Technical data

85108L System Manual
Avoiding the Effects of Spurs
Spurious Responses
D-2
Spurious Responses
There are measurement frequencies at which combinations of LO and RF frequencies will
potentially produce measurement results that are not desirable. This is due to the spurious
responses (harmonics) created by the LO and RF mixing process. This information is presented so
that, if the user requires measurements at those frequencies, the LO and RF frequencies can be
adjusted to eliminate the spurious response that can cause measurement errors.
Spurious responses occur at harmonics of the 20 MHz 85110L IF frequency (n ∗ 20 MHz). The effect
of the spurious response is an increase in noise content at multiples of 20 MHz, specifically at
60 MHz, 80 MHz, and 100 MHz. These effects can raise the noise floor to about −60 dBm at
100 MHz, less at higher multiples of 20 MHz. When making S
21
and S
12
measurements on high loss
devices, these spurious responses may impact the accuracy of the measurements. To eliminate these
effects, move the measurement frequency up or down by at least 100 kHz.
Of these frequencies, 100 MHz is the only frequency measured during the performance verification
process. The excess noise will cause the performance verification program to indicate failure for
both the 20 and 50 dB attenuator’s S
21
and S
12
measurements at 100 MHz (the first point). This
failure is expected, so the 85108L system is considered to have PASSED the performance
verification procedure if the only failed points are the 100 MHz measurements of the attenuator’s
S
21
and S
12
parameters.