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==========================================
 HP3562A Dynamic Signal Analyzer - Part 3
==========================================

:Author: David Douard
:Category: Electronics
:Tags: HP3562A, repair, test equipment, DSA
:series: HP 3562A
:series_index: 3

This is the part 3 of the series about my
`HP 3562A Digital Signal Analyzer <{filename}/hp3562a.rst>`_,
quickly describing the Analog Section of the instrument.


Analog Section
==============

The analog section consist in 6 boards:

- A30: Analog Source
- A31: Trigger
- A32, A34: Input ADCs
- A33, A35: Input

Analog Source
-------------

.. image:: {static}/images/hp3562a/hp3562a_a30_block_diagram.png
   :alt: A30 - analog source block diagram

This board is mainly a DAC converting signal signal from the Digital
Source board (for sin waves). It also generates Pseudo Random Noise
and square waves (used for input calibration).

The analog signal produced by the DAC is filtered by a 100kHz low pass
filter, then pass throught a step attenuator (5mV per step). The way
this is implemented is quite interesting: the analog signal from the
main DAC is used a reference voltage for a multiplying DAC.

.. image:: {static}/images/hp3562a/a30_analog_source.jpg
   :alt: A30 - PCB

.. image:: {static}/images/hp3562a/a30_analog_source_dac.jpg
   :alt: A30 - PCB (details)

Trigger
-------

A first section of this board produces the trigger signal (for A1
Digital Source and A5 Digital Filter boards) from one of the 4
possible trigger sources: external, channel 1, channel 2 and the
calibration.

.. image:: {static}/images/hp3562a/trigger_level.png
   :alt: A31 - Trigger circuit block diagram

The trigger clock circuit produces the 20.48MHz clock using a
VCXO. From this signal are derived the 10.24MHz clock used by many
boards (A1, A4, A5, A6, A30, A32 and A34) and the 256kHz clock signal
used as internal sample signal.

When an external clock is provided, a PLL is used to lock this
20.48MHz. The external signal can be 1, 2, 5 or the standard 10MHz.

.. image:: {static}/images/hp3562a/trigger_clock.png
   :alt: A31 - Trigger circuit block diagram (ext ref)

.. image:: {static}/images/hp3562a/a31_trigger.jpg
   :alt: A31 - Trigger PCB


Input ADC
---------

.. image:: {static}/images/hp3562a/hp3562a_a32_block_diagram.png
   :alt: A32 - ADC block diagram

The ADC board converts analog data from the input board into 13-bits
serial data words. The Analog to Digital convertion is done in 2
passes by a 8-bits ADC. At each digitilization step, the signal value
is frozen by a Track and Hold circuit. This frozen value is digitized
(with a 8-bit resolution), then the digitized value is substracted
from the input (hold) signal and the result is multiplied then
digitized a second time to produce the remaining 5-bits of resolution.

.. image:: {static}/images/hp3562a/a32_input_adc.jpg
   :alt: A32 - PCB

.. image:: {static}/images/hp3562a/a32_input_adc_bb.jpg
   :alt: A32 - PCB (details)


Input
-----

.. image:: {static}/images/hp3562a/hp3562a_a33_block_diagram.png
   :alt: A33 - Input block diagram

The input assembly implements the voltage ranges and conditions the
input signal. It mostly consist in a pair switch attenuators (the
input is balanced), followed by a pair of amplifier/signal
conditionners. The balanced signal is then fed into a differential
amplifier followed by a amplifier and an attenuator.

.. image:: {static}/images/hp3562a/a33_input.jpg
   :alt: A33 - PCB

Next
====

In the `next part <{filename}/hp3562a_4.rst>`_, we will describe the
HP 1345A Digital Display used in the instrument.