Telit Modem Power Supply Problem

Hi,

I have built a circuit using a Telit GE866. It works OK in full power mode and low power mode, but when I put it in low power mode, and put a multimeter in series with the modem power supply, the supply current oscillates between 5mA and 80mA. Is this common for a modem to do this, or is this my £30 multimeter not reading the current correctly?

I have cut the track and removed the modem, and only the micro is running and the power consumed is a steady 1mA.

If I isolate the whole circuit and only power the modem, it still oscillates. I have put a 100uF Tantalum before the modem as required in the datasheet, and tried numerous batteries, but the current still oscillates.

Does anybody have any ideas what I might be doing wrong, or how I can overcome this problem?

This is my multimeter, Precision Gold Academy PG10B

Thanks,

RocketMan.

Is the voltage ripple acceptable or causing any problems ? I suspect it is due to excessive phase shift in the LC filter with high Q resonance affecting the loop stability.
What happens when you shunt the choke with a 5 Ohms?
What happens if you apply a 1 A pulse load with 100mA steady load? Is it stable?
Does it ring at the same frequency after the 1A pulse?
What is the LC resonant frequency relative to the oscillation?
What happens if you shunt the R feedback with a small RC filter with a breakpoint near the oscillation using the an R/10 value? This gives lead-lag compensation.

The modem is good stability test on a SMPS design with a 2A TX pulse load.