Agilent Technologies 856290216 service manual If Section, A4 Log Amplifier/Cal Oscillator Assembly

IF Section

The IF section processes the 10.7 MHz output of the RF section and sends the detected video to the ADC/interface section. The following major assemblies are included in this section:

A3 interface assembly

A4 log amplifier/calibration oscillator assembly

A5 IF assembly

The spectrum analyzer uses trace-data manipulation to generate the 5 dB/DIV scale from the 10 dB/DIV scale. The A3 interface assembly amplifies and offsets the 10 dB/DIV video to generate the 2 dB/DIV scale. The 1 dB/DIV scale is generated from the 2 dB/DIV scale through trace data manipulation. The first 50 dB of IF gain (log and linear mode) is achieved using the A5 assembly linear step-gain amplifiers. The A4 assembly video-offset circuit provides the remaining 60 dB of log mode IF gain. The A4 assembly linear amplifiers provide 40 dB of linear mode gain. IF gain steps of less than 10 dB (regardless of the reference level) are accomplished on the A5 assembly.

A4 Log Amplifier/Cal Oscillator Assembly

The A4 log amplifier has separate log and linear amplifier paths. After amplification, the signal path consists of a linear detector, video log amp, buffer amplifier, video offset, and video buffer amplifier. Other auxiliary functions include the frequency counter prescaler/conditioner, the AM/FM demodulator, and down-conversion to 4.8 kHz for digital resolution bandwidths of 1 Hz through 100 Hz.

The cal oscillator, which is part of A4, supplies the stimulus signal for automatic IF adjustments. Normally, the oscillator operates only during retrace (for a few milliseconds) to adjust part of the IF. (All IF parameters will be re-adjusted approximately every five minutes.) With continuous IF adjust on, a group of IF parameters are adjusted during each retrace period (non-disruptive). If continuous IF adjust is off, the most recent IF calibration data will be used.

The IF parameters adjusted include step gains, log amplifier gain and offset, bandwidth centering, 3 dB bandwidth, bandwidth amplitude, crystal-filter symmetry, and oscillator frequency used in 1 Hz through 100 Hz resolution bandwidths.

The cal oscillator output has three forms (all -35 dBm):

n10.7 MHz

m 9.9 to 11.5 MHz in 100 kHz steps

nFrequency sweeps from 20 kHz to 2 kHz centered at 10.7 MHz (lasting 5 to 60 ms respectively)

The purpose of these signals is to:

nAdjust gains, log amps, and video slopes and offsets.

nAdjust 3 dB bandwidth and center frequencies of LC resolution BW filters (30 kHz through

1MHz).

nAdjust 3 dB bandwidth, symmetry, and gain of the crystal resolution BW filters (300 HZ through 10 kHz).

nAdjust gain and gain-vs-frequency for digital resolution bandwidths (1 Hz through 100 HZ).

7-46 General Troubleshooting