Click here for the enhanced PDF version of this article

Global system mobile (GSM) or enhanced data rate for GSM evolution (EDGE) modulated signal delivered by the mobile station (MS) uses the random access channel (RACH) link to initially communicate with the base transceiver station (BTS). Once the BTS to MS instructions are decoded via the slow associated control channel (SACCH), the RF output power level in the MS is dynamically set. The purpose is to provide the minimum necessary output power^[1], in such a way that the BTS receives signals from many MS at approximately the same received signal strength (RSS). The end effect is that overall parameters such as talk time and standby time are vastly improved, with minimum interference. Hence, it is paramount that the RF power amplifier (PA) be tested and evaluated at all required power levels.

High efficiency is accomplished by operating the amplifier in class D for GSM (non-linear mode) and class AB₂ in EDGE (quasi-linear mode).

The RF input and RF output of the amplifier device mounted in a typical evaluation PC board are connectorized with SMA-type miniature coaxial connectors. These are part of the 50 Ω impedance system together with the microstrip traces on the board. Hence, it is important that all RF connections are low-loss 50 Ω coaxial type.

To minimize any source of error or uncertainty during the measurement, it is important that the RF coaxial contacts are clean. Use laboratory-grade alcohol to remove any contaminant. Due to “skin effect,” most if not all, of the RF current flows near the surface of the conductor.

A GSM/EDGE modulated signal from a suitable generator is applied to the RF amplifier evaluation board via the bench setup as shown in Figure 1. The RF output under a 50 Ω load is monitored via an RF power meter, spectrum analyzer and an optional vector signal analyzer.

The spectrum analyzer displays the transmitted spectrum. Due to the pulsed nature of the GSM signal, it has two main components: the modulation and switching (or transient) spectrum. These two must be measured separately and carefully measured following^[3] and compared to^[2] for a pass/fail criteria. While the vector signal analyzer gives the value of the modulation quality in error vector magnitude (EVM) and phase error.

The transmitted EDGE spectrum contains additional energy due to AM/AM and AM/PM conversion by virtue of the quasi-linear class AB₂ amplification (spectral re-growth). The figure of merit is usually adjacent-channel power ratio (ACPR), given in [dBc] at an offset frequency that corresponds to the first RF adjacent and first RF alternate channels.

The RF PA is dc biased under typical operating conditions as given in the data sheet for the particular device under test (DUT). Typical values are: Vcc1 = Vcc2 = +3.5 V_dc; Vreg = +2.7 V_dc.

The minimum acceptable levels are given in references 2 and 3. Also, Table 1 provides a summary of the relevant portions of the ETSI standard and its application to GSM/EDGE RF PAs.

1 2 3 Next

RSS    Save to Del.icio.us  Digg This