ISOMET RFA920T Series User manual

Feb 2017
1
Acousto-Optic Deflector Driver
Including: Basic Deflector Alignment
Instruction Manual
RFA920T Series
Models -
RFA920T-80 : 60-100MHz 10V Tuning Input, 10V Analog Modulation Input
RFA920T-110 : 90-130MHz 10V Tuning Input, 10V Analog Modulation Input
RFA920T-150 : 125-175MHz 10V Tuning Input, 10V Analog Modulation Input
ISOMET CORP, 5263 Port Royal Rd, Springfield, VA 22151, USA.
Tel: [phone], Fax: [phone], e-mail: [e-mail]
www.ISOMET.com
ISOMET (UK) Ltd, 18 Llantarnam Park, Cwmbran, Torfaen, NP44 3AX, UK.
Tel: [phone], Fax: [phone], e-mail: [e-mail]
ISOMET

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1. GENERAL
The RFA920T combined Driver and Power Amplifier, Figure 1, is a modular, swept-frequency RF
power source specifically designed to operate with Isomet acousto-optic deflectors. The driver
accepts a tuning voltage between approximately +0 volts and +10 volts and provides an RF output to
at the selected frequency. The driver also accepts an analogue modulating signal and generates a
modulated RF output to the acousto-optic deflector. This provides for intensity control across the
scan range.
A summary of the driver specification is shown in the following table:
Model Output Frequency Output Power
RFA920T-80 60 - 100MHz > 25.0 Watt
RFA920T-110 90 - 130MHz > 25.0 Watt
RFA920T-150 125 - 175MHz > 15.0 Watt
Figure 2 is a block diagram of the Driver. The output frequency of the driver is controlled by a
hyridized varactor voltage controlled oscillator. The output frequency is proportional to the input
tuning voltage with a scale factor of approximately 2 - 4MHz/volt depending on the model. The start
frequency corresponding to a tuning voltage of 0.0 volts is defined by the frequency offset
potentiometer (Multi-turn put to the right of the LED’s) This permits adjustment of the start frequency
from approx -5% / +90% of the range The start frequency corresponds to a tuning voltage of 0.0 volts
and is pre-set to a start frequency as listed above. A table of output frequency versus input tuning
voltage is supplied with each driver and is pseudo-linear. The frequency linearity is typically less than
±2.0%, and frequency stability is typically ±0.25%.
A high-frequency, diode ring mixer is used to amplitude-modulate the RF carrier. A “video”
modulation input swing of 10volts (positive with respect to ground) will result in 100% depth of
modulation. Figure 3 shows the modulation control.
The video input level must not exceed 20 volts

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The amplitude-modulated RF signal is input to a Class A power amplifier stage. This amplifier is
designed to operate at full rated power into a 50

load with 100% duty cycle.
Ensure adequate air flow across the heatsink.
The heatsink temperature must not exceed 70

C.
SERIOUS DAMAGE TO THE AMPLIFIER MAY RESULT IF THE TEMPERATURE EXCEEDS 70

C.
SERIOUS DAMAGE TO THE AMPLIFIER MAY ALSO RESULT IF THE RF OUTPUT CONNECTOR
IS OPERATED OPEN-CIRCUITED OR SHORT-CIRCUITED.
A DC power supply is required for operation of the RFA920T. The required voltage is +24Vdc ** at a
current drain of approximately 2.8A. The external power source should be regulated to

2% and the
power supply ripple voltage should be less than 100mV for best results.
The output power level is set by the power adjust potentiometer (PWR ADJ)
NOTE : Maximum power = fully clockwise
2. LED INDICATORS
Lower Green LED:
Shows GREEN when the RF output is live, PROVIDED:
a) the modulation duty cycle is more than 20% (approx).
b) the RF CW power is > 20% (approx) of the driver maximum power
c) Red LED is on
Upper Red LED:
Shows RED when the DC power is applied AND the AO and Driver Thermal Interlocks are good
** Note:
The driver will operate from +28Vdc giving an ~8W increase in output power but increased thermal
dissipation

