A Manual Gain Control buffer and window comparator provide a means for the DSP
processor to modify the analog AGC function. When a digital-to-analog converter on
the DSP BOARD reduces the MGC voltage below the existing analog AGC voltage,
feedback regulates the AGC voltage to the new set value. The ANALOG and DIGITAL
outputs inform the DSP which driving source is asserting analog AGC gain reduction,
the analog envelope detector or the digital-to-analog converter. This mechanism allows
software implementation of both manual gain control and slower analog AGC decay
rates.
Another function enabled by this circuit arrangement, useful in narrowband detection
modes, is DSP correction of analog AGC action caused by strong signals located outside
the digitally implemented selectivity bandwidth. Because the analog AGC detector
samples the entire 300 kHz analog IF bandwidth, some signals causing AGC action may
be invisible to the DSP when narrow selectivity filtering (partially implemented in a
AD6620 Digital Down Converter) precedes the digital detection algorithm. The
information provided to the DSP by the AGC, ANALOG, and DIGITAL outputs allow the
software to identify and compensate for the effects of these strong "out-of-band"
signals. As a result, signal strength calculation and overall AGC action is based only on
those signals that are inside the chosen selectivity bandwidth. Fast, Medium and Slow
AGC modes with fixed, pre-determined, attack and decay times are provided, along with
a Programmed AGC mode where the operator can select the attack, hang and decay
times in milliseconds.
5.2.6 First Local Oscillator
The FIRST LOCAL OSCILLATOR module contains a single-loop fractional-N frequency
synthesizer producing 2272-4090 MHz in 2 MHz steps. Serial data from the DSP board
configures synthesizer to divide the provided 100 MHz reference by five and the
selected VCO by a MODULO 10 fractional N-number of 113.6 to 204.5 for phase
comparison. Integrated phase pulses from the synthesizer chip develop a DC tuning
voltage for the active VCO. The tuning voltage output is sampled and delivered to the
DSP board for possible charge pump gain calibration based on VCO tuning voltage slope
variation. A loop filter reduces reference and phase-correction ripple on the tuning
voltage before it is applied to the VCO control input. The active VCO output selected by
a switch splits to two buffer amplifiers. One amplifier provides feedback to the
synthesizer chip, completing the loop. The second amplifier develops the Mid/High
Band First LO output which splits between the module output and MODULO 16 divider
to provide the Low Band First LO of 142-228 MHz.
The 100 MHz master frequency reference signal is developed on the DSP board and is
delivered through this module to the SECOND LOCAL OSCILLATOR module.
PN 74446
5-5
