about GSM mobile phone design!

As mentioned, modern test equipment performs the necessary signal processing
automatically, making these measurements straightforward and fast. It is
also useful to view phase error versus time?especially in R&D and when fault
finding. For example, a phase and frequency error test might fail the prescribed
limits at only one point in the burst ? at the beginning. This could indicate a
problem with the transmitter power ramp or some undesirable interaction
between the modulator and power amplifier.
what's the meaning of interaction
could you take something for example?
thanks!

The power amplifier will present a load impedance to the oscillator. This load impedance will change as the power amplifier is brought from the standby to the full operational mode.

By analogy of the old days of magnetrons whose frequency was partially determined by the load impedance it was driving, this is called load pulling.

if load pulling occur,what kinds of result will you get，when you check the phone's performance?
phase error, PVT (low level,high level),or something else?

for different TX chains ,such as direct-up-convertion or offset PLL ,which is easier to be affected by load pulling?

thanks.

Due to load pulling, ACPR of the transmitter deteriorates. Another effect is that EVM also gets bad.

There are some methods to overcome this.
One is to use a harmonic LO(eg 2/3 LO) for transmitter modulator. By doing so there is no oscillator components in transmit frequency or in its harmonics. Effectively the interaction between transmitter and vco can be reduced.
thanks

Hi ,how to improve the pulling ?The frequency pulling maybe cause by the power feed back to the vco from the PA . Any guys knows how to fix this issue ?

VCO pulling it is a change in impedance seen by the VCO output that can induce changes in the DC voltages across junctions of the VCO's active device.

A change in the collector-to-base voltage (Vcb) of a bipolar device, for instance, affects the collector-to-base capacitance (Ccb), which in turn can alter the oscillation frequency by affecting the overall tank resonance.

Buffers can improve poor VCO output isolation.