In certain situations the 89410A front panel over-range indicators (amber
LEDs) may be lit, even when the reference level has been set above the level of
your input signal. This indication is conservative, and in most cases there is no
significant degradation of the measurement. Even so, if the over-range
indicators are lit, you should verify your setup and signal levels. This situation
is most likely to occur if the input frequency is below 50 MHz and the selected
reference level is below about –20 dBm. With this combination there is a
chance that some LO feedthrough, and other mixing products from the MMS
downconverter will be large enough to contribute significantly to the energy
appearing at the 89410A input and activate the over range indicators. In most
cases these signals are out of band (above 10 MHz) and will not affect the
measurement. If the out-of-band signals are larger than the desired signal,
there is a possibility of intermodulation products being generated. In almost all
cases, the out-of-band signals will be too small to cause any measurement error.
Resolution Bandwidth
The example program selects a 100 MHz resolution bandwidth for the down
converter. At low measurement frequencies, this setting may allow signals at
the IF above 10 MHz to pass with only a small amount of attenuation. The RBW
setting is a conscious trade-off between selectivity and calibration accuracy. If
the RBW filter were set to a smaller value, you might be able to eliminate some
of the unwanted conversion products for certain measurements. However, the
frequency rolloff of a narrower filter setting would reduce the accuracy of the
measurement.
DC Offset and LO Feedthrough
DC offset and LO (local oscillator) feedthrough will cause a residual response
at the center of the screen. The contribution of the 89410A to this response is
from DC offset in it’s input circuits. This term is measured and removed on a
periodic basis automatically by the 89410A unless you explicitly disable this
‘Auto-zero’ function (not recommended). When the program initializes the
89410A, this function is enabled, and should be left enabled to achieve the best
and most stable system calibration.
Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth
16 - 16
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