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Signal leakage mires conversions

Posted: 01 Nov 2000 ?? ?Print Version ?Bookmark and Share

Keywords:mixed-signal? superhetrodyne receiver? if? local oscillator? signal leakage?

Using a combination of analog and mixed-signal technology, new tricks have made direct conversion a viable solution to high-performance radio problems. Direct conversion effectively eliminates the intermediate frequency (IF) and many of the passive tuning components associated with today's superheterodyne receivers. For example, a GSM receiver built by Analog Devices uses a tunable synthesizer that requires no IF SAW filtering.

The concept of direct conversion is so appealing that over the years there have been numerous attempts to use the technique. However, many have failed because of the second-order problems that can cripple a direct-conversion receiver. These problems are mostly due to signal leakage on a PCB. This article reviews superheterodyne tuning techniques and offers pointers for more streamlined implementations.

The superheterodyne, which dominates current radio architectures, was invented by Edwin H. Armstrong in 1917. Armstrong's approach was simple: Since it was not practical to build a receiver with a broadband amplifier and tunable high-selectivity filter, he chose to move the signal of interest in the frequency domain to an IF. The IF stage of the receiver included a narrowband filter centered at that fixed frequency and a fair amount of gain, which could also be optimized for that frequency. The frequency-translation process, called mixing, required a tunable local oscillator (LO) and a device of some kind that would produce output signals at the sum and difference of the RF input and the LO.

The superhet receiver evolved over the decades as new technologies became available for the active and passive devices. Gain blocks evolved from vacuum tubes to transistors and later to integrated transistors. Filter devices evolved from LC networks, to quartz crystals, to ceramic resonators, to mechanical disks, to SAW devices, many of which are still used today. The ever-changing universe of available devices gave radio engineers numerous variables to juggle; these engineers made careers out of optimizing superhet receiver design trade-offs.

For a vivid example of such trade-offs consider the following. A receiver designed to be low cost might use only one IF?for example, 455kHz in AM broadcast receivers or 10.7MHz in FM receivers. A receiver for higher-performance systems might distribute the gain over a few different IFs, with adequate selectivity in each stage to prevent distortion and the resulting undesired spurious signals. For example, a high-performance HF communications receiver might use as many as four IFs: 70MHz, 8.8MHz, 455kHz and 100kHz.

Mathematically and spectrally, the superhet receiver appears fairly straightforward and ingenious. The mixer, treated for this exercise as a multiplier, accepts input signals at FRF and FLO and delivers outputs at the sum and difference frequencies, (FRF+FLO) and (FRF-FLO). These frequencies are usually quite far apart in the frequency domain and the unwanted signal is easily removed with a simple filter. As FLO is varied, different signals in the RF band are converted to FIF and are filtered and/or demodulated there. Consider the case shown below, with a signal around 900MHz and a tunable LO around 800MHz.

On closer inspection, a problem is revealed: With an 800MHz LO, another RF frequency at, say, 700MHz can create a "phantom" or "image" signal at FIF (100MHz) at the output of the mixer. Superimposed on the 900MHz signal, these two would be impossible to separate at the IF.

This means that a signal present at the mixer input separated from the desired signal by twice the IF frequency will be indistinguishable from the desired signal when it gets to the IF. Any signal at this "image" frequency must be removed by filtering before the input of the mixer, or it will create an unresolvable interferer in the IF passband. Techniques have been developed to reduce this effect?special mixers known as image-reject or single-sideband mixers can reduce it significantly, but they require duplicating most of the mixer circuitry and consume more power than a normal mixer.

Demodulation in most modern wireless systems is accomplished by decomposing the signal into its in-phase and quadrature (I/Q) components. These I/Q signals are generally converted from analog to digital, and digital signal processing is used to extract the modulation. The I/Q demodulation is performed on the IF signal in a modern superhet receiver, with the demodulating LO usually at a fixed frequency.

An interesting effect occurs when the demodulating LO is made equal to the incoming RF signal's frequency (instead of the IF signal). If FLO equals FRF, then the mixer outputs are at FLO+FRF, or double the signal frequency, and FLO-FRF, which is dc. In practice, a pure tone at FRF is not interesting since it carries no modulation. A modulated signal creates a signal band centered at FRF; mixing this with an LO at FRF moves the modulation sidebands to a dc-centered spectrum. A quadrature LO decomposes the incoming RF signal into its I and Q components directly. Interferers in adjacent channels are also translated to baseband and are filtered by using low-pass filters, not the IF bandpass filters used in superheterodyne receivers. This is an intriguing result and allows the SAW, ceramic or other passive devices used for IF filtering to be replaced by filters that can be integrated on a chip. These filters can be analog, digital or a combination.

Direct conversion also means that the LO required in a superhet to perform the frequency translation from RF to IF disappears. Moreover, the attendant passive components and their accompanying board space disappear with it.

The first problem is that LO radiation can be a show stopper. Consider a cellphone system, where the handsets listen for signals from the base station. Since the LO is operating on the same frequency as the base-station transmitter signal, any leakage out of the antenna port is indistinguishable from a real transmitted signal by other receivers in the same vicinity. Most cellular systems include tests in the equipment-approval process for spurious emissions in the receive band. This leakage is a tough problem to solve at higher frequencies (gigahertz range), since even a short interconnect trace on a circuit board can be a reasonably efficient "antenna." Abundant shielding is one solution, but can add cost and weight to a portable product and is something of a brute-force approach.

RF port leakage

A second problem is caused by leakage from the RF port to the LO's VCO. If we think of the oscillator as an amplifier with a lot of gain at one frequency, any external signal coupling into that amplifier/oscillator can perturb the phase of the VCO, which can cause serious problems in a phase-modulated system. If the LO's phase varies, it causes apparent phase shifts in the received signal and will degrade the demodulation accuracy. Additionally, nearby large-signal interferers in the receive band may leak enough to pull the LO VCO off frequency, further degrading the receiver performance.

The obvious solution is to somehow desensitize the LO VCO to leakage from the antenna and somehow prevent the LO VCO from radiating in the receive band. Several tricks have been used to reduce this problem. One idea is to operate the VCO in the synthesizer at a multiple or fraction of the needed LO frequency and then perform either a division or multiplication to produce the actual LO. Another approach is to create the tunable LO by means of mixing a tunable VCO with a fixed-offset oscillator. This is similar to the method of using a half- or double-frequency oscillator, but the trade-offs in power may be more favorable. An interesting twist on this method uses regenerative division to produce the desired LO from an oscillator operating at a different frequency without a second VCO. This method, originally published in 1939, uses a part of the signal from a mixer's output as the LO input to the same mixer. In the original version, energy at half the input frequency is coupled back to the LO input of a mixer.

? Doug Grant

Business Development Director

RF and Wireless Systems

Analog Devices Inc.

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