ZIF

Zero IF

The Zero IF (ZIF) radio transceiver is a direct-conversion architecture, meaning that it utilizes one mixer stage to convert the desired signal directly to and from the baseband without any IF stages and without the need for external SAW filters. Most ZIF radio designs also integrate the low noise amplifier, voltage-controlled oscillator (VCO) and the baseband filters on a monolithic die. In fact, such integrated single chip ZIF transceivers have been proved for many years in cellular and pager applications and they are beginning to emerge in WLAN radio designs as well. Some of the common RF problems inherent with the ZIF architecture are dc offset, flicker noise and LO pulling. Dc offsets are mainly generated by the LO leakage, which self-mixes, thereby creating a dc component in the signal chain that affects the receiver performance and can cause the RF stages to saturate. Flicker noise, also known as 1/f noise, is low-frequency device noise that can corrupt signals in the receiver chain. Flicker noise is more pronounced with the ZIF architecture because of the direct conversion to low-frequency baseband. Another concern with direct conversion is the pulling of the LO by the PA output, which affects the direct upconversion process. This is because the high-power PA output, which has a spectrum centered around the LO frequency, can disturb ("pull") the transmitter VCO. Recent advances in radio and modem designs are able to resolve these matters through a combination of proprietary radio design techniques and system algorithms in the baseband. For instance, dc offset can be addressed via a compensation scheme in which the offset is measured and reduced via unique radio and baseband algorithm. In contrast, the superheterodyne (or dual-conversion) transceiver is considered the classic radio architecture in which the received signal is downconverted to baseband frequency in two stages. This two-stage receiver and transmitter architecture uses an RF block to convert an incoming signal to an IF where image suppression and channel selection are performed with a narrow channel-select filter, such as a SAW, or ceramic filters. The now-filtered signal is then further downconverted to the baseband frequency, which is then digitized and demodulated in a DSP. This radio architecture has been used for decades, in part due to its excellent sensitivity and selectivity characteristics. This, however, come at the expense of more complexity and cost, for such radio implementation typically requires an RF chip and an IF chip as well as discrete SAW filters and VCO/synthesizers. The bill of materials for a dual-conversion radio design is more expensive than for a direct-conversion design for a single-band WLAN network interface card, but the cost discrepancy is increased when the chip set has to handle two bands. A dual-band design using a superheterodyne transceiver needs a 5-Ghz RF stage and a 2.4-GHz RF stage, discreet IF synthesizers/VCO, two SAW filters for image rejection and for channel selection for each band and a common IF block driven by a discreet IF VCO. Assuming that this architecture uses an integrated modem/MAC IC, a dual-band chip set solution will require nine components. On the other hand, the lowest cost DB approach is to combine the two ZIF transceivers into a single DB radio built on CMOS. This is entirely possible because of the direct conversion of the ZIF transceiver architecture and the integration capability of CMOS process. In fact, the 2.4- and 5-GHz ZIF transceiver circuitry can be laid side by side on a monolithic die without much impact to the die size or package cost. This approach results in a two-chip solution without the need for external components such as SAW filters. In fact, assuming that the dual-conversion chip set and the ZIF chip sets are priced the same, the latter architecture's materials cost at least $7 less, which is very significant in relation to the overall cost-reduction effort of the complete dual-band system design.

http://www.eetimes.com/electronics-news/4164144/Dual-band-issue-super-heterodyne-v-s-zero-IF