A frequency tuning circuit is introduced for VCOs (voltage-controlled oscillators) so that the final relationship between oscillating frequency and input control voltage is fixed and independent of nonidealities. This tuning loop is applied to an OTA-C sinusoidal VCO. Such an oscillator has an output frequency-input voltage relationship that depends on temperature, process parameters, and even amplitude of the oscillations. It is shown that, by adding the tuning loop, nonideal dependences will be minimized

Huertas Díaz, José Luis

Linares Barranco, Bernabé

Rodríguez Vázquez, Ángel Benito

Sánchez Sinencio, Edgar

English

idUS. Depósito de Investigación Universidad de Sevilla

FREQUENCY TUNING LOOP FOR VCOS B. Linares-Barranco’, A. Rodriguez-Vbquez2, E. Shchez-Sinencio’ and J. L. Huertas2 ’ Department of Electrical Engineering Texas A&M University, College Station, TX 77843-USA Dpto. de Diseiio Analbgico, Centro Nacional de Microelectrbnica, 41012-Sevilla, Spain Abstract-For a Voltage Controlled Oscillator (VCO) the oscillating frequency is controlled by an external input. The relationship between the oscillating frequency and the input control volt- age is, in most cases, expected to be linear and independent of parasitics, process parameters, temperature, and other non-idealitites. In this paper we introduce a frequency tuning circuit for VCOs so t4at the Anal relationship between oscillating frequency and input control voltage is Axed and independent of the previous mentioned non-idealities. This tuning loop is applied to an OTA-C sinusoidal VCO. Such an oscillator has an output frequency-input voltage relationship that depends on temperature, process parame- ters, and even amplitude of the oscillations. It is shown that by adding the tuning loop all these non-ideal dependences will be minimized. I. INTRODUCTION Voltage Controlled Oscillators are key components in many signal processing and communication systems [l-31, and are also used for the tuning of monolithic filters [4-91. They alsomay be used for instrumentation applications [2]. In many cases it is necessary that the relationship between the actual oscillating frequency of the VCO and its input control voltage is very precise and does not change with temperature, process parameters or parasitics. Consider a certain VCO (sinusoidal, triangular, or square wave,) that has an “uncontrolled” input voltage- output frequency characteristics. We will show that by adding the frequency tuning loop described in this pa- per the input voltageoutput frequency relationship can be made very linear and completely independent of sec- ondary parameters (temperature, amplitude, parasitics, and process parameters) within the tuning range of the original VCO. The tuning loop proposed in this paper is valid for any type of VCO. We are going to verify its operation by applying it to an OTA-C sinusoidal VCO [lo]. Sinusoidal OTA-C VCOs have a nonlinear input voltageoutput frequency relationship that depends on process parameters, temperature, parasitic components and even amplitude of the oscillations. By adding the proposed tuning loop we will show that this relationship will become linear and independent of the previously mentioned parameters. 11. THE FREQUENCY TUNING LOOP Figure 1 shows the basic structure of the frequency tuning loop [11,12]. vco “OU? I Square- wave Converter Frequency Divider by N (Optional) Fig. 1. Block Diagram of Frequency Control Loop. The output of the oscillator (if not square) is con- verted to a square wave, divided by N (optional) and applied to a Frequency-twVoltage (FTV) converter. Usually the maximum operating frequency of an oscil- lator is higher than the maximum possible frequency in a Frequency-to-Voltage Convertor. For this reason the inclusion of the frequency divider in Fig. 1 is many times mandatory. As we will see in the next section, the FTV con- verter is a device such that if fin is the frequency of its input signal, its output voltage will be given by f i n  f R E F  Vz = n VBIAS- Where n is a positive integer, VBlAS is a reference volt- age and f R E F  the frequency of a reference square wave CH 30064/91/0000 - 2617 $1.00 C IEEE signal (like a crystal oscillator). If f is the output fre- quency of the VCO and a frequency divider is used with a rate N ,  then fin = f / N .  