Theory of Operation
3-38 2715 Spectrum Analyzer Service Manual
Log AmplifierThe Log Amplifier system is a high performance, intermediateamplifier system
that provides display laws of lineardetection and logarithmic detection of the
incoming signal. The log display has scale factorsof 10 dB/div, 5 dB/div, and
1 dB/div. The linearmode has the ability to magnify the top portion of the
waveform to enhance the measurementcapability. The module also contains an
FM detector, which, when usedwith the built in audio amplifier, can be used to
monitor the modulated input signal for enhancedsignal identification. The audio
amplifier can also beused to monitor the output of the displayed waveform
regardless of the video source. Thereis an alternate audio input for the push
button clicks generated by thesystem controller. This audio signal is also used
for the vertical amplitude reference, to check the sw eep speed, and for internal
triggers. An amplitude limited output is available for the period counter for
determining the actual frequencyof the signal present in the IF system. External
video inputs are provided for the use of external detectorsand the rear panel. A
provision is made for clamping thedisplay when the 1st LO is tuned out-of-
band. Frequency dependent amplitude(flatness) correction is also performed on
this board.
The Log Amplifier system is configuredby using a 4.5 by 8.75 circuit board
mounted in the card cage. TheMicroprocessor interface is through three 8 bit
shift registers.
The signal level at the Log Amplifierboard (J190) for full screen deflection is
-- 10 dBm. The first stage of the Log Amplifier is an amplifier that providesgain
between 18 dB and 24 dB, depending on the frequencyof the 1st LO. The
amplifier is adjusted such thatthe gain is 18 dB at low frequencies and is up to
24 dB at the high end of the frequency range, becau se the frequency dependent
amplitude error is always greater at the hig her frequencies. The feedback is
adjusted by changing the amount of currentflowing through PIN diodes CR290
or CR291. The correction voltage, SWPSLOPE, reflects the frequency of the 1st
LO. Provision for a second correction voltage from a futureDigital S ignal
Processing module is included. This second correctionvoltage, which provides
an interpolated voltage, will be derivedfrom a flatness table. At present, only
SWPSLOPE is used.
The output of the first stage, a gainslope amplifier, is then fed to a two pole
noise filter to limit the noise to 5 MHz andto provide two more poles of 5 MHz
bandwidth resolution filtering. The output of this filteris then sent to the log
stages through a set of jumpersso the signal can be broken for calibration
purposes and bufferamplifier Q170 (in a common base configuration) to form an
auxiliary IF output.