Theory of Operation
1705A Spectrum Monitor 4-11
MicroprocessorDiagram 5The 1705A is a microprocessor-controlled instrument. Circuitry on Diagram 5
shows the microprocessor,the front-panel LED drive, the crt readout drive, the
graticulelight circuit, and the trace rotation circuit.
The processor(U2) is an 8-bit, 3-port microprocessor running at 12 MHz. U8is
the lower orderaddress de-multiplexer for the Program PROM U9. U9 is a 64K
UV ErasableC MOS PROM. R5is the bus termination assuring TTL levels (0
=0 V,1 = 5 V). Output enable forthe P ROM is the PSEN output from the
microprocessor(U2, pin 29). The lower-order addressesfrom the processor
(AD0 --AD7), which are de-multiplexed by U8, are from Port 0; the higher-order
addresses(AD8 --AD12) are from Port 2. Thehigher-order addresses are not
multiplexed.
Port 1 is a multifunction input/output port. Lines0 through 4 are used to poll the
front-panelpush-button switches (momentary ground closures) to set up the
measurementprogram. Line 6 outputs the clock (U2, pin 7) that is usedby the
Readout DAC and the NVRAM (Non-VolatileRandom Access Memory).
Line 5 is the data transfer(U2, pin6) for the NVRAM, U4. TheReadout Enable
(/RO--EN)that turns on the crt readout is output through Line 7 (U2, pin 8). The
Center FrequencyReadout data from U1, the readout ADC, is input through
Line 5 also (U2, pin 6).
U1 is an A-to-D Converter(ADC), with successive approximation register. Itis
used to convertthe analog voltage level from the front--panelCENTER
FREQUENCY control to a digital signal forthe microprocessor and the Readout
Digital-to-AnalogC onverter(DAC). R4 is the calibration adjustment to ensure
correctreadout.
U4 is the NVRAM that stores theinstrument condition when power is turned off
or lost, to ensure that the instrument will come back up in the correctoperating
condition. Thepower down detection circuit consists of a comparator, U11,and
a +5 volt regulator,U3. U3 input voltage is from the +15 volt supply. C5
chargesup high enough to allow U3 to continue to power U11 so that it can store
needed dataduring the power-down sequence. U11monitors the +5 volt supply
on pin 2. Pin3 is set to approximately 2.5 volts and has a large capacitor,C13,
to provide a slow decay. Under normal conditions, pin 2 is slightly higher than
pin 3, keeping the output (pin 7)high. When instrument power starts to go
down, pin 2 goes below pin 3, which forcesthe output (pin 7) low to enable the
/STO input to U4. When/STO goes low, the currentconditions, as input from
the microprocessorD-Out output, are stored in the U4 Non-Volatile RAM.