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Direct Conversion Radio

Direct-conversion (a.k.a. Zero-IF)is an alternative wireless receiver architecture to the well-established super-heterodyne receiver, particularly for highly integrated, low-power terminals. Its fundamental advantage is that the received signal is amplified and filtered at baseband rather than at some higher intermediate frequency. This results in lower current drain in the amplifiers and active filters and a simpler task of image-rejection. There is considerable interest to use it in digital cellular telephones and miniature radio messaging systems.

Radio Receivers
DCR Overview
Superheterodyne Vs DCR

Superheterodyne
DCR

Characteristics

Benefits
Disadvantages

Usage

WiMAX Transmitter
DCR Receiver

News
Glossary
See Also
Links to references

Radio Receivers

A radio receiver is an electronic circuit capable of receiving and decoding modulated electromagnetic waves (radio signals) from an antenna, providing the transmitted information in the form of audio/video signals, pictures, etc. Although, the receiver (and the transmitter) may deploy any of the available modulation/demodulation techniques available today (like AM, FM, PSK, OFDM, etc), the important attributes of a good receiver design remain common:

Sensitivity:The ability of a receiver to pick up weak signals close to the noise floor
Selectivity:The ability of receiver to Reject unwanted signals either in band (co-channel interference), close to the wanted band (adjacent channel interference) or well away from the wanted band.
Low Current Consumption

Some of the popular receiver architectures available today are:

Tuned Radio Frequency Receiver
Super Regenerative Receiver
Superheterodyne Receiver
Direct Conversion Receiver

DCR Overview

The DCR is similar to the super-heterodyne in underlying concept: The receiver radio frequency (RF) signal is translated in frequency by non-linear mixing with a local oscillator (LO) signal. Figure 1 shows the basic block diagram for the front-end of both types of receiver. The mixer is a non-linear element that combines the two signals

and

. The output of the mixer contains a number of different frequencies that obey the relationship:

where,
is the output frequency
is the frequency of the received radio signal
is the frequency produced by the local oscillator
m and n are integers (0, 1, 2, 3 ...)
(All frequencies are in the same units.)

Figure 1:Basic mixer-local oscillator circuit.

All frequencies other than and are product frequencies. In general, we are only interested in the cases where m and n are 0 or 1, so the output frequency spectrum of interest is limited to and plus the product frequencies + , and - . The latter two are called sum and difference intermediate frequencies (IF).

In a super-heterodyne radio receiver, a tuned bandpass filter will select either the sum IF or the difference IF, while rejecting the other IF, the LO and RF signals. Most of the gain (which helps determine sensitivity) and the selectivity of the receiver are accomplished at the IF frequency.
On the other hand, in a DCR, only the difference IF frequency is used (see Figure 2). Because the DCR LO operates at the same frequency as the RF carrier, or on a nearby frequency in the case of CW and SSB reception, the difference frequency represents the audio modulation of the radio signal. Amplitude-modulated (AM) signals are accommodated by zero-beating the LO to the radio signal. making = (carrier).Thus, only the recovered upper and lower sidebands will pass through the system, and they are at the audio frequency.

Figure 2:Partial block diagram for a direct-conversion receiver.

For CW signals (Morse code on/off telegraphy) and single-sideband (SSB) signals, it is necessary to offset the LO frequency slightly to recover the signal.
As was true with the super-heterodyne receiver, the majority of the gain and selectivity in the DCR is provided by the stages after the first mixer. Although the super-heterodyne uses the IF amplifier chain for this purpose, followed by second detection and audio amplification, the DCR must use only the audio amplifier chain. Thus it becomes necessary to provide some very high-gain audio amplifiers and audio bandpass filtering in the DCR design.

The block diagram of a direct conversion radio is given in Figure 3. The local oscillator is at the center of the desired passband and frequencies close to it will show up as audio frequency signals in the mixer outputs. An ideal mixer produces the sum and the difference frequency only while a real mixer also responds to overtones of the local oscillator.
The relative phase between LO and RF must differ by about 90 degrees between the I and Q channels. Modest errors in the phase can be compensated but the relative phases and amplitudes must be very stable.
The audio amplifiers amplify the very low voltage present at the mixer output to the desired level.

Figure 3:Block diagram for direct conversion radio.

Superheterodyne Vs DCR

Superheterodyne

Heterodyne simply refers to the inclusion of a mixer in the chain to convert the incoming high frequency RF signal to a low frequency (usually a fixed intermediate frequency (IF)), where much improved selectivity is possible.

To achieve the mixing function, a local oscillator (LO) running at or near the incoming carrier frequency is needed. The difference between the LO and input signal frequency results in the intermediate frequency (IF) desired. By making the IF a fixed frequency and tuning the LO to select a given channel, the IF selection filter and additional amplification can be carefully optimized for good selectivity with small size and low cost. Typical IF’s used are 455kHz, 10.7MHz and 45MHz. Filters implemented using ceramics, SAW technology or resonant crystals are all available at these frequencies suiting a range of channel spacings.
A further advantage of using an IF is that more gain can be introduced into the receiver chain without instability occurring due to feedback into the input circuits.

A drawback with the simple superheterodyne design is that the receiver cannot distinguish between signals appearing either above the receiver LO or below the receiver LO. In practice, the unwanted ‘image’ signal on one side of the LO must be filtered in the front end before reaching the mixer.
To alleviate the image problem and aid selectivity, multiple frequency conversion stages are often employed, with a high IF chosen as the first stage to make image rejection easy, and a low IF chosen for the second (and third) stages to make channel selection more precise. Commercial FM receiver IC’s typically incorporate two IF’s at 10.7MHz (45MHz) and 455kHz for this very reason.

