By: Arnoldas Bagdonas, Field Applications Engineer (FAE), Future Electronics
Zero-IF (homodyne) receivers are an increasingly popular form of radio receiver, offering several notable advantages over older and more complex architectures. But zero-IF receivers (IF = Intermediate Frequency) suffer from degradation of sensitivity for a wide variety of reasons. Armed with knowledge of the causes of such degradation, design engineers will be well equipped to take counter-measures, and ensure their circuit enjoys reliable radio reception and adequate range.
This article, from Future Electronics’ Field Applications Engineer Arnoldas Bagdonas, provides a description of the main mechanisms that cause sensitivity degradation in zero-IF receivers, and suggests techniques and components that help the developer to combat their effects.
Zero-IF receivers: a popular choice
The zero-IF receiver has won support among system designers for three main reasons:
It does not require the transceiver’s Local Oscillator (LO) to change frequency when switching between transmit and receive modes. This means that the transition between modes is quick.
In contrast to the conventional super heterodyne receiver architecture, the homodyne architecture of zero-IF receivers does not give rise to an ‘image frequency’ – an undesired input frequency equal to the desired frequency plus twice the intermediate frequency. If left untreated, an image frequency interferes with radio reception. Super heterodyne receivers therefore require image rejection, normally accomplished with additional filtering circuitry in the RF front end. Homodyne receivers require no image rejection.
Most important, signal processing takes place in the digital domain. This contributes to lower system costs. It also supports effective demodulation operation with the use of matched filtering and synchronous detection techniques.
There is a fairly extensive literature on the operation and design of zero-IF radio systems. This article, however, presents for the first time a complete overview of the mechanisms that degrade sensitivity in these circuits, as shown in Figure 1. This shows that there are two root causes of sensitivity degradation in zero-IF transceivers: mismatch of receiver and transmitter, and an increased noise floor at the receiver side.
A mismatch between the transmitted signal spectrum and the receiver’s bandwidth causes a decrease in sensitivity because some fraction of the transmitted energy fails to enter the receiver’s pass band. This occurs most commonly in the early stages of system prototyping, and is quickly fixed by an analysis of the influence of the chosen modulation parameters and schemes on the carrier’s frequency spectrum. Also it is normal to find that, in narrow-band channels, wider receiver bandwidth is used for the transmitter and receiver LO frequency-offset compensation, at the cost of slightly decreased sensitivity.
Frequency drift in crystal-stabilised oscillators, widely used as reference frequency sources, is another common cause of transceiver receiver mismatch.
Frequency or bandwidth mismatch problems have a greater impact on narrowband than on wideband systems. But in any radio design, the problems described above can be greatly mitigated by proper circuit design which allows for stable operating temperatures, minimum drive levels and, if necessary, static pre-ageing.
Raised noise floor
An increase in the noise floor on the receiver side may be caused by several different mechanisms. For instance, noise from switching digital circuits can leak in to the receiver’s input in unshielded circuits, an effect that can be mitigated through the use of good board layout practices, including the use of high-quality shielded connectors and noise-source shielding.
These techniques could be supplemented by efforts to manage the frequency spectrum of noise, distancing the frequencies at which noise occurs from the frequency of the carrier signal.
Other useful techniques include filtering, through the implementation of decoupling and bypassing circuits in the power supply, and mitigation of self-polluting interference. The measurement set-up, shown in Figure 2, can uncover noise sources by monitoring variations in bit error rate (BER) and Received Signal Strength Indicator (RSSI) levels.
Another technique, active noise cancellation, is especially effective with closely spaced antenna radiation, internal processor noise, video camera and display noise, as shown in Figure 3. This can be implemented through the use of a device such as the QHx220 interference canceller from Intersil.
Low-frequency noise in the power supply is as dangerous as highfrequency noise. For instance, if a circuit is handicapped by a low Power Supply Rejection Ratio (PSRR), LO phase noise will rise, impairing the performance of the receiver. To be more specific, LO phase noise either lowers the Signal-to-Noise Ratio (SNR) below the level that could be achieved with the ideal mixer, or it causes parasitic incidental phase modulation, when a phase-modulated carrier is used. Both effects reduce receiver sensitivity.
LO spurs could occur if the power-supply noise is periodic rather than random in nature, as shown in Figure 4. In-band spurs have the same effect as the LO phase noise described above; while out-of-band spurs, which occur at unexpected input frequencies, might in turn cause receiver spurs. Any energy in these unexpected input bands is injected as noise into the main receiver band. A common cause of unexpected spurs is the use of a low-quality reference crystal – parasitic vibrations and high drive currents often impair their performance.
The description of the power-supply noise effects above suggests that the system designer should make a calculation of the maximum noise level that can be accepted. In an effective method for designing a PLL power supply, the VCO pushing figure, the ratio of frequency change to voltage change, may be measured by DC-coupling a low-frequency square wave into the supply, while observing the Frequency-Shift Keyed (FSK) modulation peaks on the VCO output spectrum. The frequency deviation between the peaks divided by the amplitude of the square wave yields the VCO pushing number: this is used to determine the acceptable power-supply noise level, required to keep PLL phase noise at an acceptable level. The method can be adapted for measurement of receiver power-supply performance while monitoring the BER.
Mitigating the impact of noise on RF circuit performance
Armed with knowledge of the mechanisms of receiver sensitivity degradation, the RF system designer can set to the task of eliminating or mitigating the effects of noise. It is common for high-frequency power supply noise components to be filtered by a combination of passive RLC networks and shielding.
Low-frequency noise needs a different approach. To start with, high-speed Low Drop-Out regulators (LDOs), which offer high PSRR and low output noise, outperform passive circuits at low frequencies.
The signal received by the antenna is then amplified by an internal or external Low Noise Amplifier (LNA). The performance of these amplifiers has a pronounced impact on the performance of the circuit as a whole. The noise figure and linearity of the LNA, usually specified as the third or second order input intercept points (IIP3 or IIP2), should be studied carefully as the noise and intermodulation products the LNA generates can mask the received signal.
Avago Technologies provides an interesting perspective on the mitigation of noise figure degradation when the LNA is overloaded by a strong out-of-band interferer signal. Avago shows that a pre-filter to block a strong interferer signal from leaking into the receiver path gives better results than any other mitigation method, as shown in Figure 5, in GPS systems. The results may be extended to all other bands, as the degradation mechanisms of the LNA remain the same.
Another good practice to follow is to specify the receiver noise floor at a level 6dB below the calculated thermal noise floor at the receiver’s input bandwidth. If this is not done, the increased noise floor will start to dominate in the receiver sensitivity equation:
SIN = K•T•BRF + N + NF + SNR
SIN is the available input signal power (dBm)
NIN = K•T•BRF is the available input thermal noise power
K = Boltzmann’s constant
T is room temperature and
BRF is the RF carrier bandwidth in Hz
NF is the noise figure in dB
SNR (in dB) is the ratio required by the receiver in normal operation (to produce a specified output signal)
N is the additional noise level (dBm) in a real application.
Lastly, new products and technologies developed by semiconductor manufacturers can provide a marked improvement in receiver sensitivity, quite independently of the mitigation techniques described above. The new LoRa™ modulation scheme deployed in the SX1272 and SX1273 products from Semtech is an example of this. LoRa provides 10dB better sensitivity than an FSK modulation scheme can achieve when used with a low-cost, low-tolerance crystal reference. The enhanced performance of LoRa devices is due to a proprietary spreadspectrum modulation technique developed by Semtech. It also offers another benefit: each spreading factor is orthogonal, allowing multiple transmitted signals to reside on the same channel without interfering.