FM Demodulator Research Articles

A new approach to VHF/FM broadcast receiver using digital signal processing techniques

This paper describes the development of a new VHF/FM broadcast receiver using digital signal processing (DSP) techniques. The developed method gives improved receiver performance, and enables it to be integrated into general digital signal processing. This receiver realizes FM demodulation, a stereo decoder and noise reduction by using DSP. We adopted a quadrature type FM demodulator for its simple structure, and combined it with a newly developed AM (amplitude modulation) cancellation process, to reduce the demodulated signal distortion. We have also developed an extended sampling rate conversion technique to synchronize the sampling rate with the broadcast pilot signal utilizing interpolation-decimation filtering, while the stereo decoder process itself has been based on a synchronous sampling type operation.

Active antenna discriminator for frequency stabilisation

The authors show how an active antenna transmitter module can have its frequency stability improved using FM demodulation techniques. A closed loop feedback strategy is presented, in order to improve the bias pushing characteristic of the active antenna from 41 MHz/V to 5 MHz/V. A linearised feedback model for the system is also presented which predicts the frequency pushing characteristic to be 7 MHz/V.

Improvement of the positioning accuracy of a software-based GPS receiver using a 32-bit embedded microprocessor

This paper describes the improvement of the positioning accuracy of a GPS receiver, using software to apply the GPS to compact, hand-held devices. Differential GPS (DGPS) with a software FM demodulator, and a modified Kalman filter is proposed and applied to a GPS receiver. The positioning accuracy is, 35 m with a standalone GPS, 9 m with DGPS and 15 m with a Kalman filter. With both methods, the positioning accuracy is drastically improved to 2 m.

Improved instantaneous frequency estimation using an adaptive short-time Fourier transform

Instantaneous frequency estimation (IFE) arises in a variety of important applications, including FM demodulation. We present here a new time-frequency representation (TFR)-based approach to IFE based on an adaptive short-time Fourier transform (ASTFT). This TFR leads naturally to a type of short-term ML estimator of the IF. To further improve the performance, we apply a multistate hidden Markov model (HMM)-based post-estimation tracker. The end result is up to a 16-dB reduction in the threshold SNR over the frequency discriminator (FD) and an 8-dB improvement over the phase-locked loop (PLL) for a Rayleigh fading channel.

Time-varying coherent AM–FM demodulation and denoising of acoustic signals

Noise removal via linear time-invariant (LTI) filtering is most effective when the signal and noise spectra have minimal overlap in frequency. In particular, it can be difficult to extract, via LTI filtering, broadband signals from broadband noise, because often their spectra overlap. However, many broadband signals are locally narrow band (e.g., AM–FM signals with large FM and moderate to small AM), and this characteristic can be exploited to improve noise suppression for such signals. We present a method for extracting locally narrow band signals from broadband noise, based on an AM–FM decomposition of the signal and time-varying filtering. The center frequency and passband of a linear time-varying filter are determined from estimates of the instantaneous frequency and instantaneous bandwidth of the signal. Results on both synthetic signals and recorded whale sounds in ambient noise demonstrate a significant improvement in SNR compared to LTI-based filtering. [Supported by ONR Grant N00014-98-1-0680.]

FM demodulation in the presence of multiplicative and additive noise

A theory of noise in ideal frequency demodulators is developed for the case when the input carrier is a Gaussian process rather than having a constant amplitude. Exact expressions for the demodulator output are obtained which include the effects of additive Gaussian noise in the intermediate frequency stage. Numerical results are evaluated for the output spectrum and noise power following baseband filtering. A simple approximation is developed which gives good agreement with numerical results for the noise power.

A discrete-time method of demodulating large deviation FM signals

A method of enhancing the performance of a numerical FM discriminator in specific receive signal conditions, using frequency feedback techniques, is developed and evaluated. The receive signal conditions of interest are: (1) frequency deviations that are large relative to the bandwidth of the message signal; (2) a low-uniform (complex) sample rate commensurate with the bandwidth; and (3) noise strengths that yield values of input carrier-to-noise ratio of the order of 10 dB or more. Through computer simulation, observed phase cycle-slip counts and phase noise variance estimation are used to compare the enhanced demodulator to the performance of a numerical FM discriminator baseline. Based upon the theoretical analyses and simulation results, the presented method of numerical FM demodulation is shown to be a viable technique for enhancing the performance of numerical FM discriminators under the given modulation conditions.

