and Q inputs and outputs, for which the
HFA3783 IF QMODEM is recommended. Differential phase shift keying modulation schemes
DBPSK and DQPSK, with data scrambling capability, are available along with Complementary Code
Keying to provide a variety of data rates. Both Receive and Transmit AGC functions with 7-bit AGC
control obtain maximum performance in the analog portions of the transceiver. The HFA3861B is
housed in a thin plastic quad flat package (TQFP) suitable for PCMCIA board applications.
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