Theory of Operation
3-24 2715 Spectrum Analyzer Service Manual
This voltage across R926 defines thefeedback current to the summing junction
of the operational amplifier.This circuit forces 1st LO main coil current to be
proportional to the sum of the currentsfrom the various control sources, which is
described later.The normal operating current range of the main coil is from
approximately 135 mA to 255 mA.
Q920 is a switch that is normally conducting andconsequently has no significant
impact on circuit operationas described. However, it is sometimes necessary to
momentarily set the main coil currentto zero in order to standardize frequency
errors due to hysteresisin the 1st LO magnetic structure. When this is desired,
the microprocessor assertsthe DEGAUS S line HIGH (by way of the digital
interface circuits),which causes Q931 and Q930 to pull the gate of Q920 to
-- 8.5 V. This opens the switch, disconnecting the normal input and feedback
paths of the circuit. R929 provides a slight negativefeedback in this situation,
ensuring that the main coil currentgoes to zero instead of to some undefined
value.
The main coil current is thesum of at least two items. First, there is a large fixed
bias current stemming fromthe reference supply and R923 (trimmed by R931
and U920) to set the 1st LO at 3.01 GHz in the absence of other inputs. Second,
there is a currentinduced by DAC U950A (and subsequent circuits), which is
used to steer the 1st LO over therange from 2.11 to 3.91 GHz (2.11 to 4.26 GHz
for Option 50 and Option 75). This current is normallya steady state value
selected by the microprocessorto coarsely set the center frequency. Third, ifthe
instrument is set to a span greaterthan 5 MHz/div, there is a sawtooth sweep
waveform summed into the main coilcurrent through switch U830A. This
corresponds to the wide span operatingmode discussed earlier in the Block
Diagram description.
FM Coil Driver.U811 and U810 together form a buffered operational amplifier
used in the inverting configuration. Inputcurrents to the summing junction come
from a variety of sources, to be described later.R905 is the current sense resistor
for the FM coil. The voltage developed across it results in the current fed backto
the summing junction through R815. Thus, this circuit forces acurrent through
the FM coil proportional to the sum of the variouscontrol currents put into the
summing junction. The normal current rangefor the FM coil is from approxi-
mately -- 120 mA to +120 mA.
Q910 and Q911 serve as a low resistance switch. When cl osed (gates held
several volts positive), the FM coil driveroperates as described above. When
open (gates held severalvolts negative), the driver is disconnected and the FM
coil gets its current fromR 714, which is thesituation when the instrument is
phase locked. Control of the switch stateis through Q703 and Q800 from the
signal PLLCON--. This signal is LOW when the instrument is phase locked.
Several other switches arealso controlled by the same command. R812 keeps
U811 out of saturationwhen the switch is open.