Centrad GF 266, Instruction Manual

Page: 23 / 52

- 23 -
4000 4 319 _EN - 02/09
ENGLISH
For example, a 15 kHz sine wave signal passing through a low-pass filter having a roll-off frequency of 20 kHz will
remain unchanged. On the contrary, a triangular signal of the same frequency will be stripped of all its harmonics,
and only the fundamental frequency (which is a sine wave) will be available at the output.
Therefore we must never lose sight of the fact that the bandwidth required to convey any non-sine wave signal
without degradation must be much wider than that for transmitting a sine wave of the same frequency.
Any filter (or circuit performing as a filter) sees the phase of the signal through it change, well before its roll-off frequency,
and all the more as the filter roll-off rate is sharper.
Therefore any non-sine wave signal passing through a steep roll-off filter will be affected by the phase changes of
its components, even though the amplitudes of these harmonics are maintained.
Consequently, the filter connected to the DAC output cannot be the same for a sine wave and other waveforms.
Signal imprefections inherent in the DDS technology
As has been seen earlier, the increase in generated frequency goes with a reduction in the number of points per period.
For a sine wave, this is compensated by the smoothing effect of the elliptic filter.
For triangle and ramp waveforms, which cannot be filtered in the same way as a sine wave, the points comprising
the signal begin to appear above 2 MHz.
This phenomenon is different according to the oscilloscope used or its setup.
For example, analogue oscilloscope (or digital oscilloscope configured with 5-second persistence):
Graph 1
In the analogue oscilloscope, the identical effect from one period to the next results from the physical remanence
of the cathode-ray tube. In the digital oscilloscope, it results from the persistence effect.
Digital oscilloscope, real-time acquisition: Graph 2
These images are two representations of the same reality, despite the terms "Real Time". What distinguishes the first
from the second is the persistence effect, which is equivalent to averaging.
The observed phenomenon results from the cyclic and progressive shift between signal peaks and sampled points.
The points "move" along the amplitude-time curve.
The corollary of this phenomenon is in the form of dynamic phase modulation, also called jitter.
2 - PRELIMINARY INSTRUCTIONS
2-1 UNPACKING AND REPACKING
The GF 266 function generator package is designed to protect the unit during transportation.
Retain it; it may be useful later.
Packing list
1 instruction manual
1 plastic protective cover 1 GF 266 function generator
2 carboard side covers 1 mains cable
2-2 SPECIFICATIONS
Sine signal : Frequency range : 11.36
μ
Hz to 12 MHz.
Harmonic distortion : < 0.5% up to 20 kHz and harmonics less than -30dB.
< 0.1% at 2V
Square signal (main output): Frequency range : 11.36
μ
Hz to 12 MHz.
Rise and fall times : 20ns max
Duty cycle : Rated to 50%, or adjustable from 10% to 90%
(ditto at TTL output).
Triangle : Frequency range : 11.36
μ
Hz to 5 MHz.
Ramp : Frequency range : 22.72
μ
Hz to 5 MHz, positive or negative ramp.
Graph 2
Graph 1