Keysight (Agilent) 8643A Service Note

8643A-02

SUPERSEDES: None

DATE: 31 March 1993

1 of 2

AUTHOR:

LHL

ENTITY:

1000

© 1993 AGILENT TECHNOLOGIES
PRINTED IN U.S.A.

SERVICE NOTE

ADMINISTRATIVE INFORMATION

INFORMATION ONLY

SERVICE NOTE CLASSIFICATION:

ADDITIONAL INFORMATION:

Continued

8643A Synthesized Signal Generator

Jitter Is Normal For Internal Modulation Source

Duplicate Service Notes:

8644A-02
8644B-02
8645A-03
8664A-02
8665A-03
8665B-02
70320A-02
70322A-02

Parts Required: None

Situation:

The internal modulation source (also known as the NSM Oscillator, Modulation
Distribution Module or STD MOD/NSM OSC) in this instrument utilizes digital wave­form synthesis to develop sine, square, ramp, triangle, and noise waveforms at rates up to
400 kHz. Sinewaves and noise produced by this technique have excellent time stability
(no jitter). Square, ramp, and triangle waveforms have a certain amount of jitter. It is nor­mal for this jitter to occur utilizing this type of synthesis.

Jitter occurs when the frequency of these waveforms is not harmonically related to the
DAC clock frequency (which is exactly [2^24]/10 or 1,677,721.6 Hz). The value of this
jitter will be one clock cycle of the DAC (1/1.677 MHz or approximately 600 nanosec­onds) for square and triangle waveforms. This value doubles to approximately 1200
nanoseconds when using ramp waveforms due to the fact that there is only one fast tran­sition per cycle of this non-symmetrical waveform. The sinewave have no instantaneous
transitions and therefore has no jitter.

For example, if we choose a frequency of 24.5 kHz, this frequency divided into the
clock yields a non-integer relationship (68.4784). Therefore the fast edges of the square
or ramp waveforms, and peaks of the triangle wave, do not always align with the clock.
The result is that an occasional fast transition occurs one DAC cycle before or after the
correct time.

The jitter produced by digital waveform synthesis does not occur on every cycle. The
severity of the jitter is predictable by calculating how far from a perfect frequency rela­tionship a particular waveform is from the clock frequency. If we choose a different fre­quency, such as 23.3 KHz, the jitter will almost be zero because the frequency relation­ship is nearly integral (72.0052). Since this frequency is harmonically related to the
clock frequency the fast edges almost always coincide with the output clock of the
DAC.

Solution/Action:
None

Page 2 Service Note 8643A-02