GW Instek SFG-830 User Manual - Manual (Page 38)

SFG-830 p.35
(9) Checking the Symmetry of Square Wave
The deviation should be <
±1% of period +4ns.
n Set conditions:
Function : Square Wave Amplitude : 10Vp-p
Frequency : 1MHz
o Use an oscilloscope with 50 ohm load to measure the output.
p Check and make sure that the output is 49:51 or 51:49.
(10) Checking the AM Envelop Distortion
The deviation should be < -35dB at 1kHz.
n Use a spectrum analyzer, with Center Frequency at 1MHz and Span
Frequency at 20kHz, to measure the output.
o Check and make sure that the differences between powers of Basic Wave and
sidebands (2k, 3k, …, Offset) are less than 35dB.
8. The Block Diagram and Description of the System
Graph 2 is the block diagram of SFG-830 system, which consists of a micro
processor unit (MPU), a direct digital synthesizer (DDS), a digital to analog converter
(DAC), a RAM module, a low pass filter (LPF), a frequency double (F.D.), a square
waveform comparator, a amplitude control, an output amplitude, an attenuator (ATT),
etc. The principles of generating waveforms are stayed as follows:
(1) Sine Waveform
The data of waveform is stored in the lookout table of DDS (Q-2334). The output
frequency can be altered by solely changing the control word K (please refer to
Chapter 2). The digital output passes the DAC and be converted to a step-shape
analog signal. This signal will then be filtered by a 9-level LPF2, and becomes a
pure sine wave. Due to the frequency response of DDS and the points of the
output waveform, the sine wave should pass the F.D. circuit (U301 and AD834),
the amplitude control, the output amplitude, ATT, and output through the Main Out
terminal.
(2) Square Waveform
The procedure of generating square waveforms is similar to that of generating sine
waveforms. The only difference is that the signal will pass a square wave
comparator circuit between the F.D. circuit and the amplitude control.