Direct Conversion RF?

To ensure reliable reception of a radio frequency (RF) signal, it is necessary to down-convert it to baseband signals, filter out noise and interfering signals, and then demodulate the signal with a modem. With the wide spread use of wireless devices, the RF bands are getting crowded with interfering and jamming signals that should be filtered out as depicted in Figure 1, where for example, advanced mobile phone system (AMPS) signal is present in the vicinity of strong interferes. It can be seen that we need a filter with a very large Q to remove the interferers, which is not generally possible in RF bands. Until recently, the problem have been addressed by heterodyne receivers, where the signal is first down-converted to intermediate frequency (IF) for efficient channel selection, and then filtered to remove interfering signals and amplified for another step of down-conversion to the baseband frequency. The heterodyne receiver provides good performance in terms of channel selectivity and sensitivity, but with the need for off-chip surface-acoustic wave (SAW) filters in combination with additional IF circuitry. Since SAW filters are fabricated using different material technology, they cannot be integrated with the IC. To replace expensive SAW filters in the heterodyne receiver, another approach is used- the low-IF architecture, where the RF signal is converted to a very low IF frequency, and then filtered using passive or active in-circuit filters. However, this architecture suffers from poor performance in terms of image rejection and channel selectivity.


For proper reception of the signal, the receiver should filter out adjacent interferers sufficiently, which can be done in the intermediate frequency or baseband.

Another promising RF architecture, direct conversion receiver or homodyne receiver, eliminates the intermediate frequency stage, and directly down-converts the RF signal to a baseband signal. Without the IF stage, bandpass filters and other elements can be eliminated to effectively reduce the bill of materials (BOM). As depicted in Figure 2, the direct conversion receiver cuts the number of required parts drastically compared to the heterodyne receivers.

Figure 2

Comparison of heterodyne receiver and direct conversion receiver

In spite of the advantages, for the past several decades, direct conversion architecture has frustrated RF engineers with its inherent problems such as self-mixing, 1/f noise and sensitivity. Self-mixing comes from the LO signal making its way to the input of the mixer, which generates DC as a mixing product, and possibly saturating the following filters and gain amplifiers as shown in Figure 3. GCT's patented fast-acting DC offset cancellation algorithms can suppress the DC sufficiently to keep the receiver from saturation and possible performance degradation. The sensitivity problem and 1/f noise have been solved by optimally controlling a number of amplify-and-filter stages in the receiver chain, where the controlling algorithm is one of the key patents of GCT's direct conversion receivers.


Self-mixing may saturate the circuits degrading the performance

GCT has been a pioneer in direct conversion technology to provide industry-leading single-chip direction conversion transceivers for Bluetooth, W-LAN, and WCDMA RF ICs. GCT succeeded in providing mass-production samples based on the patented direct-conversion architecture for Korean WCDMA-based wireless local loop (WLL) transmitter and receiver as early as 1999 based on the patented direct-conversion architecture. Subsequently, GCT developed Bluetooth receivers in year 2000 based on the direct conversion architecture, while many of the chip-makers are still using receivers based on low-IF architecture that suffers from image problems and low channel selectivity. Going forward, GCT is also developing direct conversion 802.11b transceivers. As the 2.4GHz band becomes more crowded due to Bluetooth and 802.11b, the high channel selectivity promised by the direct conversion receiver will certainly prove most desired features.
As the market for wireless LAN, personal area network (PAN) and cellular systems are rapidly growing, there are unprecedented demands for multi-mode, multi-band, multi-standard solutions. The direct conversion architecture is undoubtedly the best choice for serving these demands with the lowest BOM, the smallest form-factor and the most efficient manufacturing process (CMOS). GCT will continue to be the leader in the direct conversion receiver design with its state-of-the-art technologies and strong market ownership.

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