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3. INSTALLATION AND ADJUSTMENT
(See Figures 4 & 5)
3.2 With no d-c power applied, connect the + 24Vdc in to the center terminal of the feed-thru
terminal 1. DO NOT APPLY POWER.
3.3 Connect the RF output SMA jack to the SMA or BNC RF input of the acousto-optic deflector
(or a 50

RF load, if it is desired to measure the modulator RF output power).
3.4 Connect the amplitude control source to the the D-type connector.
Connect the modulation signal ( 0,0 - 10.0V) to pin 8
Connect the signal return (0V) to pin 3
To begin, apply a constant voltage of 10.0V.
3.5 Connect the external Interlock of the acousto-optic modulator (if fitted) to the enable inputs on
the 9-pin D-type connector. If no interlock is fitted then these inputs MUST be connected
together.
Connect pin 9 to the floating contact 1 of the interlock switch.
Connect pin 4 to floating contact 2 of the interlock switch. (Grounded at driver)
The interlock connection becomes open circuit disabling the RF output, if the temperature of
the modulator exceeds 30ºC or the internal driver temperature exceeds 70ºC. An LED
indicator illuminates when the Interlocks are closed and the RF is enabled.
3.6 If required apply a 12V CMOS compatible or open collector driver to the Digital Gate input.
Connect the tuning signal ( 12v maximum) to pin 7
Connect the signal return (0V) to pin 2
This input is optional and may be left NC (Not Connected).
A closed contact on this input will disable the RF Output. Switching response time 10msec.
3.7. Connect the tuning voltage source to the the D-type connector
Connect the tuning signal ( 0,0 -10.0V) to pin1
Connect the signal return (0V) to pin 6
3.8 Adjustment of the RF output power is best done with amplifier connected to the acousto-optic
modulator

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The optimum RF power level required for the modulator to produce maximum first order
intensity ( or minimum zero order intensity) will be different at various laser wavelengths.
Applying RF power in excess of this optimum level will cause a decrease in first order
intensity (a false indication of insufficient RF power ) and makes accurate Bragg alignment
difficult. It is therefore recommended that initial alignment be performed at a low RF power
level.
3.9 Locate the PWR ADJ. Single turn pot on the left side of the access aperture.
3.10 With an insulated alignment tool or screwdriver rotate the PWR ADJ potentiometer fully anti-
clockwise (CCW) i.e.OFF, then clockwise (CW) approx 1/2 turn.
3.11 Apply + 24V DC to the amplifier.
3.12 Apply a 10.0V constant modulation signal to the modulation input, pn 8 of the D-type
connector.
3.13 Apply a 5.0V constant tuning voltage to the tuning input, pn 1 of the D-type connector. This
will operate the deflector at its mid-scan position.
3.14 Align the deflector head to insure that the incident light beam is centred in the active aperture
of the deflector. The following explanation assumes the light beam is directed slightly toward
the transducer (connector end) of the deflector
Observe the diffracted first-order output from the acousto-optic modulator and the undeflected
zeroth order beam. Adjust the Bragg angle (rotate the deflector) to maximise first order beam
intensity.
3.15 After Bragg angle has been optimized, slowly increase the RF power (rotate PWR ADJ CW)
until maximum first order intensity is obtained. Record this intensity value (ISAT).
3.16 To equalise deflection efficiency across the extremes of the scan, alternate between the
minimum and maximum desired frequencies ( 0.0V and 10.0V tuning voltage resp. ) and
adjust the Bragg angle to give the same efficiency at both positions. Fine tuning of the
incident Bragg angle and RF power may be necessary for optimum results.
3.17 The RFA920T is now ready for use as a variable frequency AO driver.

