LeCroy LW110 User Manual page 59

User Manual LW120
must compute the actual carrier and shifted frequencies in a similar
way as was described above for the FM function.
To generate FSK you must specify two parameters:
1. Carrier Sample Clock – This is the frequency of the sample as
displayed after you press the FREQUENCY button. Note that the
first line on the display shows Freq and some value and the second
line shows Sclk and value. This Sclk is what you need to interpret
your carrier frequency. This is also the frequency that is available
as sine waveform at the rear-panel sine output. If you do not care to
work with your sine output but want to generate FSK from the front
panel, then you must define your carrier frequency first. Here is
what you do:
When generating standard waveforms, your sample clock
frequency is computed automatically. In fact, when you program
your waveform
automatically. You can not access or modify the sample clock
field, it is there for reference only. Although you can not change
the sample clock, this is the value you'll be using for the FSK
carrier.
It is a bit simpler when you generate arbitrary waveforms. The
only value that you see on the display is Sclk and this is the
value of your carrier frequency. It is also the frequency of the
sine waveform at the rear-panel sine output connector. However,
if you want to know the frequency of your signal at the front
panel outputs, then you must computed it yourself from the sclk
frequency, divided by the number of points in your active
segment. For example, if your sclk is 100MS/s and you have a
1000-points waveform, then your output frequency is 100 KHz.
2. Shifted Frequency – Defines the shifted frequency value. The
LW120 will hop to the shifted frequency when commanded to do so.
Note that the instrument changes the frequencies at the SINE OUT
connector coherently, that is, after the shift command is accepted,
the generator completes the waveform to its last point or phase and
only then the next frequency is "kicking in". This way you can be
sure that discontinuities in the output waveforms is not disturbed.
While using the FSK IN as hop control, the instrument will shift on
positive-going transitions only. Note that the FSK input is level
sensitive, so, although the frequency shifts on transitions, the
output remains at the shift frequency as long as the level remains
true. The output will remain at carrier frequency as long as the FSK
input remains TTL low (false) and at shifted frequency as long as
the output remains TTL high (true).
Refer to Figures 3-23 throughout the following description to learn
how to program the LW120 to generate FSK modulation.
frequency, the Sclk
Using The Instrument 3-29
value is updated