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Acousto-optics from A.A Sa Tel 33 (0)1 30 52 87 17 Fax 33 (0)1 30 52 78 03 www.a-a.fr

APPLICATION NOTES RF DRIVERS

To meet the needs of its acousto-optic componentsover the last 20 years, A.A has developed acomprehensive range of fixed and variablefrequency sources with associated RF drivers,operating from 1kHz to 3GHz. A number of techniques are used.

Fixed frequency sources:QuartzFixed PLL

Variable frequency sources:VCO (Voltage Control Oscillators)PLL (Phase Locked Loop)DDS (Direct Digital Synthesizer)

The choice of a driver will be given by the type of AOdevice and will depend on the application purposes.

Quartz drivers

These drivers have been especially designed toproduce RF power levels and frequencies compatiblewith A.As modulators. Drivers can be provided at anyfrequency from 10 to 300 MHz. All models use highstability ( 10000. Frequency control isgenerally digital, controlled via a parallel or seriesinput.

VCO drivers (Voltage controlled oscillators)

Deflectors or variable frequency shifters require avariable frequency source covering a suitablefrequency which might lie anywhere between 10MHzand 2GHz. VCOs are ideal for raster scan or randomaccess. Their stability and linearity will be a limitationfor some applications.

For general purpose applications, three types of VCOdrivers are available, differing only in their sweep time( Fmin to Fmax.) which can be 1 s, 10 s or 100 s.The VCOs can be modulated (amplitude) from anexternal signal. An external medium power amplifier will be required to generate the RF power levelsrequired by the AO device.

DDS drivers (Direct Digital Synthesizers)

To get a high resolution driver with fast switching time,A.A has designed direct digital synthesizers based on

monolithic IC circuits. 2 models have already beenreleased, and different units can be designed tospecific requirements.Both models offer high frequency accuracy andstability and extremely fast switching times, generallyof a few tens of nanoseconds.The DAC circuits have been designed with utmostcare to obtain clean RF signals, with minimumspurious noise.

Power amplifiers

A.As acousto-optic amplifiers are linear with large

bandwidth and medium or high power.They are specially designed for AO devices andadjusted at factory to fit the AO frequency range,impedance and necessary RF power..Each amplifier is supplied with its heat sink and all arestable and reliable under all conditions.


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Acousto-optics from A.A Sa Tel 33 (0)1 30 52 87 17 Fax 33 (0)1 30 52 78 03 www.a-a.fr

RISE TIME / FALL TIMEThe rise time T r and fall time T f of your driver specifiedin your test sheet corresponds to the necessary timefor the output RF signal to rise from 10 % to 90 % of the maximum amplitude value, after a leading edgefront. This time is linked to carrier frequency and RFtechnology.The class A drivers from AA, offer the best rise/falltime performances.

EXTINCTION RATIOThe extinction ratio of your driver specified in the testsheet is the ratio between the maximum output RFlevel (MOD IN = max value) with the minimum outputlevel (MOD IN = MIN value).A bad modulation input signal can be responsible for the extinction ratio deterioration.

)()log(20)log(10min

max

min

max dBV

V

P P

ratio Extinction pp

pp==

FREQUENCY CONTROLS

ANALOG CONTROL (0-V max )

The analog frequency control input of your driver controls linearly and continuously the output RFfrequency of the signal from Fmin (minimumfrequency) to Fmax (maximum frequency).The minimum and maximum frequencies are set atfactory, and can be slightly adjusted withpotentiometers "OFFSET" and "GAIN".The typical linearity of the frequency versus inputcommand for standard VCOs is typically +/- 5%.

Sweeping time (VCO)This is the maximum necessary time to sweep

frequency from minimum to maximum, or maximum tominimum.

This value will be taken as the maximum randomaccess time, though it depends on the frequency step.Vpp

When applying 0 V on "FREQ IN", Frequency = F minWhen applying V max on "FREQ IN", Frequency = Fmax(Standard frequency control input : 0-10 V / 1K ).

0 10 255 8 bits code

0

0,2

0,4

0,6

0,8

1

0

0 , 2

V m a x

0 , 4

V m a x

0 , 6

V m a x

0 , 8

V m a x

V m a x

FREQ IN Volts

N o r m a

l i z e

d f r e q u e n c y

Linear

Typ + 5%

Typ - 5%

8 BITS FREQUENCY CONTROLA byte (8 bit //) controls the frequency of the output RFsignal. A D/A converter converts the 8 bits command(N) on an analog signal which controls linearly theoutput frequency.

P max90 %

10 %

0

Vmax

0 256 steps are available : refer to your test sheet for pin connexions.

When N=00000000, RF signal frequency = Fminimum

TR Tf t

When N=11111111, RF signal frequency = Fmaximum

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