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4. MAINTENANCE
4.1 Cleaning
It is of utmost importance that the optical apertures of the deflector optical head be kept clean
and free of contamination. When the device is not in use, the apertures may be protected by
a covering of masking tape. When in use, frequently clean the apertures with a pressurized
jet of filtered, dry air.
It will probably be necessary in time to wipe the coated window surfaces of atmospherically
deposited films. Although the coatings are hard and durable, care must be taken to avoid
gouging of the surface and residue of the cleaning solution. It is suggested that the coatings
be wiped with a soft ball of brushed (short fibres removed) cotton, slightly moistened with
clean alcohol. Before the alcohol has had time to dry on the surface, wipe again with dry
cotton in a smooth, continuous stroke. Examine the surface for residue and, if necessary,
repeat the cleaning.
4.2 Troubleshooting
No troubleshooting procedures are proposed other than a check of alignment and operating
procedure. If difficulties arise, take note of the symptoms and contact the manufacturer.
4.3 Repairs
In the event of deflector malfunction, discontinue operation and immediately contact the
manufacturer or his representative. Due to the high sensitive of tuning procedures and the
possible damage which may result, no user repairs are allowed. Evidence that an attempt
has been made to open the optical head will void the manufacturer's warranty.

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ISOMET
+24Vdc
to centre pin
RF Out
PWR CONTROL
ISOMET
RF DRIVER
MODEL :
S/N :
+Vdc
79
100
170
6,5876,5
4,5 161
Ø4,2
SMA
9way D-type Female
Optional
FAN and COVER
44,1
Freq Start Adj
RF Pwr Adj
Control signals - : 9 pin ‘D’ type
Tuning Voltage (0 -10V) : +sig pn1, -rtn pn6
Amplitude Control (0 –10V) : +sig pn8, -rtn pn3
Enable/Interlock Control (Low = On) : +sig pn9, -rtn pn4
Digital Gate Input (NC or High = On) : +sig pn7, -sig pn2
Internal 10K ohm pull up to 12V
DC supply connection for older drivers fitted with solder feed through
Tag or threaded case stud = 0V connection
Fig 1: RFA920T (with optional Fan and cover)

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+Vdc
+V
.
.
+V
o
Interlock sw TO-220
-
+
DB9
5
9
4
8
3
7
2
6
1
+
-
+
-
+
PATransistor
Analog Mod
Pre-Amp RF
Output
PWR ADJ
Offset
Tuning
Figure 2a: Driver block diagram (generic)
SKT
DB9
5
9
4
8
3
7
2
6
1
3 1
2
.
..
Int
GATE (Optional)
No Connect or
Open Drain
FREQ TUNING
(0.0 - 10.V, ~1Kohm)
MOD / Amplitude Control
(0.0 - 10.V, ~600ohm)
AOD INTerlock connector
(normallyclosed contact)
Figure 2b: Connection diagram

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ISOMET
-1
0
1
RF Carrier
-1
0
1
Modulation Signal Input
-1
0
1
Amplitude Modulated RF Output
Figure 3: Typical Modulation Waveforms

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Schematic of a basic acousto optic deflector
and tunable driver
RF
1st Order Scan
0th Order
Input Laser Beam
Intensity Modulation
Tuning Voltage



SEP
SCAN
BRAGG
AO Deflector
Deflector Driver
The input Bragg angle, relative to a normal to the optical surface and in the plane of deflection is
given by:  
BRAGG = fc
2.v
The separation angle between the zeroth order and mid scan point of the first order is given by:

SEP = fc
v
The first order scan angle is given by:
    
SCAN= f
v
The access time or time aperture is given by:
Tacc = d/v
where :  = wavelength
 f = scan frequency bandwidth
fc = centre frequency
v = acoustic velocity of the crystal material
Quartz 5.7mm/usec
Germanium 5.5mm /usec
d = beam diameter
Figure 4: Deflector Parameters

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RF INT Scanning First Order
Diffracted Beam
Zero Order
Input Laser Beam
AOD
Separation
Angle
If applicable, Coolant circuit not shown for clarity.
Interlock Connector
SMB Coax
or
3-pin Snap
Normally Closed Contacts : Pins 1 , 2
Not connected : Pin 3
Tune: 0 - 10V
Analog Mod: 0 - 10V
Gate: O/C
+Vdc
1
23
1st
0th
Input
Orientation options
Input
Input
Input
1st
1st
1st
0th
0th
0th
Figure 5: Typical orientations