The HP 8560A 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’s linear step-gain amplifiers. The A4 assembly’s video-offset circuit provides the remaining 60 dB of log mode IF gain. The A4 assembly’s 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 10, 30, and 100 Hz. The A4 Cal Oscillator 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 10 Hz, 30 Hz, and 100 Hz resolution bandwidths.

The Cal Oscillator’s output has three forms (all -35 dBm):

n10.7 MHz

n9.9 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:

nAdjust gains, log amps, and video slopes and offsets

1 Adjust 3 dB bandwidth and center frequencies of LC resolution BW filters (30 kHz through 1 MHz).

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 (10 Hz through

100 Hz).

The Low-Pass Filter is illustrated in Function Block AB. The Low-Pass Filter filters the square wave output of the variable gain amplifier so that harmonics do not subtly degrade the performance of the IF ADJUST process.

A5 IF Assembly

The A5 IF Assembly has four crystal filter poles, four LC filter poles, and step gain amplifiers. The crystal filters provide resolution bandwidths of 300 Hz to 10 kHz. The LC filters provide resolution bandwidths of 30 kHz to 2 MHz. All filter stages are in series. PIN diode switches bypass unwanted stages.

6-40 General Troubleshooting