Theory of Operation
3-40 2715 Spectrum Analyzer Service Manual
Immediately following the VideoFilters is the Scale Factor amplifier. The gain
of the Scale Factor amplifier can be set to 1.1, 2.2, 5.5, or 11 corresponding to
10 dB/div, 5 dB/div,2 dB /div,or 1 dB/div. The linear function is done with the
gain set to 1.1.
The AD834 (U350) is a four quadrant multiplier that is used as a quadrature
detector for demodulation of FM signals. The audio output is sent to the audio
amplifier selectorand to the video input selector.
The limited output of one of the log cellsis used to drive an isolation buffer
(U630A). This signal is used to drive the periodcounter on the Microprocessor
board for frequencymeasurements.
The TDA1013A is used as the audio amplifierfor monitoring purposes, and as a
means of providing feedbackto the user from the microprocessor.
Microprocessor audio appearingat pin 22 of J7 is used to provide feedback to
the operator for push button clicks and out of range or uncalibratedconditions.
This signal is also used to provide a signal forcalibrating the video amplifier and
display storage. The sweep triggersare also tested by this signal.
At power on, the system reset line SYSRST-- on pin 4 of J7 goes low until the
Microprocessor resetsit. During this time, the control latch’s output enable line
is held false and the outputs arein a high impedance state. This is used only by
the mute bit. The pull up resistor will force a mute condition during this time,
thus disallowing any extraneous noise from the speaker.
Display StorageThe Display Storage board digitizesboth vertical (LOGVID) and horizontal
(SWP) signals and places this dig itizedinformation in nonvolatile RAM
(NVRAM). Variousstate machines on the board then take this digitized data
from the NVRAM and generate th e horizontal and vertical signals which are s ent
to the deflection circuitryon the Power Supply board. In addition to displaying
the digitized data, the Display Storageboard also is used to display the readouts
and place markers on the CRT.
The major features of the Display Storag e board are:
HDigitize the LOGVID signal from the Log Board (8 bits)
HDigitally track the SWP signal from theS weepBoard (9 bits)
HAcquire and store up to fourwaveforms in RAM