512 IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, AUGUST 1989 FM RADIO RECEIVER FRONT-END CIRCUITRY WITH ON-CHIP SAW FILTERS P. T. M. van Zeijl', J. H. Visser' and L. K. Nanver** *Delft University of Technology Electrical Engineering Faculty Mekelweg 4, 2628 CD Delft, The Netherlands tel.:31.15.781083; FAX:31.15.785922 **Delft Institute for Microelectronics and Submicron Technology (DIMES) Abstract local oscillator (LO) harmonics, additionally limit the use- The tendency today in radio receiver design is towards a fully ful dynamic range. Therefore, another architecture must be integrated receiver, with a minimal number of adjustments chosen. Direct detection, converting the RF signal directly and external components. This paper presents the results to baseband, seems a promising solution. However, offset of a feasibility study concerning the monolithic integration of the transistors remains a limiting factor for the dynamic of an FM upconversion receiver. The on-chip selectivity is range. Upconversion receivers with frequencies of the IF and obtained with silicon-integrated surface acoustic wave filters LO signals higher than the highest RF signal, only need a by using a ZnO-SiOz-Si layered structure. non-adjustable low-pass or band-pass filter at the input to achieve a sufficient rejection of image and spurious responses 1 Introduction [4]. The upconversion principle for a high-performance FM For reasons of performance, reliability or cost, the number receiver, without any adjustments of filters or offset voltages, of components in electronic systems are constantly being seems most promising. reduced and more and more parts of the total system are The use of the upconversion principle results in an inter- being integrated in one chip. Frequency selectivity is of- mediate frequency above 108 MHz and to avoid problems ten required. At low frequencies switched-capacitor filters or such as pin-to-pin feedthrough at these high frequencies, all continuous-time active filters can be made on-chip, but these high-frequency circuits and filters should be monolithically are filters with a limited dynamic range. Until now, high dy- integrated on the same chip. Only the input signal, some namic range selectivity has been achieved by using external bias voltages, tuning voltage and the output baseband sig- (bulk or surface) acoustic wave devices or LC filters. On-chip nal should occur at the pins of the IC. By using a silicon- selectivity can be obtained by the monolithic integration of integrated SAW filter as IF filter, a monolithically integrated surface acoustic wave (SAW) devices in a ZnO-SiOz-Si lay- FM upconversion receiver is feasible. ered structure. Such an integration of a SAW delay line In section 2 system design considerations for FM upcon- version receiver front-ends will be discussed. In section 3 the has been carried out as part of a delay-line oscillator in a design of silicon-integrated SAW filters and their equivalent physical-electronic system for sensors [l]. ZnO as a piezo- electric layer has been used to fabricate SAW filters 121, but circuits will be discussed. In the receiver system optimal sig- these filters have thus far not been realized in a ZnO-SiOz-Si nal transfer, in contrast to power transfer, is desired. Using structure. Such SAW filters (monolithically integrated with the equivalent circuit model it is concluded that the SAW fil- electronic circuitry) would be attractive for example in AM, ter should be driven by a balanced voltage and loaded with FM and digital radio receivers, satellite broadcast receivers a differential short circuit. In the present study the integra- and television (HDTV) systems. Furthermore, on-chip SAW tion of SAW filters and electronic circuitry is realized in the resonators can be used in low-phase noise oscillators for com- 3 GHz BIFET process. As described below, only mi- munication applications. nor process modifications were necessary for achieving this In this paper, the monolithic integration of an FM upcon- goal. Measurements on silicon-integrated SAW devices and version receiver front-end with on-chip SAW filters will be electronic circuitry are presented in sections 5 and 6, re- discussed. The commonly applied downconversion configu- spectively. Section 7 ends this paper with a discussion and ration for FM receivers uses 10.7 MHz as an intermediate conclusions. frequency (IF). Because the image channel is only 21.4 MHz away from the desired radio frequency (RF) signal, a tuned 2 System design considerations band-pass filter of sufficiently high order is needed to obtain For fabrication with a minimum of external adjustments and an adequate image response suppression. An adequate on- peripheral components, upconversion is the method which chip replacement for the tuned filter at the input has not seems