A HIGH GAIN OPERATING REGIME FOR THE FREQUENCY TO VOLTAGE CONVERTER *

Transcription

1 APPLIED PHYSICS ELECTRONICS A HIGH GAIN OPERATING REGIME FOR THE FREQUENCY TO VOLTAGE CONVERTER * O.S. STOICAN INFLPR Bucharest, ostoican@k.ro Received December 21, 2004 A high gain operating regime for a frequency to voltage converter is analyzed. Such kind of regime is useful to detect very lo variations of a signal frequency. Another possible application is as a marker generator for the seep lo frequency oscillators. The converter gain is theoretically calculated and compared ith experimental results. Key ords: frequency to voltage converter, frequency comparator, frequency demodulation. In order to observe certain physical effects it is necessary to demodulate a frequency-modulated signal (e.g. NQR effect [1], RFSE effect [2], various biophysical applications [3], etc). In this purpose, there are many possible methods. Due to its simplicity, the frequency to voltage converter is one of the most idespread. Frequency to voltage converter provides to its output a voltage U 0 proportional to the input signal frequency ν. Although frequency to voltage converters are not as accurate as the frequency counter circuits, they remain useful in many applications. This is the case here it is required to record only the variations of a signal frequency but it is not necessary to kno very precisely the signal frequency. In this paper, e propose a ne operating regime for frequency to voltage converter used as a frequency comparator. Such a regime can be useful in order to detect very lo variations of a signal frequency. Generally, a frequency to voltage converter must accomplish to main requirements: Output voltage versus input signal frequency to be linear Converter gain K= U 0 / ν to be as high as possible. * Paper presented at the 5 th International Balkan Workshop on Applied Physics, 5 7 July 2004, Constanţa, Romania. Rom. Journ. Phys., Vol. 51, Nos. 1 2, P , Bucharest, 2006

2 58 O.S. Stoican 2 If the goal of the experiment is to detect lo variations of the frequency then only the second requirement is important. The block diagram of the frequency to voltage converter discussed here is shon in the Fig. 1. Fig. 1 Block diagram of a frequency to voltage converter based on a monolithic monostable multivibrator. The monostable is triggered by positive edge of the input pulse. The external resistor R1 and capacitor C1 settle the length t of the output pulses. The frequency to voltage converter consists of a monostable circuit folloed by a lo-pass filter. The monostable circuit provides to the output one pulse of fixed length t for each pulse applied to the input. The function of a monostable circuit is accomplished by the dedicated integrated circuits (called monolithic monostable multivibrators) made by various manufacturers. Usually, an external resistor R1 and capacitor C1 settle the length t of the output pulse (timing components). Monostable circuit is triggered either by the positive or negative edge of the input pulse and may operate either in retriggerable or non-retriggerable mode. The choice of the operating mode, the sign of the trigger pulse edge and necessary values for external components R 1 and C 1 must be performed accordingly to the respectively integrated circuit datasheet. In the simplest case, lo pass filter consists of a single RC cell. The signal to be frequency demodulated is applied to the input of the monostable circuit hile the output voltage U 0 is collected to the output of the lo pass filter. In order to suppress the output voltage ripple RC>>t. If RC >> t then the voltage U 0 is average value of the pulsed voltage occurring to the monostable circuit output.

3 3 Frequency to voltage converter 59 Further ill be discussed the case of a frequency to voltage converter based on a monostable circuit triggered by the positive edge of the input pulse and operating in the non-retriggerrable mode. Results do not change if the monostable circuit is triggered by the negative edge of the input pulse. As a function of the input signal period T to the pulse length t ratio, to cases can be distinguished. 1) Case T> t t U E Et T 0 = = ν (1) here ν and E are the input signal frequency and amplitude, respectively. E is approximately equal to the supply voltage of the monostable integrated circuit. The frequency-voltage characteristic is linear, output voltage varies in the range 0 < U 0 < E corresponding to a frequency variation in the range 0<ν<ν max = 1/t and the converter gain has a maximum value: U 0 K = = Et = ν For the most applications such kind of the frequency to voltage converter is employed in the frequency range 0<ν<ν max = 1/t. 2) Case T< t Once triggered the output state of a non-retriggerable monostable circuit are independent of further transitions on the input and depends only of the timing components R1 and C1. Timing chart of a non-retriggerable monostable circuit if T< t, is shon in Fig. 2. The monostable circuit input is disabled during the time period t referenced from the corresponding transition of the input trigger pulse. K max (2)

