LMS7002Mr3 Calibration Using MCU

Introduction

This document provides a brief description of the main calibration procedures used to improve the RF performance of LMS7002M IC by using the built-in microcontroller (MCU). It also contains information needed to program and run LMS7002M IC internal MCU, which handles the calibration procedure execution routines, thus greatly simplifying the calibration porting procedure to other systems.

General MCU calibration description

This section focuses on a brief description of the calibration procedures themselves that are handled by the program stored in MCU image. The calibration procedures for the LMS7002M chip are listed below:

• DC offset and LO Leakage, and Quadrature (IQ) chain imbalance calibration;
• Analog filter -3 dB bandwidth calibration.

Calibration procedures are used for both transmitter (TX) and receiver (RX) chains, although the individual calibration setup and flow is different for each of them.

For the ideal TX case, if the continuous wave (CW) signal is up-converted to radio frequency (RF), the TX output ideal spectrum has no unwanted components, as shown in Figure 1 upper graph. In non-linear conditions, the TX output spectrum has unwanted tones as shown in Figure 1 lower graph. Descriptions of the tones are given below:

• Wanted Signal (1) – wanted TX sideband, with an offset f_wanted from the carrier frequency f_carier;

• LO Leakage (2) – primarily caused by DC offsets in the TX baseband and LO leakage at the TX frequency mixing stage;

• IQ Image (3) – unwanted TX sideband, with an offset -f_wanted from the carrier frequency f_carier. Primary caused by gain and phase imbalances in different IQ signal blocks.

Figure 1. TX spectral tones looking at the TX output. Here: (1) – TX Wanted Signal; (2) – TX LO Leakage; (3) – TX IQ Image.

For the ideal RX case, the down-converted CW signal does not have unwanted tones at the input of the analog-digital-converter (ADC) as shown in the Figure 2 upper graph. However, under non-linear conditions the RX spectrum has unwanted signals as shown in the Figure 2 lower graph. Descriptions of the tones are given below:

• Wanted Signal (4) – wanted RX sideband, with an offset f_wanted from the DC;

• DC Offset (5) – Unwanted RX DC offset. Primarily caused by DC offsets in the RX baseband and LO leakage at the RX frequency mixing stage;

• IQ Image (6) – Unwanted RX sideband, with an offset -f_wanted from the DC. Primary caused by gain and phase imbalances in different quadrature (IQ) signal blocks.

Calibration procedure 1 is used to calibrate unwanted tones. The goal of the MCU program executing the calibration procedure is to find suitable LMS7002M DC offset, gain or phase correction control values for TX and RX, at which the unwanted tones are reduced to levels close to ideal cases shown in Figure 1 and Figure 2.

Figure 2. RX spectral tones looking at the ADC input. Here: (4) – RX Wanted Signal; (5) – RX DC Offset; (6) – RX IQ Image.

Calibration procedure 2 is used for analog filter bandwidth calibration. The procedure can be used to tune the desired filter(s) to a new bandwidth setting or to perform re-calibration on an active setup. Figure 3 shows two cases of analog filter tuning:

• Case 1 – General case. Filter -3dB bandwidth is tuned to a particular frequency or re-calibrated for a new setup;

• Case 2 – Multiple filter stage tuning. On RX side, the transpimedance amplifier (TIA) acts as a 1^st order low pass filter (LPF), while the baseband LPF provides a 2nd order Chebyshev response with a 3 dB peak. Combining both filters can yield a 3rd order LPF with a tunable in-band response. Both stages need to be calibrated separately to produce the desired filter response. A similar setup is used in the TX low band stage.

The MCU program automatically selects the appropriate analog filter bandwidth calibration case in respect to the wanted bandwidth. It also handles the appropriate power down (PD) or enable (EN) controls of the involved blocks (manual setup is not needed).

Figure 3. Analog filter tuning

General MCU calibration limitations and options

The internal LMS7002M MCU has 16384 bytes of memory, thus the size of the calibration program is limited. Furthermore, it takes time (further elaborated in Section 2) to program the MCU, hence only the main (used most frequently) calibration procedures (see Section 1.1) are stored in the MCU image. Other calibration procedures, such as PLL frequency, TX gain tuning, should be implemented as separate baseband processor routines and should be ported separately using the latest source code found on LimeSuite GitHub webpage. Thus, time for MCU image upload is reduced and the overall calibration speed is increased.