Note: The same LO cable type and length is
required for both the reference and test mixer
modules. This is to ensure that the inser-
tion losses through the reference and test
mixer module LO paths are the same. Using
the same LO cable type also optimizes cable
phase tracking versus temperature and there-
fore, system phase measurement, stability,
and accuracy.
When a rotary joint is used, the equivalent
cable length must be added to the reference
mixer LO cable due to the rotary joint inser-
tion loss. To determine the equivalent cable
length, first determine the insertion loss from
the input to the output of the rotary joint at
the maximum LO frequency. Then, using in-
sertion loss curves for the LO cables between
the 85309B and the mixer module, calculate
the equivalent length in meters at the maxi-
mum LO frequency. The reference LO cable
length must be increased by this amount.
Select the LO Source (continued)
Before calculating cable loss, you must first determine the LO frequency. If using PNA
Series Option H11, the LO frequency must be set so that an 8.33MHz IF is produced. The
PNA Series’ LO is offset from its RF by 8.33 MHz automatically if the PNA is operated
below 20 GHz and frequency offset is turned off. Refer to “Setting up the PNA LO for an
8.33 MHz IF”, later in this document, for more information.
The PNA Series’ internal LO can be accessed through a rear panel output port if
Option H11 is installed. Its frequency range is limited to 1.7 GHz to 20 GHz. The signal on
the rear panel is very low power and always requires an amplifier to achieve the required
power level at the 85309B. The front panel RF can only be used as the LO for the 85309B
if it is not used as the system RF.
Calculate required power of LO source
P
s
= cable length (meters) X cable loss (dB/meter) + P
in
(85309B)
where P
s
= Power out of the LO source (dBm)
P
in
= Required power into 85309B (0 to 6 dBm)
Select a source that meets your individual preferences and needs. Higher-output power
sources or an amplifier must be used if P
in
is insufficient.
Reference signal level
The reference mixer provides a phase reference for the measurement and a reference
signal for a ratioed measurement (test/reference), to ratio out any variations in signal
levels from the system. If you select RF and LO sources that are synthesized, or use the
internal source of the PNA, then phase locking the receiver is not required. The only
requirement for the reference channel is that the signal level be high enough to achieve
the desired accuracy for the measurement. Figure 9 shows the magnitude and phase
errors as a function of signal-to-noise ratio; this also applies to errors contributed by the
reference channel. For most applications, it is desirable to maintain a 50 to 60 dB signal-
to-noise ratio.
Determine Cable Length from 85309B unit to mixers
Mixers require a certain LO drive power level; the output power of the 85309B LO/IF dis-
tribution unit and the RF loss of the cables will determine the maximum allowable cable
lengths. To assure you have enough power at your mixers, use the following equation to
calculate the maximum cable length allowed for your setup:
Cable length (meters) = (P
out
85309B – P
in
mixer)/(cable loss/meter at frequency)
High quality, low loss, phase stable cables are recommended.
31 | Keysight | Antenna Test – Selection Guide