ISOMET 1205C and 232A1 Driver AOM

[mixedgas]

Gogu Sent me a ISOMET 1205C1 and a 232A1 Analog Driver for review. While I've had 1205c in the past, I've only had the on/off TTL driver.

I'm in the process of building a mount, but I started with the driver.

The factory specification calls for 1 volt at about 10 mA into 100 Ohms.
While 100 Ohms is the ILDA standard input impedance, its a bit low for some systems, and you would loose resolution reducing your output in software.

So...
I took a look at the innards of the driver, and found many parts that would be instantly familiar to any RF technician, but would look like voodoo to a non RF tech. I do as much RF as I do lasers, so I'm at home here. Rather then map out the whole Driver, I've done a partial schematic showing blocks, but a actual schematic for the key parts. The BNC connector marked RF goes to the AOM. The connector marked VIDEO is the drive signal to the AOM. The +28 volts to the driver and the ground are obvious. You can get away with 24 volts , but I would NOT exceed 28.5 V to preserve the lifetime of the CA2830C RF amplifier, which is about 90 to 135$ to replace. The optimal drive to the crystal is a little less then 1 watt of RF if you have a RF wattmeter that covers the 80 Mhz range. A high speed scope with a low loading 10X probe might be a option here for measuring the RF.

Do not run the AOM DRIVER without the AO Cell attached. This is very bad for the expensive RF AMP module used as the output stage. Good heatsinking is required, as the RF amp runs hot. Its mounted inside the case adjacent to the mounting flange, so mounting the AO Driver on a metal plate is enough. I'd guess 6x6 inches (15 cm by 15 cm) is enough metal. Also the cable from the driver to the AO cell MUST be a 50 Ohm RF grade coaxial cable, anything else such as bare wires may cause back reflections of power into the amplifer. This is VERY bad for the amplifier module.

To raise the input impedance, you can either mount your resistor in series with your wire to the center connector of the video port, or in Antistatic environment, remove the 4 screws that hold the lids on the case, unsolder the purple wire from the back of the "video" BNC, solder on your 270 to 470 ohm resistor, with one lead to the BNCs center terminal, and leave one lead floating. Onto the floating lead, solder the purple wire. Make sure you cover the resistor with heat shrink tubing. Do not run the driver with the covers off or without a heat sink. You can increase the resistor above 470 ohms, but I would NOT lower it below 270.

It helps if you mount and align the AO crystal before doing the driver modification. (Or cable modification, if your not opening the driver)
In its factory configuration, apply +1 volt to the video port using a adjustable bench power supply or a D cell. The 1.5V from a "D" cell is just fine and it can easily source the 10 mA required. Align the crystal for the brightest possible 1st order beam. Then shut down the system and disconnect the driver.

Perform the modification and reconnect the driver.

You may need to now adjust the bias pot. Make sure the cover is closed so you do not interfere with TV or RADIO services in your area.
I'm assuming you mounted and aligned your crystal before you did the modifications.
Apply 100-200 mV to the video port. Adjust the bias pot till you just see a faint beam, Apply the full 5 volts, and make sure you have at least the power you had in the diffracted beam when you applied 1 volt. You may now check for linearity and back off the bias potentiometer if needed. DO NOT adjust the "power Adjust" potentiometer. You want the one located over the word "OPTIC" on the cover.

Hint: turning on the diodes in the SRA1 mixer module causes it to conduct more RF from the buffer amp to the final amplifier. Over a certain range of current this can be fairly linear. Above a certain current, the diodes will be slammed full on, and nothing more is to be gained. You might see additional "ghost" beams at that point.

Make sure your DAC can source 10 mA or so. Some dacs may need a buffer opamp between them and the AO driver, but most can easily source 20 mA.

MUCH MORE TO FOLLOW!


SEE ATTACHED BMP

notes:

http://www.minicircuits.com/pdfs/SRA-1+.pdf
http://store.americanmicrosemiconduc...m/ca2830c.html

Steve

[sugeek]

MUCH MORE TO FOLLOW!

Steve

Ka-BUMP

Steve, I was wondering about the adjustment of the power and gain, can you go more in to detail about these adjustments? Feel free to call me if you don't feel like posting!

-Adam

[higgy]

Great write-up! Do you know what the crystal oscillator is?

I have a dead 232A driver opened up in front of me, and all that's printed on the oscillator is "500 HJB FM 0308". Haven't had much luck looking for the part online.

I was hoping to try swapping the oscillator for a different one at 90 MHz on a working 232A, and seeing if it works fine at the different frequency.

[mixedgas]

Great write-up! Do you know what the crystal oscillator is?

I have a dead 232A driver opened up in front of me, and all that's printed on the oscillator is "500 HJB FM 0308". Haven't had much luck looking for the part online.

I was hoping to try swapping the oscillator for a different one at 90 MHz on a working 232A, and seeing if it works fine at the different frequency.

It needs at least 7 dBm of RF to drive the mixer. I do believe that it is a true sine wave oscillator. Do you have a oscilloscope fast enough to see 80 Mhz?
I'm on the road, it will be some time before I can tear into one.

Steve

[higgy]

It needs at least 7 dBm of RF to drive the mixer. I do believe that it is a true sine wave oscillator. Do you have a oscilloscope fast enough to see 80 Mhz?
I'm on the road, it will be some time before I can tear into one.

A true sine wave oscillator would make sense. Since I can't find the exact part number, I looked around for other 80MHz sine wave crystal oscillators. There aren't as many as I expected, and they're not cheap ($40+), at least on my first pass.

7 dBm after the transistor would make sense, as the SRA-1 is a Level 7 mixer.

I have the equipment to test the oscillator (100 MHz scopes, resistors, transistors, caps, breadboard, etc), but I have a strong suspicion that I fried it before I knew what I was doing.

I drew up what I believe to be part of the circuit. It is attached (note: parts of this may not be correct, and I left out values for those components that I could not find). Apparently the oscillator wants 5V. Is a transistor necessary for testing a crystal oscillator?

[mixedgas]

The drawing you posted is a oscillator for a 80 Mhz 3rd overtone crystal

The replacement transistor is at Mouser or on Ebay:

http://www.mouser.com/ProductDetail/...t5ieMcGkiPE%3d

Its not a module, its a discrete component oscillator. If its not oscillating odds are the 2N5179 died.

Also check your schematic, you have the DC bias going into the base. I'd bet on the collector having some drive.

If its not at Mouser and its a RF transistor, try RFPARTS.COM

I'd probe it with a small coupled loop, about 1-2 turns of enameled wire in a ~ 1 cm loop. You may see the oscillator dip to 26.7 Mhz fundamental or stop if you draw too much energy out of it with the coupling loop. The loop should couple a few hundred microwatts into the scope.

Long ago and far away when I was poor, Dr. Wenzel did this for me when I could not afford a proper RF voltmeter.

You may find it of use with a coupled loop, a long wire antenna, or a 50 Ohm terminating resistor (NOT WIREWOUND).

http://www.techlib.com/electronics/detect.htm

If it works for you send him a thank-you email.

It will work into your scope besides a voltmeter.


Don't run the AOM driver unloaded, leave the crystal off it and tie in a 50 ohm 2-3 watt metal film or carbon resistor on the output stage. Keep the leads short on the resistor and run coax to it, it does not take much inductance to short the amp output at 80 Mhz.


Steve