4 60 O.S. Stoican 4 Fig. 2 Monostable timing chart for 1/t <ν<2/t. Upper trace-waveform of the input signal. Bottom trace- Waveform of the output signal. Only edges x marked trigger output pulses. In fact, monostable is triggered by a 2T period signal. Thus, if 1/t <ν<2/t then the monostable circuit is triggered by an input signal having frequency equal to ν/2 (only the positive edges of the input pulses pointed ith x trigger output pulses). The output voltage U 0 ill be equal to: t Et ν U 0 = E = (3) 2T 2 varying in the range E/2<U 0 <E for 1/t < ν< 2/t. The converter gain ill be equal to K max /2. Similar considerations developed in the general case n/t < ν < (n+1)/t, here n= 0, 1..., demonstrate that the monostable circuit is triggered by an input signal having a frequency equal to ν /(n+1). In the general case, the output voltage ill be given by the relation: U 0 t = E ( n + 1) T Etν = ( n + 1) (4) ne varying in the range <U0 <E for n/t < ν < (n+1)/t. n +1 The converter gain is given by relation: K Et K max = = (5) n + 1 n + 1

5 5 Frequency to voltage converter 61 The frequency-voltage characteristic based on above relations is shon in Fig. 3. If ν>ν max =1/t then frequency-voltage characteristic remains linear but the converter gain decreases. Thus, it is not useful to use such kind of the converter for ν>ν max =1/t. Fig. 3 The complete theoretical characteristic output voltage-input frequency of a frequency to voltage converter. It is interesting to analyze the response of the circuit hen the input signal frequency ν is placed in the close proximity of n/t (n=1, 2...). When the input signal frequency passes through n/t value then an output voltage step equal to E/(n+1) occurs. Thus, the converter gain becomes very high. Theoretically, K(ν n/t ). In this ay, the frequency to voltage converter discussed in the paper can be used as frequency comparator detecting the very lo variations of the input signal frequency. Another possible application is as a marker generator for the seep lo frequency oscillators (Fig. 4). In that case markers are spaced ν=1/t apart. Actually, the converter gain for ν n/t is finite. In order to estimate the converter gain for ν n/t must kept in account that it is necessary a finite time to change the state of the monostable output. There is a minimum required time t k beteen to consecutive output pulses. As a result, response of the monostable circuit is identically for any input signal having frequency in the range n/t <ν< n/(t +t k ). Thus, the converter gain for ν n/t can be estimated as: U 0 E /( n + 1) K max t = = = + K( ν n / t ) 1 (6) ν n / t n /( t + tk ) n( n + 1) tk here K max =Et. For n=1 the converter gain reach a maximum value. As a rule t /t k >>1 so that K(ν n/t )>>K max. Generally, value of t k depends on the part type used as monostable circuit. For example, in the case of CMOS integrated

6 62 O.S. Stoican 6 circuits, value of t k is of the order of hundreds nanoseconds. If e further consider the orst case, i.e. t k 1µs and choosing t =50 µs, then K(ν 20kHz)/K max 25. In Fig. 5 is shon the frequency-voltage characteristic of a frequency to voltage converter using a type CD4047 CMOS monolithic monostable multivibrator. The pulse length t =100µs and supply voltage E=5V. The input signal frequency sept in the range 10 khz - 45 khz, seep time as equal to 1.8s, and the time constant of the lo pass filter RC 600µs. Results are in good agreement ith the theory. Fig. 4 Marker generator for a seep lo frequency oscillator. Fig.5 Characteristic voltage-frequency of a frequency to voltage converter recorded by a digital oscilloscope. Channel 1 (upper trace) - Output voltage versus input signal frequency. Channel 2 (bottom trace) - A voltage proportional to input signal frequency. Channel 1 and channel 2 voltage sensitivity, 1V/div and 2V/div, respectively. Time base is 200 ms/div. The input signal frequency varies in the range 10 khz - 45 khz, seep time being 1.8s i.e khz/s. Calculated K max =0.5V/kHz.

7 7 Frequency to voltage converter 63 The ork as supported by IFA, contract number CERES 99/2001. REFERENCES 1. E. P. A. Sullivan, Easily constructed pure NQR spectrometer, Rev. Sci. Instrum., Vol. 47, (1976). 2. P. A. Probst, B. Collet, W. M. MacInnes, Marginal oscillator optimized for radiofrequency size effect measurements, Rev. Sci. Instrum., 47, (1976). 3. K. P. Cohen, D. Panescu, J. H. Booske, J. G. Webster, W. J. Tompkins, Design of an inductive plethysmograph for ventilation measurement, Physiol. Meas., 15, (1994).