IBM 7220 3.3 Accuracy

3-8

Chapter 3, TECHNICAL DESCRIPTION

and Y channel output filters. The outputs of these in turn drive two 16-bit digital to

analog converters (DACs) which generate the instrument’s FAST X and FAST Y

analog outputs. In addition, the signals are fed to further low-pass filters before

subsequent processing by the instrument’s host microprocessor.

Although shown on the block diagram as a separate entity, the output processor is in

fact part of the instrument’s main microprocessor. It provides more digital filtering of

the X and Y channel signals if required, calculates the vector magnitude, R, where

R = √(X2 + Y2) and phase angle, θ, where θ = tan-1(Y/X), and routes any of these

signals via two further 16-bit DACs to the unit’s CH1 and CH2 output connectors. It

also allows one of the two auxiliary analog inputs, ADC1 and ADC2, which are

digitized by a 16-bit analog to digital converter, to be used in ratio calculations.

All functions of the instrument are under the control of a microprocessor which in

addition drives the front panel displays, processes front panel key operations and

supports the RS232 and GPIB (IEEE-488) computer interfaces. This processor also

drives the instrument’s 8-bit digital programmable output port, which may be used

for controlling auxiliary apparatus.

The microprocessor has access to memory which may be used for storage of the

instrument’s outputs as curves prior to transferring them to a computer via the

computer interfaces. In addition to using this function for the normal outputs, such as

the X and Y output signals, it may also be used with the auxiliary ADC inputs to

allow the instrument to operate as a transient recorder. The internal oscillator

frequency sweep function is also controlled by the microprocessor.

A particularly useful feature of the design is that only part of the controlling

firmware program code, which the microprocessor runs, is permanently resident in

the instrument. The remainder is held in flash EEPROM and can be updated via the

RS232 computer interface. It is therefore possible to change the functionality of the

instrument, perhaps to include a new feature or update the computer command set,

simply by connecting it to a computer and running an Update program.

3.3 Accuracy

When the demodulator is operating under correct conditions, the absolute gain

accuracy of the instrument is limited by the analog components in the signal channel,

and the absolute phase accuracy is limited by the analog components in both the

signal channel and the reference channel. The resulting typical accuracy is ±0.5

percent of the full-scale sensitivity and ±0.5 degree. When the higher values of AC

Gain are in use, the errors tend to increase in the upper part of the frequency range

(above 25 kHz).