R&S ZVB - Page 324

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ZVT GUI Reference
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Operating Manual 1145.1084.12 – 30 309
 In the first measurement step no measurements for port group 1 will be performed, measurements
for port group 2 will be performed at 100 MHz.
 In the second measurement step measurements for port group 1 will be performed at 100 MHz,
measurements for port group 2 will be performed at 101 MHz.
 In the third measurement step measurements for port group 1 will be performed at 101 MHz,
measurements for port group 2 will be performed at 102 MHz.
 …
 In the 100th measurement step measurements for port group 1 will be performed at 199 MHz,
measurements for port group 2 will be performed at 200 MHz.
 In the 101st measurement step measurements for port group 1 will be performed at 200 MHz, no
measurements for port group 2 will be performed.
Please note that Simultaneous Measurement with Frequency Offset is completely transparent to the user:
all port groups will be measured in the requested frequency range and the results are available in the
same form as if they were obtained in separate measurements without frequency offset.
How the firmware applies the frequency offset
The firmware uses the minimum frequency offset specified as explained above to determine an actual
frequency offset for each port group. The first port group is always measured without any frequency offset.
For all other port groups there will be an offset (which may be positive or negative) applied to the
generator frequency of the driving port(s) which is chosen in such a way that
 within each simultaneous measurement the source frequency for any two port groups differs at
least by the minimum frequency offset
 and that the actual offset is an integer multiple of the frequency spacing of the sweep; in a
segmented sweep this may result in different actual offsets due to different frequency spacings in
different segments.
The frequency offset applied to the generator must of course be balanced by a corresponding offset on
the receiving side. Since for technical reasons there can only be one LO frequency within a simultaneous
measurement the offset on the receiving side is applied to the IF so that with a single LO frequency but
different IFs one obtains different receiving frequencies. A simple example would be a situation in which a
port group uses a generator offset of +4 MHz which must be balanced by an offset of +4 MHz applied to
the receiving frequency which is achieved by applying an offset of -4 MHz to the IF (f_LO = f_rec + IF <=>
f_rec = f_LO – IF so that decreasing the IF will increase the receiving frequency).
The limiting factor concerning the magnitude of the actual frequency offset is the range of possible IF
values. The VNA can handle IF values in the range from 5 MHz to 35 MHz. For the first port group (which
is measured without any offset) the firmware will use the IF that would be used in setups without
Simultaneous Measurement with Frequency Offset. This IF is chosen by an algorithm which considers
different factors among which the receiving frequency is the most important one. When Simultaneous
Measurement with Frequency Offset is activated the firmware will automatically try to choose sign and
absolute value of the IF offsets for the different port groups in such a way that all resulting IFs will be in the
valid range between 5 MHz and 35 MHz.
Please note that because the LO frequency may lie either in the upper or lower side band of the receiving
frequency an IF offset that will yield an IF in the range -35 MHz to -5 MHz is valid as well. In certain setups
the combination of frequency spacing in the sweep and the chosen minimum frequency offset may lead to
IF offsets outside the valid range. In such a situation Simultaneous Measurement with Frequency Offset
will not be activated and an according error message will be displayed.
The following example shows how a Simultaneous Measurement with Frequency Offset is done in the
following setup: