A c

~~~~~~~~~~ Already ~ : n n w n VVI denaoilula!~r cfrcitits are l i n ? ~ ! ~ ~ ? in t'x ?x!-g.aw of sonit' 3 ~ ~ ~ a t e ~ d ai awbacks. Inherent nonlinearity md response to undesired amplit lde variations are example of these limitations. Also, realization of such circuits 011 a single chip is impossible because they need to employ transformers, inductors and/or large capacitors- A novel FM detector circuit, which is entire11 integrated using MOSFEIT tsch"ogy, on a single chip is developed in this papeq. It is precise, linsar Q V ~ I - a vi7itle Erequency range (> 200 H-lz), reliable and providing stable charackristics. It is also simple io design, easy to use and inexpensive to realize. hforzover, it is compatible with the recent scaling $ow11 trends.

1. XNT'RODUCTIOK :

The detection andl demodulation of frequfncy modulated signals needs ai discriminator which i s chara,:::rizcd by good linearity (better thm 3%) over a u.Xe ~ a ~ l d n i d l h (\;l?ider than 2 ~ 4 ) ,'&?I ;; s tm& also be sfable (6YS : Y ~ ~ S T tiian - ~ # ~ ~ ~ ~ jrisc'ns tive to ampiitudt. i ariatiois. 3lroiem er it must be leak I , e. offv:i. drift z 4 I ' I C ~ I V fisc. t'aiiety of circuits iiave been \fidei) in i f s t . Hov,c\er t h ~ i i p h y s ~ ~ i l cnnilmctiisns add limita~ions IO their performance and operation -41. I h c w e d it?

t i L Iisfolmers and ao cmploy inductors aid large capacitors a e cxmip le~ of t Irmitatims. characteristic spreading. presence of offset, drift and noise are also senous drav backs.

Although these techniques employ automatic gain control, they stili buffer from :some amount of instability due to temperature, aging and de~ice spreading. Narro7~7 dynamic region of operation, nonliiiearity and sensitivity to amplitude variations of the FM signal are also examples of the still existing dra\;l.backs.

Howcver great efforts have been devoted to the de\-elopment and realkariun of a new hlOSFET 1C FM discriminator which is free from the ma-jority of the above mentioned drawbacks and limitalions.As mentiomcd above. Section (2) presents t k construction arid explains rhe mechaiiisni of opcwtion of the proposed disc?-imlnator

NATIONAL RADIO SCIENCE CONFERENCE Feb. 24-26 1998, Melwan, Cairo Egypt.

The theory and modelling of it are given in section (3). Experimental and simulation results are presented in section (4). Conclusion are finally given in section (5).

2. THE MOSFET IC FM DISCRIMINATOR :

This section introduces the construction and explains the mechanism of operation of the new FM Discriminator. It is entirely integrated on a single chip using the standard 2pm MOSFET technology. Supplementary simulation a characterization work is developed to study the compatibility with the recent scaling downtrends (down to 0.2 pm technology).

2.1. CONSTRUCTION :

The construction of the new MOSFET IC FM Discriminator possesses many advantages and manifests excellent performance. It is precise and offers a stable performance since it incorporates a powerful leakage compensation and also a new noise cancellation techniques. It is linear owing to the negative feedback loop used, and the supplementary circuitries used to sense occurrence of nonlinearity and forces scale changement so as to keep its operation within its linear region. It has a wide dynamic range of operation since it employs cascode connected to operational amplifiers with speeding up cross coupling techniques. It is reliable because of its simple construction and the usage of the well known MOSFET technology.

Fig.( I) shows the block diagram of the novel detector chip. It consists of two basic \

loops :

1. The first is the active loop which is sensitive to the FM signal to be demodulated and also to the circuit leakage current and the technology defects.

2. The second one, called the compensation loop which is sensitive to the local oscillator frequency, the circuit leakage current and the technology defects.

The first loop is constructed of :

a) Frequency controlled current source. Its circuit diagram is shown in Fig.(2). It converts the frequency of the input signal into a corresponding current signal. If the frequency is varying with time, the amplitude of this current will also be varying with time.

The operation of this unit is as follows :

When 4 is high, Tz is switched on and TI is turned off. The capacitor Ck will be charged to V,, then acquiring a charge.

When 4 is low, 7'2 turns off and TI switches on. Then the charge QI will be modified (el+ @) so that Ck acquire the new potential VR

'H NATIONAL RADIO SCIENCE CONFERENCE Feb. 24-26 , 1998 , H e l ~ a n , Cairo , Egypt.

Q2 c k V R (2)

AQ QI - !2 (3)

This process repeats f times per second. Every time the capacitor Ck transfers a charge:

from the V, to the V, source. This means that a frequency controlled current &flows through tlze TI, T2 MOSFET string, it is given by :

[f=@kfl>-vR)f (4) Equation (4) shows that the sensitivity S of the current to the frequency variation increases as Ck is increased. Additional capacitors 9 C;, and 90 Ck are introduced in parallel with Ck to increase S by 10 and 100 times respectively. Two MOSFET switches SI, S2 are therefore needed.

Operational Amplifier, shown in Fig. (3). The detailed analysis of op. amp. are given elsewhere [5-9]. It is cascade connected and incorporates a dc negative feedback loop to stabilize its operation against temperature excursions and device aging,. It also uses a new technique to compensate for the expected technological error 3 guarantee adaptive cancellation of noise. Beside special cross coupling technique can be used to increase further the amplifier bandwidth (-3GHz). For reasons of stability margins, another feedback loop is employed. A gain o f 400 at 2 ,uv offset and 28 ,m /e drift and 20 pV RhlS at 44dB signal to noise ratio which is sufficient for the present applica-tion.

Stabilized depletion mode MOSFET structure to provide thc feedback resistor for the 0p.Amp. blocks. (see Fig (&I). It provides a resistance RD which i s smoothly constant and independent of any Op.Amp. voltage swing. A great advantage of this structure is the capability of smooth and easy control of the value of RD via varying the MOSFET threshold voltage [lo] and consequently enable to control the FM discximinator sensitivity.

d) Driver which is simply a MQSFE'T push pull stage. It is needed to protect the sysiem against loading and to improve its output impedance.

The second loop is constructed o f :

a) Frequency controlled current source (FCS).

b) Opxational Amplifitx (Op. Amp).

The first and second loops are both monitored by a common sensitivity, linearity and zero adjustment control units as shown in Figs. (I) and (4). 2.2. OPERATION :

NATIONAL RADIO SCIENCE CONFERENCE Feb. 24-26,1998, Helwan Cairo, Egypt.

The operation of the FM discriminator is as follows. The active loop receives the FM signal from a preamplifier whose central frequency is ganged with a local oscillator which feeds the compensation loop with the carrier to be selected. The

active loop gives an output voltage VFM whose amplitude varies proportionally with the frequency of the FM signal, while the compensation loop gives an output voltage vc is constant and proportional to the carrier frequency. Each loop gives in addition parasitic signal voltage V L and accelerate parasitic signal current IL. Both V L and IL are stocastically variable and can not be estimated or compensated by any technique other than that presented in this paper. A comparator amplifier is employed whose output will be parasitic, leakage and noise free and having amplitude VAF which exactly equal the modulating information signal.

3. THEORY AND MODELING :

The FM signal is fed to the active loop. Where it commands the frequency controlled current source FCCS 1 and accelerates a current I1 whose amplitude varies in accordance with the frequency variations of the FM signal. Loop leakage current IL will also be accelerated and should be taken into account, then.

I1 = IC + SI + IL with IC the current accounting for the carrier frequency FC

61 the current accounting for the modulating frequency SF

IL the loop leakage current.

The compensation loop is fed from a local oscillator whose frequency fo is maintained equal to the FM carrier frequency& . It accelerates therefore 12 whose amplitude is constant and corresponds to&. Taking the circuit leakage current IL into consideration.

12 = IC + I L (6) IL in equation ( 5 ) and (6) should be equal because the two loops are identical

and implanted close to each other on the same chip [7, 8, 9, lo]. The circuit is provided with a zero adjustment potential V,, to be used to compensate for any mismatching between the two loops and to maintain 1, the same in both of them.

These currents are reconverted to proportional voltages V F M ~ VL and VC 2 VL as they cross the resistance RD feeding back the output of 0p.Amps GI and G2 to their inputs. These latter voltages are to the comparator amplifier G3 to yield an out put V,, which purely corresponds to the modulating frequency 6F. It is also leakage free. The output audio voltage is given by

N NATIONAL RADIO SCIENCE CQNFERENCE Feb. 241-26 1998 ? Helwan , Cairo, Egypt.

P

> The discriminator sensitivity S is given by :

S VAf/6F = ck A V R D (8) Equation (9) shows that S increases by increasing all or any of all the circuit adjustable parameters Ck, Ax RD. The easiest one is to vary C k .

4. EXPERIMENTAL WORM AND SIMULATION :

Work of this paper is focused on :

1. Computer Aided Acquisition of MOSFET parameters and measurement of the evoluiion of these parameters with device geometry, biasing conditions and technology of fabrication, ASTIC program was used.

2. The experimental results OF this stage are fed to the SPICE simulation program which we have used to characterize the present discriminator design and to evalu,Tte its performance.

3. Necessary modifications which give better performance are introduced into the design. Afterwards the circuit layout is made using GARANIT graphics program according to the modified design.

Test specimens were realized by the French society thomson microelectronics, Grenoblc, France.

Fig (5) shows the experimental VAFGF transfer characteriestics of the presented MOSFET IC discriminator for two different values of RD, with RD being the Op. Amp. feedback resistance. We observe that VAF increases linearly as 6F increases. The rate of increase of V . F is greater the greater is the value of the resistance RD. Some nonlinearity problems are noticed.

a) Devke Characterization at High Frequency

We notice that the sensitivity S begins to increase as the frequency exceeds a certain threshold value. This is related to the frequency controlled current source which delivers greater current to RD. referring to Fig (2), this phenomena may be explained as follows :

,At the end of a certain positive half cycle, the switch TI does not turns off. It instead remains closed till T2 is subjected to a successive positive half cycle. This means that the two switches TI and 72 remain closed during a certain interval of time every cycle. This interval becomes relatively longer at greater frequencies. This erronous behaviour makes the VAF,J~ transfer curve to deviate upwords as shown in Fig. (6) Or, in other words, giving VAF values greater than those which we expect to obtain i%t a certain Frequency. This problem is treated by inserting a high frequency saturation stage which dekects the frequency at which this problem QCCUTS and forces the disc riminator to another but linear VAF/SF transfer characteristic curve by closing S,

Feb. 2424,1998, Helwan , Cairo, Egypt.

for example which varies Ck to 10 ck see Figs (2) 22 (6). This problem is seen to occur at a much lower frequency (200 Kplz) when the compensation loop is not inserted.

bj

At frequencies smaller than I Hz, we observe that VAF does not decrease further as SF decreases. V A ~ : remains constant and independent of 6F. This is referred to residual leakage current due to an uncompensated mismatching. This current is forced by the frequency controlled current source. Fortunately this frequency range is not important for the present application.

5. CONCLUSIONS :

Device characterization at low frequency

The paper presents a new FM discriminator which is entirely integrated on a signal. chip using the standard 2pm MOSFET technology. It is compatible with the recent scaling down trends.

1.

2.

3. Reliable.

It is simple to realize.

Easy to use without precautions.

4. Very sensitive ( IHz)

5.

6. Leakage free (1/1000 compensation)

7. Has a good linearity (better than 2%)

8. Has a wide dynamic (about !06) range.

Although some nonlinearity problems are observed, the presented discriminator functions well and shows a very good agreement between theory and measurements.

6. ACKNOWLEDGMENT :

Stable performance against temperature and aging,

the authors would like to express his deep thankful to Prof. J. Bore1 technical president of the socity thomson microelectronics, Grenoble, France for his help in experimentation and also for the valuable discussions and comments in reviewing this manuscript.

7. REFERENCES :

[ 11 G.KENNEDY :

ELECTRONIC COMMUNICAITION SYSTEMS

McGraw Hill, 3rd Edition, 1985.

TI3 NATIONAL RADIO SCIENCE CONFERENCE Feb. 24-24 1998, Helwan , Cairo, Egypt.

C.M.MILLER

MODERN ELECTRONIC COMMUNICATIONS

Prince Hall Int. 3rd Edition, 1988.

A, .B .CARES ON

COMMUNICATION SYSTEMS

McGraw Mill, 3rd Edition, 1986

YOUNG, PAUL H. J.E.

ELECTR0NIC COMMUNICATION TECHNIQUES.

Prentice Wall, 3rd Edition, 1994

ROGER. CONANT

ENGINEERING CIRCUIT ANA1,YSIS WITH PSPICE AND PROB.

McGraw Ilill, International Editions, 1993

P. ANTOGNETTI AND

[ IO] A. EL-HENNAWY

.DESIGN AND SIMULATION OF A HIGH R E L i m r L r Y NON- 1701.ATILE CMOS EPROM MEMORY CELL COMPATIBLE WITH SCALING- DOWN TRENDS

Int.3. Electronics, 1992, vol. 72. No. I , 73-87.

[I 1) A. EL-HENNAWY AND W. SHAHAB

NEW LEAKAGE COh/lPENSATION TECHNIQUE FOR SUPERIOR CFC (10-15 TO 10-8 A)

W.SHAHAB AND AEL-HENNAWY [ 12 J NEW TECHNIQUE FOR OFFSET COMPENSATION AND NOISE WDUCTION OF MOSFET WLSf OP. M S .

bit. 3. Elec., Vol. 68, Ne. 4, 1990

cLuar l+

w

FIGURE (4) : Circuit Diagram of FM Discriminator