[Xiaomi Mi 5 Guides, News, & Discussion] greasymonkey1960:
RF Calibration

As mentioned I would try to write some tips on the RF calibration in the Mi5 and similar devices.

This information should not be treated as absolute fact, it is based on my experience in RF lab work on a range of equipments over 40 years, reading on the Mi5 and other devices using the Qualcomm WTR3925, and "rough as guts" experimentation.

Transmitter Calibration.

The basis of transmitter calibration is to ensure that the device is transmitting on the correct band, within the specified power and frequency envelopes and is using the minimum electrical power to achieve the required specifications.

The modern RF transmitters for LTE and WCDMA have extremely tight performance criteria (particularly linearity and phase error) that need to be met for correct performance, but if that is to be designed into the device, they become extremely expensive, so the technique used is to make a "cheap as chips" device, work out the charactieristics of it, then apply non-linear curves to the input signals to achieve the desired performance - it is one of the few cases where "two wrongs make a right" in the world of RF.

To achieve this, there are a series of tests run where different variables are "constrained", such as current, voltage or modulation scheme, and the resulting performance (Tx power, AM-AM distortion, AM-PM distortion, etc), then these are stored in the NV RAM. the main one for this information is the RFNV_LTE_Bx_TX_MULTI_LIN_V3_DATA_I.

Currently for the Mi5, these Items are only listed for the operating bands, so to enable new bands (it is not guaranteed, may only be part of the requirements) is to ensure that there are correct (or near-to-correct) calibration values. This has to be done partially by guess-work, and observing how the calibration curves are changed from one band to another, as the frequency move from a notional "centre-point frequency" - for the Mi5 it appears that B3 (1800MHz) is usually the goal for designers as the centre-point frequency.

It can be seen when examining the curves for the different items in the calibration data how the device changes as the frequency is decreased, from 1800 -> 900 -> 850MHz, and then one can infer what may be the required values for 800 and 700MHz, which is the lower operating limit for the RF devices.

Receiver Calibration

The general aspects of the recveiver calibration are similar to the Transmitter, except the goal is to maximize the signal level and QUALITY (low amplitude and phase distortion) of the signal which is detected from the base-station.

To calibrate the Receiver, the main NVRAM Item is the NVRF_LTE_Bx_FBRX_MODE:VS_GAIN_VS_TEMP_VS_FREQ_I,

This NV Item provides the calibration data for for the defined range of frequencies in the band, at a range of different operating temperatures, for four different operating_modes and four different gain_states.

The Operating Modes which are used in this are CW (no modulation), 20MHz carrier, 20+20MHz 2CA carriers and narrow-band carrier, for 1.4, 3, 5 and 10MHz carrier bands.

The Gain Modes appear to be related to different number of gain stages switched in - these devices use RF amplifiers with quite narrow gain ranges, with multiple stages "ganged" together to obtain a more linear gain curve across a wide dynamic range.

These values are stored and used to "adjust" the gain & bandwidth of the receiver to provide a linear signal to the demodulator, allowing for less-stringent design and manufacturing requirements of the Rx silicon wafers.

The Rx signal is then "corrected" using a set of calibration curves to ensure that the I and Q signals from the demodulator out to the Modem have correct phase and amplitude balance. These are defined in the NVRF_LTE_C0_Bx_FBRX_DC_CAL_VAL_I, for the applicable operating Mode and Gain_states for the band being calibrated.

The same estimation techniques can be used for deriving the values from the existing curves of other bands, and observing the roll-off or change as the bands become further away from the "center-point frequency".

There are other calibration values that are used in the devices, but are simpler in terms of grasping what they do, and what the settings should be, but these ones mentioned above are the critical ones that will determine if the mobile device is better for frying eggs than communications......

There is much more than just these values to be determined, and will take many more pages than this short passage to describe each one - besides, I am a consultant, and should be charging for this knowledge, so I shall stop here. :cool:

Cheers,

GM.