Innovative demodulation method for SSB technique

A single sideband (SSB) demodulation method is described, in which frequency mixing is not required. The proposed method is based on the phase shift method, but no local oscillator is employed. The IQ demodulator is replaced with an envelope detector and an FM demodulator. It can replace the synchronous demodulation method, which requires an expensive phase-locked loop for carrier recovery. Not only is less interference encountered in the proposed circuit, but the production costs of the SSB and independent sideband receiver are low. Furthermore, the Doppler effect is not a serious problem since the carrier transmitted by the transmitter is used for demodulation.

FM demodulation of a speech signal using an extended Kalman filter with a third-order message model

The authors propose a solution to the problem of demodulation of a frequency modulated speech signal using an extended Kalman filter. They discuss the justification for, and generation of, the third-order model and compare the results to those obtained through using a simple single pole message model quoted extensively in the literature.

PLL FM demodulator performance under Gaussian modulation

A widespread test of the phase-locked loop (PLL) FM demodulator under Gaussian modulation is simulated using the Monte Carlo method. Unified noise-free and noise performance analyses of the PLL FM demodulator are presented. Substantial reduction of the modulation limit by the input bandpass filter is reported in the region of the input bandwidth of practical interest. Bessel bandpass filters of order greater than two are shown to compare favorably with Butterworth filters in front of the PLL FM demodulator relative to the intermodulation (IM) distortion. A lower bound on the loop noise bandwidth is found by minimizing the output click rate for given IM distortion specifications. FM threshold of 4 and 7 dB for the root-mean-square (RMS) frequency deviation-to-message bandwidth ratio 0.1 and 1.0, respectively, is reported on the worst-case IM distortion of 45 dB.

Analog direct-modulation behavior of semiconductor laser transmitters using optical FM demodulation

In this paper, we report a theoretical investigation of the analog modulation performance of a semiconductor laser transmitter which employs the direct optical FM demodulation. This analysis is based on the rate equations in which Langevin noise functions are included. The optical FM response has firstly been derived using a chirp-power relation deduced from the rate equations. The chirp-to-power ratio (CPR) has secondly been computed and illustrated showing the effect of the novel damping parameter phenomenologically introduced in the rate equation for the optical phase. The curves show that the conventional CPR can be notably reduced using FM demodulation. Beside this, the comparison of the FM responses with prior studies show that our model may be more appropriate to describe optical FM behavior of many types of single-mode lasers. The relative-output noise (RON) at the transmitter has also been determined and subsequently used to calculate the carrier-to-noise ratio (CNR) available. It is shown that the transmitter inherent noise would significantly degrade as a result of the conversion of the optical frequency noise into intensity noise. Consequently, intrinsic noise is expected practically to contribute the most to CNR at high powers. This feature can be more easily appreciated through curves illustrating CNR and is in contrast to previous studies in which the receiver shot noise was assumed to be the main noise term.

Stochastic fm models and non-linear time series analysis

An important model in communications is the stochastic FM signal st = A cos , where the message process {m t} is a stochastic process. In this paper, we investigate the linear models and limit distributions of FM signals. Firstly, we show that this non-linear model in the frequency domain can be converted to an ARMA (2, q + 1) model in the time domain when {mt } is a Gaussian MA (q) sequence. The spectral density of {St } can then be solved easily for MA message processes. Also, an error bound is given for an ARMA approximation for more general message processes. Secondly, we show that {St } is asymptotically strictly stationary if {m t } is a Markov chain satisfying a certain condition on its transition kernel. Also, we find the limit distribution of st for some message processes {mt }. These results show that a joint method of probability theory, linear and non-linear time series analysis can yield fruitful results. They also have significance for FM modulation and demodulation in communications.

