Posted on mar. 03 janvier 2017 (last updated on jeu. 10 novembre 2022)

This is the part 2 of the series about my HP 3562A Digital Signal Analyzer, quickly describing the Digital Section of the instrument.

Digital Section

The unit is quite capable, since the main CPU is a 68000 (not exactly of small CPU for the time).

Picture of the main 68000 CPU of the HP3562A

The 8 boards are:

  • A1: Digital source and front-end interface (rev B, ref: 03562-66501)
  • A2: CPU & GPIB controller (rev D, ref: 03562-66502)
  • A3: Memory (rev B, ref: 03562-66538)
  • A4: Local oscillator (rev C, ref: 03562-66504)
  • A5: Digital filter (rev B, ref: 03562-66505)
  • A6: Digital filter controller (rev D, ref: 03562-66506)
  • A7: Floating Point Processor (FPP) (rev B, ref: 03562-66507)
  • A9: FFT (rev B, ref: 03562-66509)

The service manual also describes an A8 board with additional RAM, but my device does not have such a board. It looks to me that my version of the DSA has both extra ROM and RAM on the A3 memory board.

Digital Source


The Service Manual available on the net lacks a few pages in the Digital Source section, so I do not have details on the Timing Control section.

Block diagram of the A1 Digital Source Board

This board is mainly responsible for generating the digital signals that are used as input for the source DAC. It generates all sort of noise figures, bursts, sweeps, and so on.

Picture of the A1 Digital Source PCB of the HP3562A

The Timing Control section looks like:

A1 Timing Control block diagram

The Phase Resolution is used in external and internal triggered measurements, ensuring that the phase of a triggered measurement is accurate. Since the trigger moment does not always occur on a sample and hold edge, there is a time delay and phase error in the data. This phase resolution circuit counts the time between the samples and a trigger.

A1 Phase Resolution block diagram

The Burst Control circuit controls the burst length and generates the pulse signal to the Local Oscillator (A4). It provides the gating signal that gates the analog source on and off during the burst and chirp modes.

A1 Burst Control block diagram

Other parts of this board are the LO Input Receiver that synchronizes the local oscillator input data to the sample rate, the Multiplier that multiply the LO data by the noise and send it to the analog source, and the Noise Generator that produces binary random sequences that is used for band-limited random noise and burst noise signals.


The processing unit is a MC68000. It aims at telling each assembly which process to execute and monitor the overall functionning and data processing of the instrument.

Blocke diagram of the A2 board of the HP3562A

The main CPU board, with the beautiful MC68000P9 DIP64 package:

Picture of the A2 CPU board of the HP3562A

It comes with 2 populated M5M256BP static ram chips (32k x 8bits), for the CPU, but seems to be capable of holding 4 more of them. The CPU runs at 8MHz.

There is also some ROM, the main "boot loader" program. Unfortunately, neither of the ROM chips are mounted on sockets but are directly soldered on the PCBs, so I did not took the risk of damaging one of them by desoldering them for dumping their content.

Most of the remaining of the circuit is to manage the global data bus, the system bus, the IRQs and the GPIB bus.

The blue connector is for the GPIB connector.


Picture of the A36 memory board of the HP3562A

This board is described as an "extension of the read only memory of the system CPU board" and read/write memory used by most of the other assemblies.

Block diagram of the A38 memory board of the HP3562A

The ROM section stores most programs for the HP 3562A except the startup routines (which are on the ROM ships of the A2 CPU bloard).

The board allows flexibility in the number and type of ROM chipes used. ROM density is selected by placement of jumpers.

Block diagram of the RAM/ROM selection mecanism on the A38 board of the HP3562A

The RAM section of the assembly consist of 4 32k by 8 bits static RAM chips as well as the arbitrer section which controls access requests to the global RAM from six devices (FFT, both Digital Filters, Display, FPP and the system CPU).

Block diagram of the memory bus pipeline of the HP3562A

The Display Controller section also lies on this A38 board.


On older versions of the instruments, this board was splitted in 2 (A3 and A8) and used less dense chips (thus, much more chips).

Local Oscillator

The Local Oscillator produces digital sin and cosine signals (synchronized with the sample rate). The sinusoidal signal is generated from a table of values stored in the ROM.

Block diagram of the A4 local oscillator board of the HP3562A

Block diagram of the A4 local oscillator board of the HP3562A

Picture of the A4 bloard of the HP3562A

Picture of the A4 bloard of the HP3562A

Digital Filter and Digital Filter Controller

The Digital Filter Assembly consists in both A5 and A6. It processes two channels of serial data coming from the instrument front end (ADC) ans stores the results in global RAM. Processing consists of conversion from a serial format to a parallel format and, if required, digital filtering or zoom (a combination of frequency shifting and filtering). The processed data is transferred on the global data bus.

Digital Filter Assembly block diagram

Digital Filter Assembly block diagram.

Digital Filter: Each digital filter consists of a control IC and 2 filter ICs, one for the real data and one for the imaginary data.

Digital Filter for the zoom mode

The Digital Filters are fed with a kind of LF I/Q demodulator, used for zooming or actual digital filtering.

Picture of the A5 Digital Filter board

Picture of the A5 Digital Filter board.

Digital Filter block diagram

The A5 Digital Filter board block diagram.

Clock Generator: The clock generator creates two complementary clock pulses from the 10.24MHz system clock.

Overload Detect: The first 3 bits in the ADC serial data stream contain overrange information that must be stripped off the serial data.

Picture of the A6 Digital Filter Control board

Picture of the A6 Digital Filter Control board.

Digital Filter Controller block diagram

The A6 Digital Filter Controller board block diagram.


The Floating Point Processor board is a fast arithmetic unit which carries out real and complex arithmetic operations on blocks of data stored in the global RAM. The processing ALUs are six AM2903 bit-slice microprocessor ICs and one AM2910 microprogram controller. Instructions are provided to the ALUs by an address sequencer and seven microcode PROMs.

Floating Point Processor block diagram

Floating Point Processor block diagram.

Picture of the A7 FPP board

Picture of the A7 FPP board.

This Floating Point Processor is capable of handling 16 bits integers, 32 bits and 64 bits floats.


Block diagram of the A9 FFT board of the HP3562A

Block diagram of the A9 FFT board

The FFT board performs windowing, FFT and Inverse FFT directly from and to the RAM. It's built around a TMS230 microprocessor runningat 5MHz

Picture of the A9 FFT board of the HP3562A

Picture of the A9 FFT board


Block diagram of the keyboard processor

Block diagram of the keyboard processor