Operation 19
CG635 Synthesized Clock Generator
For example, if the outputs are currently at +3.3 V CMOS levels, setting V
HIGH
to 6.5 V
will cause the CG635 to briefly display “Volt Error” and leave the outputs unchanged,
because 6.5 V exceeds the upper limit for V
HIGH
. On the other hand, setting V
HIGH
to
0.25 V will cause the CG635 to briefly display “Lo is -0.25” and to set V
HIGH
and V
LOW
to
0.25 V and –0.25 V, respectively. V
LOW
is adjusted in addition to V
HIGH
in order to satisfy
the amplitude limits.
Frequency
The CG635 can output frequencies in the range 1 µHz to 2.05 GHz with up to 1 pHz of
resolution and 16 significant digits. The frequency may be set to arbitrary values, or
stepped up and down by configurable step sizes by following the instructions described in
the Front-Panel User Interface section at the beginning of this chapter. All sixteen
significant digits can be entered via the front panel. If the user enters more than sixteen
digits, the result will be truncated to sixteen digits. If the user enters an invalid frequency,
the CG635 will briefly display “Freq Error” and leave the frequency unchanged.
While the frequency can be set to 16 significant digits, the CG635’s main display is only
13 digits wide. For most users, this will not be a problem because the extra digits will
usually be zero. Nevertheless, the user can view the extra significant digits by pressing
and holding the ‘FREQ’ key down. After a brief delay, the CG635 will display all
significant digits below 1 Hz. For example, if the frequency is 1234567890.123456 Hz,
the CG635’s main display for frequency will be 1234567890.123 Hz. If the user then
presses and holds the ‘FREQ’ key down, the display will show 0.123456 Hz after a
brief delay.
Similarly, the extra significant digits for the frequency step size can be viewed by first
pressing ‘FREQ’, and then pressing and holding ‘STEP SIZE’ down.
The frequency resolution is limited by the available significant digits. For frequencies
less than 10 kHz, the resolution is 1 pHz. Starting at 10 kHz, the resolution is reduced by
a factor of ten for each decade of frequency from 10 kHz to 1 GHz where the resolution
is 1 µHz.
The CG635 truncates both the frequency and the frequency step size to the available
resolution when the frequency changes. Thus, if the current step size is 1 pHz, and the
user changes the frequency to 1 GHz, the CG635 will also modify the frequency step size
to be 1 µ Hz, which is the minimum valid step size for frequencies of 1 GHz.
Normally, when the user steps the frequency up and down by a small amount, the CG635
will seamlessly slew the output to the new frequency. If the user crosses an octave
boundary, however, the CG635 will disable the output momentarily, forcing it low for
about 10 ms before re-enabling it at the new frequency. The octave boundaries are a
consequence of the fact that the CG635 generates all output frequencies by dividing
down the output of an RF VCO that operates from 960 MHz to 2.05 GHz, which is a little
more than one octave of tuning range.
An example should make the operation clear. When the frequency is 1.5 MHz, the
divider is 1024, and the RF VCO operates at 1024×1.5 MHz = 1.536 GHz. Now, suppose
you increase the frequency in steps of 0.1 MHz to 2.1 MHz. From 1.5 MHz to 2.0 MHz,
