3.4.2 Spurious Signals
Typically, the spur with the highest amplitude will be the image response, located 40-120 MHz
below the actual RF signal. This will typically be around -63 dBc below 6 GHz, -57 dBc above 6
GHz.
Spurious signals also arise from spectral impurities in the LO, as well as undesired mixing
products. The translation loop architecture tends to have low level spurs around 30-60 MHz from
the carrier. These will have minimal impact when measuring signals of 25 MHz bandwidth or less.
There may be spurs inside of 30 MHz at some frequencies. Undesired mixing products typically
show up at multiples of (LO – RF).
Another source of spurious is subharmonics of the LO, above 6 GHz for the SM200 and above 8
GHz for the SM435. For most frequencies, these will be too low to interfere with typical
measurements, and are several GHz away from the signal of interest.
3.5 Scalloping Loss
An FFT-based spectrum analyzer uses digital resolution bandwidths rather than discrete analog
filters. Moving from analog to digital introduces some new terms important to measurement
accuracy, like FFT bins, window functions, spectral leakage and scalloping loss. To sum up, an
FFT produces an array of discrete frequency bins and their associated amplitude. Real-world
signals rarely line up exactly with a single frequency bin, which can result in some ugly behavior
unless a window function is used. Many different window functions are available, with various
strengths and weaknesses.
For the SM200, swept modes default to a flat top window, which offers excellent amplitude
flatness and therefore very little scalloping loss, in exchange for a wider resolution bandwidth and
longer processing time. Most RBWs used by the SM200 are from flat top windows, so scalloping
loss is negligible.
In real-time mode a Nuttall window function is often used, which has a narrower bandwidth to
reduce processing time and level out impulse response. However, when a signal falls halfway
between two “bins,” the energy is split between adjacent bins such that the reported “peak”
amplitude may be lower by as much as 0.8 dB.
To get an accurate CW reading using “Marker peak”, flat top RBW shape in swept mode is
recommended.
In either mode, the “channel power” utility, which integrates the power across any channel
bandwidth you specify, also eliminates this scalloping loss, giving you a full accuracy amplitude
reading even in real-time mode.
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