most promising. A block diagram of the FM receiver yet been found. Furthermore, spurious responses, due to is shown in Fig.1. The voltage from the whip antenna passes Manuscript received June 9, 1989 0098 3063/89/0200 0512%01.000 1989 IEEE van Zeijl, et al.: FM Radio Receiver Front-End Circuitry with On-Chip Saw Filters 5 13 through a band-pass filter to the input amplifier (amp.1). ters have been realized. For a 40 dB attenuation of signals By using the local oscillator (LO) signal and the mixer, the at the image frequencies and at the spurious frequencies a RF signal is converted to a signal at the intermediate fre- fourth-order non-tuned LC band-Dass filter is sufficient. quency (IF). The second amplifier is used to drive the IF A mixer is used to perform the frequency conversion of filter with the mixer output signal. The IF filter is loaded signals from the FM broadcast band to signals at the inter- with an amplifier (amp.3), which drives succeeding stages for mediate frequency. The mixer has to handle the total FM the detection of the FM signal. broadcast range of signals and must, therefore, be extremely The demands placed on each block specified in the dia- linear to maintain sufficient selectivity. The linearity can gram of Fig.1 will now be discussed, beginning with the be described in a figure-of-merit: the intermodulation-free choice of the intermediate frequency. The IF filter is an dynamic range (IMFDR). It is determined by the dynamic on-chip SAW filter, either a transversal filter or a resonator range in which the intermodulation cannot be distinguished filter. Because the SAW filter is fabricated in a threelayered from the noise. A bipolar current switching mixer is the best ZnO-SiOz-Si structure, higher order Rayleigh-wave modes circuit with respect to noise and distortion performance [4]. are present. The most dominant (spurious) pass-band in the A balanced version is preferable to compensate for local os- filter is caused by the second Rayleigh-wave mode at approx- cillator feedthrough. Both input and output signals of the imately 1.7 times the fundamental pass-band frequency. The mixer are balanced currents. signals passing through this spurious pass-band may disturb The input amplifier also has to be extremely linear. A the detection of the desired signal. For instance, FM detec- noise floor of 1 pV (B=180 kHz) and a maximum input level tors using phase-lock loops may thus be pulled out of lock. of 100 mV for linear operation of the input amplifier are de- A low-pass filter between the IF SAW filter and the FM de- sirable. Furthermore, this amplifier has to terminate the LC tector will attenuate these undesired signals. However, such filter at the input to obtain a signal transfer which is suffi- a filter cannot be made in a form compatible with the wish ciently independent of the impedance of the whip antenna, to fully integrate the receiver. The signal frequencies arriv- which can vary, dependant on the position of the antenna ing at the antenna, from which the disturbing signals may relative to the ground plane of the receiver. The output sig- originate, can be placed far away from the FM broadcast nal of the amplifier must be a current for driving the bipolar range by choosing the proper intermediate frequency. Thus switching mixer. Amplifiers employing two feedback loops maximum attenuation by the input filter is achieved. Choos- are to be preferred for a low-noise, linear input impedance ing these spurious frequencies higher than the FM broad- for the termination of the filter and driving the bipolar mixer cast band leads to intermediate frequencies above 500 MHz, with a current [14]. which puts impractical demands on the electronic circuitry Amplifier 2 in Fig.1. has a balanced current as input sig- that is realized in a 3 GHz process. However, the spurious nal and must deliver a balanced voltage for the SAW filter. frequencies can be set lower than the FM broadcast band by As this amplifier has to handle the total upconverted FM choosing an intermediate frequency of about 160 MHz. broadcast range this amplifier must also be extremely lin- The local oscillator frequency may be chosen lower or higher ear. In section 6 a balanced transimpedance amplifier will than the intermediate frequency of 160 MHz. A LO fre- be presented to fulfil these demands. quency higher than the intermediate frequency gives much As an IF filter either a SAW transversal filter or SAW fewer spurious responses in the receiver [4]. The local os- resonator filter may be applied. A properly designed SAW cillator should then be tunable from 247 MHz to 268 MHz. transversal filter shows little variation in group delay and, Oscillators using only small capacitances, like regenerative therefore, does not distort the audio signal. Resonator filters types, are to be preferred.