Configuring the RF signal
R&S
®
SMM100A
306
User Manual 1179.1341.02 ─ 05
7.8.1
Required options
The equipment layout for using the LO coupling function includes:
●
Base unit
●
Option phase coherence (R&S
SMM-B90)
7.8.2
Phase coherence
Phase coherence of RF signals designates a defined, constant delta phase between
two or more RF carrier signals with the same frequency or a multiple of the frequency.
If two signal generators are coupled via their 10 MHz reference, they are generating
the same frequency but only from the long term perspective. Having a closer look into
the instantaneous differential phase (“delta phase”) of these two RF signals, the follow-
ing are possible causes for instabilities:
●
Phase noise of the two synthesizers
●
Weak
coupling at 10 MHz and a long synthesis chain up to the RF domain
●
Temperature differences which cause a change of the effective electrical length of
some synthesizer components
Most critical for a stable delta phase is the thermal RF phase fluctuation between multi-
ple RF synthesizers. These fluctuations can be minimized by using a common synthe-
sizer, that means a common local oscillator (LO) signal for all RF carrier. The LO signal
is internally used for upconverting the baseband signal to the RF. Only if the LO signal
is the same for all carriers, you can achieve a stable phase relationship between the
RF signals.
Local oscillator coupling
