Pulse Width Modulation with Frequency Changing Milan Stork, Petr Weissar, and Kamil Kosturik Department of Applied Electronics and Telecommunications/RICE, Faculty of Electrical Engineering, Umiversity of West Bohemia, Czech Republic [email protected], [email protected], [email protected] Abstract generated using a simple oscillator) and a comparator. For the demodulation of the PWM signal the classical concept employs Pulse width modulation (PWM) is widely used in different a simple low pass filter. Another possibility is described in [3, applications. PWM transform the information in the 4]. There the demodulation is made in two steps. First the PWM amplitude of a bounded input signal into the pulse width signal is transformed into a pulse amplitude signal (PAM). As a output signal, without suffering from quantization noise. result we have an equidistant pulse train where the amplitude of The frequency of the output signal is usually constant. In the pulses contains the information. In the second step the PAM this paper the new PWM system with frequency changing signal is processed with a low pass filter. This reconstructs the (PWMF) is described. In PWMF the pulse width and also original signal waveform. These two step demodulation is frequency is changed, therefore 2 independents information exactly the reverse way to the uniform sampling process. The are simultaneously transmitted, and PWM and frequency main disadvantage is lowpass filter because slow response. The modulation (FM) are simultaneously used. But such system similar (speed limitation) is for FM signal. needs fast demodulator separately for PWM and FM. The In this paper is described new approach for fast circuit for fast demodulation of PWMF signal is also demodulation of PWM signal and also for FM signal. The new described and measuring results are presented. All system therefore allows transmit simultaneously PWM an FM described circuits were constructed and measured. signals as 2 independents information in one PWMF signal and also fast demodulation. The fast demodulator can be also 1. Introduction separately used as PWM demodulator, FM demodulator and demodulator for asynchronous sigma-delta modulator (ASDM) PWM, or pulse-duration modulation (PDM), is a technique [5-12]. used to encode a message into a pulsing signal. It is a type of modulation and therefore this modulation technique can be used 1 to encode information for transmission. Pulse-width modulation T T uses a rectangular pulse wave whose pulse width is modulated 1 2 resulting in the variation of the average value of the waveform. 0.5 w If we consider a pulse waveform f(t), with period T low value w 2 PW MF 0 1 ymin, a high value ymax and a duty cycle D, the average value of the waveform is given by: 0 10 20 30 40 50 60 70 80 90 100 0.8 T 1 0.6 yftdt O () (1) V 1PWM T 0 0.4 PWM V 0.2 2PWM where f(t) is a pulse wave, its value is ymax for 0<t<D.T and ymin for D.T<t<T. The above expression then becomes: 0 0 10 20 30 40 50 60 70 80 90 100 1 CSDT T 4 (2) 0.3 yydtydtDyDyDTOOmax min max(1 ) min T EU0 DT 0.2 f V This latter expression can be fairly simplified in many cases 2f 0.1 where y =0 as 4 and for y =1 as . From V min yDymax max yD 1f this, it is obvious that the average value of the signal y is 0 0 10 20 30 40 50 60 70 80 90 100 directly and linearly dependent on the duty cycle D (or width w), where D+>0, 1N. Different methods for PWM modulation can be classified if Fig. 1. The principle of PWMF modulation and fast we consider the sampling process [1, 2]. The simplest way to demodulation (in every cycle – on rising edge of PWMF). Top - generate a PWM signal is the intersective method, which PWMF modulated signal, middle - PWM signal demodulation, requires only a sawtooth or a triangle waveform (easily bottom - demodulation of frequency modulation. 