Stochastic fm models and non-linear time series analysis

An important model in communications is the stochastic FM signal st = A cos , where the message process {mt} is a stochastic process. In this paper, we investigate the linear models and limit distributions of FM signals. Firstly, we show that this non-linear model in the frequency domain can be converted to an ARMA (2, q + 1) model in the time domain when {mt} is a Gaussian MA (q) sequence. The spectral density of {St} can then be solved easily for MA message processes. Also, an error bound is given for an ARMA approximation for more general message processes. Secondly, we show that {St} is asymptotically strictly stationary if {mt} is a Markov chain satisfying a certain condition on its transition kernel. Also, we find the limit distribution of st for some message processes {mt}. These results show that a joint method of probability theory, linear and non-linear time series analysis can yield fruitful results. They also have significance for FM modulation and demodulation in communications.

Performance analysis of a COFDM/FM in-band digital audio broadcasting system

In this paper, we present a performance analysis of a coded orthogonal frequency division multiplexing DAB system that is power multiplexed over a commercial FM station. We model the mobile channel with a time-frequency scattering function. Results show that the performance of the FM demodulator is crucial, since the level of the residual FM signal has a strong influence on the BER performance of the COFDM system. As with other mobile communication systems, the use of interleaving greatly enhances the performances.

Low-drift digital SWFM demodulator

A new pulse-counting squarewave FM demodulator structure is described that exhibits significantly lower drift, permitting the design of simpler wideband analogue SWFM modems for fibre data links with improved performance.

A linear, low cost and versatile FM demodulator

The experimental behaviour of a low cost and versatile FM demodulator is described, designed for field applications where high linearity of the device is required in order to obtain a very low harmonic distortion of the signal to be measured. The circuit solutions which have been taken in order to reduce noise effects and to improve the dynamic range of the device are also discussed with reference to a vibration measuring system. A linearity uncertainty of less than can be attained with reference to a frequency decade with input frequency to the demodulator in the range 50 kHz to 25 MHz.

Ultrasonic Transducer Array for Structural Intensity Measurements

Measurement of structural intensity, which is a vector quantity defined as an energy flow of vibration per unit area and is calculated from vibration velocity distribution, is a key technique for radiation noise reduction and vibration control. An ultrasonic Doppler vibrometer array is proposed for structural intensity measurement. A useful large array probe can be constructed. Since the ultrasonic system is much cheaper than an optical one, and has no loading effect on specimens during measurement, in constant with conventional methods that use an accelerometer and a strain gauge, a system for 1-dimensional measurements is made which is composed of ultrasonic transducers and an FM demodulator. The validity of the proposed system is demonstrated experimentally by the 1-dimensional structural intensity measurement of a beam.

Comments on "A one volt four-quadrant analog current mode multiplier cell" [with reply

For original paper see C. Chan, H. Ling and O. Choy, ibid., vol.30, no.9, pp.1018-19 (1995). The goal of the original paper was to develop a novel FM demodulator based on a Gilbert multiplier that can operate with a single 1.2 V battery. In this Comment the author suggests that the original paper contained some errors and that the cell developed was not at all novel and is actually very well known. A reply by the original authors is included.

DSP-based digital FM demodulation for GMSK signals

Demodulation of Gaussian Minimum Shift Keying (GMSK) using a limiter-discriminator is a low complexity alternative to coherent demodulation. This so-called digital FM demodulation is followed by clock recovery, sampling, and thresholding. Conventionally, clock recovery is done in hardware, and matched filtering is usually not possible when the Gaussian pulse is wider than a bit duration. We propose a clock recovery technique based on discrete-time processing of the demodulated baseband signal. This technique couples very nicely with a new maximum likelihood sequence estimator for the data that uses a whitening filter followed by a Viterbi decoder. The entire detection algorithm can be implemented in an efficient manner on a Digital Signal Processor (DSP). Computer simulation results are presented to show that the new algorithm performs better than the conventional slicer by as much as 5.5 dB.

Power spectral density of phase noise for afrequency modulated signal plus additive whiteGaussian noise after sampling and filtering

In an FM(PM)-transmission system the demodulator transforms the complex baseband noise into amplitude and phase noise components. Without modulation, and at moderate to high carrier-to-noise ratio (CNR), the shape of the phase noise power spectral density (PSD) is modelled by the input noise PSD. The authors derive an approximation for modulated signals, which determines the PSD of phase noise near the threshold of FM demodulators.