Hi,
DESCRIPTION OF PROBLEM
When the AFE7071 RFIC is at low temperatures after power on and initialization, modulation does not occur until the RFIC has warmed sufficiently, at which point modulation then spontaneously starts without further intervention from the host processor. Once modulation has started, the RFIC can then be cooled again without loss of modulation and continues to work as expected. I.e. this is purely a start up issue.
When modulation does not occur, only carrier leakage present on RF output.
Issues has been observed with circuit starting from temperatures ranging from -5'C to +7'C. Above +7'C appears to function reliably as expected. Below -5'C has not been tested due to limits of our thermal chamber.
DESCRIPTION OF CIRCUIT
A 10MHz VCOCXO clock provides all clocking to the circuit. The OCXO feeds a NB3N551 1:4 clock distribution IC which in turn feeds:
- ADF4360-9 Synth to provide baseband clock of 17.333 MHZ, which is then converted to differential by Si53341.
- ADF4350 Synth to provide RF LO signal in the 1800MHz band.
- NB3N502 to provide 40MHz to MCU.
Both Synths then feed the AFE7071 RFIC. RF output of AFE7071 is then filtered and amplified using ADL5240.
The ADF4350 RF Synth power is derived from 5V0 via two off ADP150AUJZ-3.3 LDOs. Both have been verified to be operating correctly across temperature.
The other ICs obtain their 3V3 & 1V8 rails from the board's main 3V3 & 1V8 rails (ADP5052 SMPS spread by DS1090U-16+ at 320KHz) with local filtering as required. Rails are operating correctly across temperature.
DESCRIPTION OF SOFTWARE
MCU boots and then enters a 3 minute wait whilst the OCXO frequency stabilizes before then programming the two synths, the RFIC, and FPGA to generate IQ data. After initialization the MCU's only purpose is to perform environment monitoring for over temperature shutdown. It makes no further interaction with any components and sleeps most of the time. I.e. Board Initialization is a one time set-and-forget methodology.
DESCRIPTION OF DEBUGGING
- Board tested in temperature chamber. Board allowed to reach ambient temperature before being powered. Board monitored on RF Tx output for signal via coax cable to vector signal analyser.
- Board found to malfunction at low temperatures.
- Power Rails, reset lines and clocks verified. Clocks / Synths found to be be present and locked at all times. Signals at expected levels.
- RF Output power from ADF4350 verified and also experiments altering the output power via register writes to the Synth also have no effect on AFE7071 modulation issue, so it is not low LO power.
- Problem is believed to be internal to AFE7071 as experimentation with chilling the board and holding it there whilst AFE7071 is locally heated with application of a finger to the top of the RFIC allows modulation to start very quickly after finger is applied. Other ICs held at low temperature with light application freezer spray.
Any ideas?
Thank you
-Tim
