What is a Direct Conversion Receiver?

A Direct-Conversion Receiver (DCR), also called Zero-IF receiver, is a type of receiver in which the Intermediate Frequency (IF) conversion stage is not present. The signal intercepted by the receiving antenna is converted directly from RF to baseband using synchronous detection driven by a local oscillator whose frequency is identical or is close to the carrier frequency of the RF signal. Since only one single stage for frequency conversion exists, the architecture of a direct conversion receiver is less complex than conventional superheterodyne receivers, which converts the signal to an IF Frequency first and includes an image rejection filter prior to feeding the signal to the mixer.

Figure 1: Simplified Version of a Direct Conversion Receiver

Common Technical Challenges Faced by Direct Conversion Receivers

1. DC offsets from strong out-of-band interference

As discussed, the issues arising due to image rejection stage has been eliminated, since the IF is zero. Thus, only a local oscillator is required, which implies that only a single-phase noise will be contributed to the section. The need for bulky filters is also removed since majority of the filtering is now done only at baseband with a higher amplification.

 Under non-ideal conditions, strong out-of-band interferers or blocking signals exist and should be eliminated prior to down-conversion to avoid generating higher-order harmonics followed by other inter-modulation distortions that are likely to occur. A filtering process such as this may be included after the Low-Noise Amplifier so as to avoid driving the LNA to high loads causing damage to the circuitry.

Another issue that can arise in DCR systems - Since the incoming RF signal is converted directly – without prior pre-filtering and signal processing other band-selection – various phenomena contributing to DC offsets can occur, which appear as interfering signals at the desired band of interest. In fig. 1, the LO signal might leak through an unintended path to the mixer’s RF input port and mix with itself upon return, thereby producing unwanted DC components causing an offset. Numerous mechanisms contribute to this problem such as poor conductor or cable isolation, substrate coupling, and bond wire radiation.

2. DC offsets from strong in-band interference

Strong in-band interference can also exist due to signal leakage from the LO to the mixer input port, which leads to self-mixing, as discussed before. Adjacent in-band receivers may interfere with the desired channel and cause severe interference with the RF signal, thereby violating the emission standards set by the governing technical organization. Additionally, the radiated LO leakage signal can also be reflected off from urban buildings or moving objects such as cars and are then re-intercepted by the antenna.

How to the issues of DC offsets be addressed?

One way to address this problem is to use AC capacitive coupling at the mixer output to eliminate the DC component before feeding it to the LNA and other baseband subsystems. However, proper care must be taken when selecting the capacitance value and should assist the intended modulation scheme and target application. While certain modulation schemes show low degradation for low frequency components, others that exhibit a peak at DC will undergo significant degradation and will lead to information/data loss for example, in TDMA-like systems. This, in turn, corresponds to a significantly lower Bit Error Rate (BER). Therefore, the capacitance value must be chosen such that it should be large enough to avoid a large and wide notching at the DC and at the same time, the value is small enough to ensure that the undesired transients are ruled out before the data is received.

Another alternative would be to utilize the idle timeslot before data reception to store the value of the DC offset in the capacitor and then subtract it from the output signal to obtain the desired response. Further improvements can be made by using appropriate digital signal processing in TDMA systems along with the DC cancellation/correction schemes explained. A classical BER simulation shown below points that by using a DSP based DC cancellation technique, the BER can be substantially reduced to improve the packet efficiency in TDMA systems.

Figure 2

3. Non-Linearities

Like in superheterodyne receivers, DCR receives present spurious responses too primarily owing to faulty isolation of cables and poor bond wiring. When a blocking or interferer’s signal frequency superposes with the spurious responses, the resultant signal undergoes a shift in bandwidth in baseband. This shift depends on the spurious order and its intensity determines the severity of the distortion.

How to address the issue of non-linearities in direct conversion receivers?

During the designing stage, it should be made sure that the bond-wire radiation and other isolation faultiness are addressed or that the radiation is well within the tolerable limits that do not outweigh the optimum performance of the signal. Therefore, it can be alleviated via a well-balanced circuit design that ensures linear operation with a wide operating range.

4. Concentrated Noise at Low-Frequency Bands

In DCR, since the incoming RF signal is converted directly to the baseband, the impact of low-frequency flicker on narrowband signals can be significant. When MOS-based devices are used, the impact of this noise is particularly severe and presents a reasonable degradation and that the noise components lie in-band of the desired signal. This noise is generated due to additional electron-energy states existing at the boundary of Si and SiO2, which overcomes the energy of the electron and hence releases them from the channel. Since this process is slow, this noise is present usually in the lower frequency range of the device. Therefore, each transistor and hence each circuit contributes to the flicker noise and therefore can be addressed by choosing an optimal physical dimension of the transistor that improves the device transfer function of the MOSFET.

Although technical challenges exist in direct-conversion receiver systems, a careful designing of the circuit followed by an optimal architecture pave the way to implement the said system in a variety of applications including satellite receivers, mobile phones, pagers, and in other communication technologies.

Operation Principle of an FSK Based Direct Conversion Receiver

Figure 3

A block diagram of a Frequency Shift Keying (FSK) based direct conversion data receiver is shown in fig. 3. The output of the RF amplifier is fed to two separate mixer circuits that receive a signal with in-phase component and quadrature components (signal shifted by 90o). The in-phase (I) and Quadrature components (Q) are included in this section to allow the luxury of selecting an arbitrary amplitude and phase of the output signal. Upon frequency conversion, each of the outputs of the current stage is passed to low-pass filters and limiters and then fed to the phase detector for demodulation. A delay of 1/4th the period of frequency offset is chosen as the optimal period and is inserted into the quadrature component relative to the in-phase component. 

Upon demodulation, the FSK-keyed signals appear at the response, thereby, giving a binary representation in terms of mark and space, depending on whether the received signal is above or below the local oscillator frequency. The result is a demodulated output very similar to what is obtained using a superheterodyne receiver which uses both a synchronous detector followed by an Intermediate Frequency (IF) stage.