Operating basics
The rear panel external modulation input supports bandwidths of 500 kHz, but the
modulation bandwidth is limited to 100 kHz for fc greater than 62.5 MHz (93.75
MHz for the TSG4106A). The sensitivity is set such that a 1 V signal results in a
full scale deviation (depth) in the output. For example: in
Φ
M, if the deviation is
set for 10°, applying –1 V produces a –10° shift; applying 0 V produces no shift;
and ap1 V produces a +10° shift.
When modulation is enabled using an internal source, the rear panel modulation
output will provide a waveform of the selected function with a full scale range
of ±1 V. When external modulation is selected the modulation output tracks the
applied signal.
Pulse modulation
In pulse modulation, the RF signal is turned on by the internally generated or
externally applied signal.
The internal pulse modulation source is a digital waveform with period settable
from 1
μ
s to 10 s with 5 ns of adjustability, and on time settable from 0.1
μ
s to
9999.9999 ms. The period of the digital waveform is set using the
Period
setting
in the
Mod
menu. The on time (for pulse mode) is set using the
Width
setting
in the
Mod
menu.
When an external input is selected the rear panel external modulation input is set
for a threshold of 1 V. The resulting signal is used in place of the internal source.
In pulse mode, the modulation output is a 3.3 V logic signal, which tracks the
pulse waveform.
Example.
The following image shows the front panel BNC and Type N outputs
for a pulse modulated carrier frequency of 50 MHz. The internal pulse modulator
was set to 1
μ
s period with a 300 ns pulse width (or a 30% duty cycle). The
output amplitudes were set to 2 V
PP
into 50
Ω
. The top trace is the rear panel
Modulation Output signal. The middle trace is the BNC output. The bottom trace
is the Type N output. Both traces show about 50 ns latency in their response to
the gating signal. The Type N output also shows some gate feedthrough at the
leading edge of the signal.
Linear Noise modulation
For AM, FM and
Φ
M, the noise source is pseudo random additive white Gaussian
noise (AWGN). The bandwidth of the noise and the RMS deviation are set using
the
Rate
and
Deviation
settings in the
Mod
menu, respectively.
The peak deviation will be about
fi
ve times the set RMS deviation. This forces
limits on the maximum allowed deviation corresponding to one
fi
fth of the
non-noise counterparts. For example, at a carrier frequency of 500 MHz the
maximum FM deviation for a sine wave function is limited to 4 MHz, and so the
maximum deviation for noise modulation is limited to 800 kHz.
For linear modulation, the rear panel output will provide 200 mV
RMS
that will
be band limited to the selected modulation rate. Again, the peak deviation will
be
fi
ve times this, or ±1 V
PP
.
TSG4100A Series RF Signal Generators User Manual
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