Tektronix PFG 5105, Инструкция по эксплуатации

Страница: 41 / 89

Operating Instructions
MODULATION
Two kinds of modulation can be used to alter the
output waveform: frequency modulation, and ampli­
tude modulation. For each, the appropriate external
signal is applied to a special connector on the front
panel. Refer to Specifications section regarding limita­
tions on external signals applied to the AM IN and
VCO/FM connectors.
Frequency Modulation
A signal applied to the VCO/ FM IN connector can
modulate the frequency of the output waveform; the
output signal becomes the FM carrier or a voltage­
controlled frequency signal (VCF). The polarity of the
external input signal determines the direction in which
the output will deviate in frequency: a positive-going
input signal increases output signal frequency. The
amplitude of the external input signal determines the
percent of frequency change in the output signal.
Range Lock. Special function 260 controls Range
Lock ON or OFF (either INCREMENT key toggles
ON or OFF). For either Range Lock setting, the output
frequency of the triangle generator (the PFG oscillator)
can be changed by a signal at the FMNCO input.
The Range Lock should be OFF for frequency modula­
tion operation; or ON for operation as an externally
swept frequency generator. To enable either frequency
modulation or externally swept output, set Range
Lock appropriately and press the FM key (LED on).
Entering a frequency parameter will have no effect
on output frequency unless Range Lock is turned
OFF.
The selected output frequency (FREQ key se
t ting) is
the "center" frequency above and below which fre­
quency modulation swings. The PFG 51 05/5505 is
capable of 3 decades of sweep within each frequency
range. Frequency ranges change at 1 2 in decade
steps.
NOTE
The allowable maximum deviation from the cen­
ter frequency is dependent upon the location
of center frequency within the frequency range.
If center frequency is close to the upper end
of the range, only a small increase in frequency
can be achieved before the maximum range
frequency is reached.
2-1 4
As an example, it is OK to set carrier signal
frequency to 1 10 kHz and modulating signal
amplitude to produce 20 kHz deviation. However,
if the modulating signal amplitude
is
increased
to produce 40 kHz deviation, distortion will occur
since the carrier signal cannot go higher than
about 140 kHz in this frequency range. Frequen­
cy modulation or FM/VCO input sweep operation
cannot drive output frequency from one fre­
quency range to another. Frequency modulation
can however drive the output frequency about
10% higher than the top of the range, as
explained below.
With Range Lock ON, the frequency control DAC of
Fig. 2-6 is turned off and only the VCO/FM input
voltage controls the VCO triangle generator. With
o
volts VCO/FM input, output frequency is at its lowest
of the selected range. To move the output frequency
to the top of the range, apply approximately 7 volts.
With Range Lock OFF (default set
t ing), the DAC
output supplies a DC voltage to set output frequency.
The amplitude of a signal applied to the VCO/FM
input when in FM mode will be added to the DAC
DC output at the summing junction and applied to
the VCO triangle generator. Approximately 7 volts is
still required to move the output frequency to the top
of the range, but it can consist of the sum of the
DAC DC output voltage and the peak amplitude of
the FMNCO signal input. Thus, to frequency modulate
an output frequency, Range Lock should be OFF,
center frequency should be entered by depressing
FREQ parameter and value, and FM mode selected.
Then a sinusoidal input signal centered around O volts
to the FMNCO port will deviate the output equally
UP and DOWN in frequency.
When in FM mode, output frequency can actually be
driven above the nominal range limit, by approximately
10%. When not in FM mode, range frequency is
precisely defined by a firmware controlled DAC out­
put voltage. For example, if an output frequency of
1 .21 MHz (0.1 MHz above the 0.1 2 MHz to 1 .2 MHz
range) is selected, the DAC output voltage would
produce an output frequency close to the bottom of
the next higher frequency range 1 .21 MHz to 1 2 MHz.
In the FM mode, the voltage to the triangle generator
is not limited to the DAC de output, but can be
changed by the amplitude of the modulating signal.
The triangle generator can be driven harder into
limiting as shown by the flattening of the upper
frequencies in Figures 1-2A and 1 -28.
PFG 51 05/5505 Instruction Manual