Traditional software defined radio (SDR) RF front-end designs involve a whole set of high complexity analog parts (mixers, filters, LNAs, etc). When combined together with precision, these components can create a configurable RF front-end transceiver which addresses the SDR need of rapidly switching between frequencies, working within different baseband bandwidths, dynamically adjusting gains, etc.
Since the early 1990s, RF component integration into ICs has seen dramatic progress, specifically in addressing the needs of commercial wireless equipment (GSM, WLAN, Bluetooth, DECT, CDMA). It is clear that the goal of these market players was to reduce the components system cost, reduce design complexity and also increase the production yield.
Nowadays, the wireless SDR market is looking for the same advantages that come along with an integrated SoC (system on chip) architecture. Additionally, with emerging markets such as TVWS (TV White Space) where agility in the RF spectrum is a must, the popularity of tunable SDR SoC RF solutions has increased dramatically in the last 5 years. Tunable SoC RF is also very attractive for 4G designs, due to its ability to support a global frequency scheme with the same HW design, reducing design complexity and its support of multiple options and inventory to accommodate different regions.
An additional important trend in the market is the avenue of the Zero-IF transceivers (or Direct-Conversion Transceivers). Basically, the concept is to mix the RF signal by a local oscillator whose frequency is identical to, or very close to the carrier frequency of the intended signal. This is in contrast to the standard approach of using a superheterodyne technique where this is accomplished only after an initial conversion to an intermediate frequency. The resulting advantages of using a Zero-IF transceiver are that:
- Unwanted additive noise & distortion created at the mixing stage are completely rejected by the use of low-pass filters in baseband.
- Reduced design complexity
- Increased receiver selectivity
Like everything in this world, these advantages also come with some disadvantages:
- Signal leakage paths can occur at the receiver, creating a DC-offset
- Required additional baseband processing for calibration
The Zero-IF architecture has also been integrated in recent RF SoC designs where almost everything from RF to digital baseband is packed into a unique silicon (including LNAs, mixing, baseband filtering, clocking as well as A/D and D/A converters). This is a large step forward in the industry and greatly helps to reduce the complexity of SDR RF designs.
Lime Microsystems are the first to introduce such integration with their LMS6002D RF SoC. The device also provides very interesting features, such as its auto-calibration programming features. It supports a fully tunable range from 300MHz to 3.8 GHz while providing programmable bandwidth from 1.5 to 28 MHz.
Nutaq’s Radio420x FMC modules uses the Lime Microsystems’s LMS6002D technology and also includes additional features such as software selectable RF bandpass filter banks and additional RF gains. Such an integration on the same FMC radio head card allows a very high tunable range to address today’s agile SDR needs covering the whole UHF band. Additionally, two cards can be stacked and synchronized together to achieve 2×2 MIMO configurations.