If p-' is the time constant of the integrator, its output voltage will be given by: V r  = ~ ( v c o  -vz) ( 2 )  On the other hand, for a CMOS OTA-C sinusoidal os- cillator we have 1131, (3) K f = r e  = - ( V  - v  ih) 2* c 2TC where K and Vth are given by the topology and tran- sistor geometries of the OTAs used as well as by the process parameters. Solving equations (l), ( 2 )  and ( 3 )  yields, Vco - V B I A S -  N f R E F  which has a stable stationary point (!$ < 0) charac- terized by N Vco f = f R E F -  ~ V B I A S  ( 5 )  The relation between f (output frequency) and V,, (in- put voltage) is shown in Fig. 2. Note that it is linear and it depends only on f R E F ,  V B I A S ,  n and N .  In the next Section we will give the implementation details of the FTV converter, which is the key compo- nent of the frequency tuning loop. , f  Fig. 2. Fkequency vs control voltage relationship for V,, with frequency tuning loop. 111. The FTV Converter Consider the circuiteof Fig. 3. It contains a monos- table that produces a pulse of duration to = 1During this pulse, capacitor C is charged by a current go ( ~ ~ V B I A S  - vz). The rest of the time it is discharged by goV,. The waveform that results at the capacitor u(t)  is shown in Fig. 4. For each cycle of u( t ) ,  Vz(t)  is the peak value of u(t) of the previous cycle. Two consecutive peak values are related by 2 f R E F  ' V,(t+nT) = V,(t)-g"Vz(NT-to)+g"(2nv~~~s-vz)t, where T = 1/ f .  If f is constant and the FTV converter C C (6) reaches its steady state, then Vz( t  + NT) = V z ( t ) ,  i.e., A stability condition for the steady state can be ob- tained by writing (6) in the Z-domain and making the poles to be inside the unity circle. The resulting condi- tion is [E], (8) Ngo f'2C A convenient IC implementation for this FTV converter is shown in Fig. 5. To improve the time response, the switch of Fig. 3 has been replaced by a differential pair and current mirrors. The current sources are imple- mented with linearized OTAs. t o  = tr, - - I +  MONOSTABLE N I 1 4 Detector Ujt, Peak &- - - Fig. 3. Fkequency-to-Voltage Converter t -  u(t) Fig. 4. Time waveforms of U( f )  inside the FTV Converter, and of V x ( t ) ,  its output. tsv 1. --cHc Ju-l- V, ItJ Fig. 5. CMOS Circuit Implementation of FTV Converter 2618 The monostable has to provide a pulse of duration to  = 1/2fREF. such a circuit is shown in Fig. 6. The duration of the output pulse to is given by the time it takes to charge capacitor CI from OV to VREF by the current IREF. VREF is given by the voltage to which Cz is charged, starting at OV by a current IREF during a time to = 1/2fREF. Therefore, if the two circuits enclosed by broken lines in Fig. 6 are perfectly matched, the duration of the monostable output pulse will be to  = 1/2fREF, independent of IREF and cl = cz = c. If there is a mismatch between the two circuits the width of the monostable output pulse will not be equal to half the period of reference signal  REF. However, besides this, the operation and characteristics of the tuning loop still remain basically invariant. r - - - - - - - - - - - - - - -  _L I C I l  I 1  I -  - - a ____-- M.tChcd C h U i t .  -rc--c I. Fig. 6. Block Diagram of Monostable with Output Pulse Width Controlled by an External Square Wave Signal of Precise Frequency. IV. EXPERIMENTAL RESULTS In order to test theoproposed frequency tuning loop strategy we used a sinusoidal OTA-C oscillator [10,12]. Its frequency dependence is shown in Fig. 7. Note that the relationship between output frequency and input voltage (Bias Voltage) is nonlinear and changes with the amplitude of the oscillations. A discrete frequency tuning loop was implemented around the previous CMOS OTA-C oscillator. Since the output frequency gdes over 13MHz, a fast compara- tor (CM710C) was used to convert the sinusoid to a