Understanding the Hardware | External 10 MHz input
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manual control is available as well. Fine amplitude control is handled digitally, providing 0.01 dB
resolution on the output amplitude.
Finally, a bank of harmonic filters reduces the amplitude of harmonics generated from the modulators
and amplifiers, typically below -40 dBc.
The FPGA in the VSG60A can quickly switch between streaming I/Q and changing LO frequency or
attenuator settings at precise intervals (200 µs and 10 µs, respectively), allowing the user to build
signals that hop across the entire frequency range of the device. This makes the VSG60A a good
choice for generating frequency hopping signals or signals that must cover a wide amplitude range.
EXTERNAL 10 MHZ INPUT
A low-jitter comparator provides low additive phase noise for an external 10 MHz reference input of 0
dBm to +13 dBm. When external reference is enabled, the internal 10 MHz VCTCXO is disabled and
the external reference disciplines the 80 MHz VCXO directly. Inputs as low as -15 dBm will work, but
phase noise degrades at lower amplitudes. A square wave provides the highest slew rate and
therefore the best phase noise.
TRIGGER OUTPUT
The trigger output is a series-terminated 3.3V logic signal, meant to typically drive a high impedance
load. The trigger output is synchronized with the RF output. Up to 1000 triggers per second can be
output, with a user-selectable pulse width. This enables the VSG60A to be connected to other
equipment in an automated testing environment.
LOW SPUR MODE (DIGITAL TUNING)
The LO uses a fractional-N PLL. This can lead to integer boundary spurs, as well as other spurious.
To mitigate these spurs, a low spur
mode has been added, which is on
by default. When low spur mode is
enabled, the fractional-N PLL uses a
very low denominator to keep the
spurs at least 2 MHz away from the
carrier and typically below -50 dBc
above 3.7 GHz, and below -55 dBc
below 3.7 GHz. This gives a coarse
LO frequency, which is then digitally
tuned to the exact requested
frequency. The advantages of digital
tuning are very low-level, predictable
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