R&S SMJ100A, Operating Manual

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Instrument Function
R&S
®
SMJ100A
213 Operating Manual 1403.7458.32 ─ 14
5.5.5.1 Phase Coherence
Phase coherence of two RF signals means that there is a defined and stable phase
relationship between two (or more) RF carriers, i.e. there is a fixed delta phase ΔΦ
between the carriers. Strictly speaking, phase coherence is only defined for CW carri-
ers with the same frequency (or for CW carriers at frequencies that are multiples of
each other).
If two signal generators are coupled via their 10 MHz reference, they are generating
exactly 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,
this is quite instable due to:
●
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 drift between multiple RF
synthesizers. This drift can be minimized down to 0.1° by use of a common synthe-
sizer, i.e. a common local oscillator (LO) signal, for all RF carriers. Only if this LO sig-
nal (which is internally used for upconverting the baseband signal to the RF) is the
same for all carriers, a stable phase between the RF signals can be achieved.
5.5.5.2 Typical Applications
The LO Coupling function can be used to generate a beamformed signal.
●
Generation of a beamformed signal with two instruments, working in External
mode.
5.5.5.3 LO Coupling Settings
RF Signal and Analog Modulations - RF Block