96 2. Principle of Pulse Width Modulation with 3. Circuit for Fast PWM Demodulation Frequency Changing For fast demodulator of PWM the IC’s LTC2644 was used Signals with pulse-width modulation are very easy to [13 - 15]. The simplified block diagram of LTC2644 is shown in demodulate. In addition to a number of other components, their Fig. 2. The LTC2644 measures the period and pulse width of the spectrum contains also the baseband spectrum of the original PWM input signals and updates the voltage output DACs after modulating signal. A low-pass filter is therefore sufficient to each corresponding PWM input rising edge The input frequency separate the useful signal. This method, however, has a is between 30 Hz and 6.25 kHz (12-bit), 25 kHz (10-bit) or 100 drawback in that the power of the original baseband spectrum in kHz (8-bit). The DAC outputs update and settle to 12-bit the spectrum of the modulated signal is small. The second accuracy within 8μs typically. The most important is that slow drawback is slow low-pass analog filter (which reduces analog filter was eliminated. The LTC2644 has a full-scale bandwidth of transmitted signal) and needs of output buffer output of 2.5V using the 10ppm/°C internal reference. It can amplifier. Another possibility is use a converter which converts operate with an external reference, which sets the full-scale the width-modulated rectangular pulse to an amplitude- output equal to the external reference voltage. Each DAC enters modulated pulse (usually applying the linear charging of the a pin-selectable idle state when the PWM input is held capacitor for the duration of the width-modulated pulse). This unchanged for more than 60ms. The part operates from a single amplitude-modulated signal is then demodulated with the help 2.7V to 5.5V supply and supports PWM input voltages from of a demodulator with pulse-stretcher. This approach can be 1.71V to 5.5V modified by using counters and digital-analog (D/A) converter. This principle is used for demodulation. It is important to note that PWMF signal change booth – pulse width and also 5 1 57 4 frequency. VCC IOVCC PD IDLSEL In this paper the new approach is used for demodulation of 9 IN PWM TO V 2 PWM in every period (every rising edge). The principle of 2 A DAC A OUTA independent information transmissions by means of PWMF and BINARY demodulation is shown in Fig. 1. On the top 2 period of modulated signal are shown, where first period is T1=50 and 8 INB PWM TO VOUTB 3 DAC B width w1=30 and second period is T2=30 and width w2=10. In BINARY the middle, the demodulation of PWM signal is displayed. The 10 new values v1PWM and v2PWM are updated on every rising edge of REF LTC2644 PWMF signal. On the bottom, the demodulation of frequency 0.1F GND GND REFSEL modulated signal is shown. The new values, corresponding 6 12 11 frequency of PWMF signal are also updated on every rising edge of PWMF signal. Fig. 2. The circuit diagram of 2 channel PWM/V converter The value of demodulated PWM signal of period T1 with LTC2644. PWM inputs are 8 and 9, voltage outputs are 2 and 3 width w1 is v1PWM (updated on the end of period T1) The demodulated output voltage VDEM can be calculated by w 30 the following equation 1 (3) v1PWM 0.6 T 50 1 t 1 VVDEM REF [V, s] (7) and for period T2 with width w2 is v2PWM (updated on the end T of period T2) where V is 2.5 V internal reference voltage (or external w 10 REF v 2 0.333 (4) reference voltage) and t1 is width of PWM signal and T is period 2PWM of PWM signal. The measured example of PWM demodulation T2 30 on every rising edge of input signal is shown in Fig. 3. The value of demodulated FM signal of period T1 (for gain coefficient kf=5) v1f is (updated on the end of period T1) 11 (5) vk1 ff 50.1 T1 50 and for T2 is v2f (updated on the end of period T2) 11 vk50.166 (6) 2 ffT 30 2 Fig. 3. Th scope of measured the PWM demodulation in every The circuit and measuring results for conversion of PVM cycle of PWM signal (time evoluation of signals). The PWM signal to voltage and FM signal to voltage in one period signal (top), demodulated voltage output (bottom). The output (updated on every rising edge) is described in next part. of the demodulator is updated every rising edge of input signal. 97 3. Circuit and Measuring Results The block diagram of circuit for PWMF modulation/demodulation (with isolation barrier) of 2 separate In this part the circuits and measuring results are presented. signals is presented in Fig. 5. For FM demodulation, on the first The PWMF modulator based on LTC6992 is displayed in Fig. 4 monostable pulse generator (MPG) is used and output of MPG [16]. VPWM is analog input voltage for PWM modulation and is connected to one of PWM/V demodulator. The MPG converts VFM is analog input voltage for FM modulation. Applying a FM to PWM. For PWM to voltage the LTC2644 (see Fig. 2) is voltage between 0V and 1V on the MOD pin sets the duty cycle. used. Results are shown in next figures. The frequency range is from 3.81 Hz to 1 MHz.