square wave signal. For the monostable a conventional 555 timer was used. In this case, the width of its output pulse is not controlled by an external frequency refer- ence, but by a precise RC constant. Also, since this circuit has very bad time response (propagation times are around lOOns the output of the oscillator had to be divided by 32. The resulting output frequency-input voltage relationship of the VCO with frequency tuning loop is shown in Fig. 8. This characteristic is the same for different values of the oscillating amplitudes, for dif- .O Bias Vdtog. (volts) Fig. 7. Frequency vs Bias Voltage for sinusoidal OTA-C Oscillator without frequency tuning loop. ferent temperatures and for different IC chips. At this time we do not have available a complete IC prototype to quantify the degree of linearity, which is highly de- pendent on the implementation. 14.0' 13.0. 12.0, 11.0 CI Y E" '"5 8. f ;:5 1 6.0 5.0- 4.0. 3.0. 2.0 + 1:O 1:1 1:2 1.3 114 115 1.6 1.7 Froquoncy Control V & a p  (Vdb) Fig. 8. Measured Oscillation Frequency vs Frequency Control Voltage of VCO with Frequency Tuning Loop. V. CONCLUSIONS A frequency tuning loop for VCOs is presented that compensates for temperature, process parameters, am- plitude, and parasitics influence in the original output frequency-input volt age relationship. This tuning loop can be applied to any type of VCO. Its viability has been demonstrated, by applying it to a sinusoidal OTA-C VCO. The tuning loop was constructed using a bread- board prototype, but it can be integrated together with the oscillator in the same IC die [14]. 2619 VI. REFERENCES [ 11 K.K. Clarke and D.J. Hess, “Communication Cir- cuits: Analysis and Design”, Addison Wesley 1978. [ 21 Analog Devices, “Nonlinear Circuits Handbook”, Ed. D. E. Sheingold, Analog Devices Inc. 1976. [ 31 B.P. Lathi, “Modern Digital and Analog Commu- nication Systems”, Holt Rinehart, and Winston, 1983. [ 41 T.L. Brooks and P.M. VanPeteghem, “Simultane- ous ‘hning and Signal Processing in Integrated Continuous-Time Filter: The Correlated Tuning Loop”, Proc. of the 1989 IEEE Int. Symp. Circ. Syst., pp. 651454, 1989. [ 51 F. Krummenacher and N. Joehl, “A 4-MHz CMOS Continuous-Time Filter with On-Chip Automatic Tuning”, IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988. [ 61 C.S. Park and R. Schaumann, “Design of a 4-MHz Analog Integrated CMOS Transconductance-C Bandpass Filter”, IEEE J.  Solid-State Circuits, Vol. 23, NO. 4, pp. 987-996, August 1988. [ 71 W.J.A. Hey, K. Hoen and E. Seevinck, “Accurate Automatic Tuning Circuit for Bipolar Integrated Filters”, Proc. of the 1990 IEEE Int. Symp. Circ. Syst., pp. 1159-1162, 1990. [ 81 Y. Tsividis, M. Banu, and J. Khoury, “Continuous- Time MOSFET-C Filters in VLSI,” IEEE J. Solid- State Circuits, Vol. SC-21, p i .  15-30, February 1986. [ 91 R. Schaumann, M. S. Ghausi, K. R. Laker, “Design of Analog Filters, Passive, Active RC and Switched Capacitor, Prentice Hall, Englewood Cliffs, 1990. [lo] B. Linares-Barranco, A. Rodriguez-VQzquez, E. Sbchez-Sinencio and J.L. Huertas, “CMOS OTA- C High-Fkequency Sinusoidal Oscillators”, IEEE J.Solid-State Circyiits, Vol. 26, No. 2, pp. 160- 165, February 1991. [ll] Date1 System Incorporation, “Designer’s Guide to V/F Converters”, 1976. [12] B. Linares-Barranco, “Design of High Fkequency Transconductance Mode CMOS Voltage Controlled Oscillators”, Ph.D. Dissertation, University of Seville, May 1990 (available in English upon re- quest). [13] A. Rodriguez-Vkquez, B. Linares-Barranco, J.L. Huertas and E. Shchez-Sinencio, “On the Design of Voltage Controlled Sinusoidal Oscillators Using OTAs”, IEEE lhnsactions on Circuits and Sys- t e m ,  Vol. 37, No. 2, pp. 198-211, February 1990. 141 T. P. Lin, and R. G .  Meyer, “A 250-MHz Mono- lithic Voltage-Controlled Oscillator with Low Tem- perature Coeff;cient,” IEEE J. of Solid-State Cir- cuits, Vol. 25, NO. 2, pp. 555-561, April 1990. 2620 

Frequency tuning loop for VCOs

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Frequency tuning loop for VCOs

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