In digital domain, it is possible to digitise the waveform at an IF up to 70MHz and then implement further channel filtering and data detection.This brings enormous benefits in flexibility of the receiver unit, but also brings a power consumption penalty with the high sampling clocks required.

DCR

DCR is can be thought of as a special case of Superheterodyne with just one mixing stage where the LO frequency is same as the incoming carrier frequency giving an IF or 0Hz.
A major advantage of DCR over heterodyne is that there are no image problems and that the IF selection filter is a simple low pass filter at baseband. This allows the filter to have even greater selectivity with better gain and phase response.

There still are two design challenges with the DCR solution. The first is the problem of local oscillator re-radiation from the antenna. Very careful design is needed to ensure that the LO, which is now at the incoming RF frequency, does not leak back through the front end mixer/amplifier/filter chain.

The second design challenge is the problem of dc-offset within the two baseband signals which, if present, can corrupt wanted information that has been mixed down around zero Hz. Causes of dc offset are either drift in the baseband components (e.g. op amps, filters, A/D converters), or dc from the mixer output caused by the LO mixing with itself or with the mixers acting as square law detectors for strong input signals . Again careful design can minimise this problem, but it can be the reason why direct conversion will not work for every application.

However, even with the added complexity of these solutions, the aggregate of direct conversion’s advantages for a miniature, low-power radio transceiver is enough to continue research and development in this area.

Characteristics

Benefits

Significant Power Savings, ideal for miniature, low power transceivers

Disadvantages

Local Oscillator re-readiation from the antenna
Corruption due to DC offset

Usage

WiMAX Transmitter

The use of high data rate OFDM modulation translates into challenging requirements for the transmitter in terms of spectral quality and EVM. This calls for low distortion, good signal balance and low phase error. With WLAN 802.11 and now WiMAX 802.16, there has been a growing interest in technologies that allow delivery of higher data rates over large geographical areas. The IEEE 802.16 family of standards (802.16-2004 and 802.16e) are intended to provide high bandwidth wireless voice and data for residential and enterprise use. The modulation used to achieve these high data rates is orthogonal frequency-division multiplexing (OFDM).
To address these challenging requirements of the 802.16 standard,the transmit signal chain may be based on radio architectures like super-heterodyne, IF sampling or zero-IF. At 2.35 GHz, a direct up- conversion architecture is attractive for the following reasons:

state-of-the-art synthesizers and IQ modulators still perform well at this frequency,
WiMAX OFDM has no active subcarrier at the origin,
direct upconversion produces less mixing product spurs,
it requires fewer filters, which is important when dealing with wideband signals, and
the lower number of parts helps minimize the current consumption.
Finally, in multicarrier modulation schemes like WiMAX OFDM, reducing the number of LO mixes is critical.

The high number of subcarriers within the OFDM signal actually makes this modulation quite sensitive to phase noise, as each of the N subcarriers will be modulated by the phase noise of the LO.
In the transmit (TX) signal chain architecture, The I and Q analog baseband signals are generated by a dual 14-bit DAC. The direct RF upconversion is done using an IQ modulator. Low-pass filters are required at the DAC output to remove the alias at the sampling frequency before the upconversion. The LO is generated by an external fractional-N synthesizer, which provides a continuous wave signal with minimal phase error. Finally, the composite RF output signal is amplified or attenuated through a variable gain amplifier (VGA) with a 50 dB of gain control range. An rms power detector ensures precise control of the output power.

DCR Receiver

As third-generation (3G) wireless networks are currently expanding in Japan (IMT-2000), in Europe (UMTS) and in the United States (CDMA 2000), the need for low-cost, low power consumption, and low form factor user equipment (UE) is becoming important for the commercial development of 3G mobile handsets. The direct-conversion radio architecture with the proper use of silicon process, circuit design techniques and architecture implementation represents a promising system solution for high integration platforms for 3G handsets. The receiver second-order input intercept point (IIP2) requirement is a key specification for the direct-conversion receiver solution.
Direct-conversion or zero-IF receiver architecture enables the pathway for a full on-chip integration of the receiver as the signal is directly demodulated to baseband I and Q signals. In a 3G WCDMA full-duplex (FDD) operation mode, only an external duplexer is needed for separation between RX and TX sections. Furthermore, the post low-noise amplifier (LNA) RF filter is needed in a FDD radio to reject out-of band blockers and transmitter leakage at demodulator input due to limited finite duplexer TX-RX isolation. In a zero-IF receiver IC, channel selectivity is achieved at baseband by on-chip low-pass filters. Following the channel filtering, I/Q signals at baseband are amplified by variable gain amplifiers (VGAs) before they get digitized in the analog baseband section of the radio modem IC.

News

Analog Devices have released a single chip GSM receiver using a direct conversion architecture

Glossary

ADC	Analog-to-Digital Converter
AM	Amplitude Modulation
CDMA	Code Division Multiple Access
DAC	Digital-to-Analog Converter
DCR	Direct Conversion Radio
FDD	Frequency Division Duplex
IF	Intermediate Frequency
LNA	Low Noise Amplifier
LO	Local Oscillator
OFDM	Orthogonal Frequency Division Multiplexing
RF	Radio Frequency
SSB	Single-Sideband
SWL	Short Wave Listening is the hobby of listening to shortwave radio broadcasts. Shortwave listeners, or SWLs, do not transmit, in contrast to radio amateurs.
UMTS	Universal Mobile Telecommunications System
VGA	Variable Gain Amplifier
WCDMA	Wideband Code Division Multiple Access
WiMAX	Worldwide Interoperability for Microwave Access