Understanding the Hardware | Linearity and compression
4
fractional-N and integer boundary spurs, and nearly infinite tuning resolution. The disadvantage is that
the hardware’s I/Q offset is no longer centered in the modulation envelope.
For applications where digital tuning is not desirable, such as protocols that require the hardware I/Q
offset to be centered, digital tuning should be disabled. This sets the fractional-N PLL for 1/6 Hz
resolution, allowing 1 Hz steps and frequencies like 833 1/3 MHz with zero additive frequency error.
Most frequencies in this mode will have fractional-N spurs below -50 dBc, but some frequencies will
have close-in spurs that exceed this level.
LINEARITY AND COMPRESSION
To keep the VSG60A USB-powered, it was designed with modulators and amplifiers that strike a
balance between linearity and power consumption. The best linearity is obtained at output levels of
-10 dBm or lower, where 3
rd
order intermodulation products are typically below -50 dBc, and generally
linearity is better at lower frequencies. For frequencies above 3 GHz at amplitudes above 0 dBm,
compression and intermodulation distortion may become an issue for some signals. Above 4 GHz, a
+10 dBm CW output may be compressed by about 1 dB in places, and compression at this level will
significantly impair EVM. Applications requiring good linearity at high output power, especially at high
frequencies, may require an external amplifier. An amplifier with a third order intercept 25 dB above
the required output power will generally preserve the linearity of the VSG60A.
IMPROVING VSWR, IF FLATNESS, MISMATCH UNCERTAINTY
The VSG60A harmonic filters have fairly high VSWR at certain frequencies. When connecting to a
high VSWR receiver or antenna, a 3 to 10 dB fixed SMA attenuator connected to the VSG60A output
significantly reduces mismatch uncertainty and VSWR, and improves the IF flatness for applications
where this is a concern.
AMPLITUDE CONTROL AND SWITCHING
Internally, the VSG60A has 3 coarse gain settings, usually controlled by the API. High gain uses an
amplifier with a 20 dB gain. Mid-gain is 0 dB (bypass both amplifier and attenuator). Low gain is a 20
dB attenuator. The amplifier is powered on for both high gain and mid gain, but powered off for low
gain. Additionally, there is a 0-30 dB step attenuator, calibrated in 2 dB steps. Fine amplitude control
is accomplished digitally by scaling the I/Q data. The final amplitude state is “off”, which powers down
the I/Q modulators and amplifiers.
The VSG60A can switch amplitudes in 10 microseconds. This is a fixed delay to allow for switches,
atenuators, and amplifiers to settle a bit. However, when switching directly from low gain amplitudes
(such as -40 dBm) to high gain amplitudes (such as +5 dBm), the output amplifier must turn on and
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