A Tale of Two PCAOMS -PCAOM PROBLEMS

[mixedgas]

Used PCAOMs often get a bad rap. Factors I often site as the cause of this are A. Insufficient attention to the the positioning stages used in mounting the cell. B. The polarization quality of the incoming light.

A third factor can be the history of the driver. For one of the two series of Neos PCAOMs, the maximum drive power per wavelength is 250 mW. This power can be trimmed for each channel by a RF attenuator control on the driver. For a typical cell, the power required decreases with wavelength. Red channels will require the most power, typically 160 mW of RF drive for the 676 nm line. Violet will require the least power, with 457 nm requiring 50-60 mW of RF driver.

TWO PCAOM drivers I measured today have the following characteristics:

Color Channel#---- RF Power Driver Sample---- I RF Power Driver Sample II
(All measurements into proper dummy load, not a cell)

1-- 170 mW----51.1 mW Deep Red
2 --151 mW----43.0 mW Red
3 -- 70 mW ----35.6 mW Yellow (sometimes)
4 -- 84 mW---- 48.9 mW
5 --71 mW---- 47.5 mW
6 --66 mW ----40.2 mW
7 --72 mW ----47.8 mW
8 -- 77 mW ----39.1 mW


RF Drive levels for Sample Driver I are what I would expect for a healthy crystal with typical efficiency. Driver I came from a planetarium system. What is going on for Driver II? Considering its history at a major theme park's 3D Indoor theatre, it has been adjusted for a rather strange color balance, on a laser that had excessive red.

Lets compare the frequency synthesizer settings for the two drivers:

CH1-- 47.17 Mhz ------47.29 Mhz
CH2 -- 49.70 Mhz ----- 49.72 Mhz
CH3 --58.353 Mhz----- 65.29 Mhz
CH4-- 65.32 Mhz ------66.39 Mhz
CH5 --66.393 Mhz -----69.72 Mhz
CH6-- 71.393 Mhz----- 71.31 Mhz
CH7--73.383 Mhz------ 73.85 Mhz
CH8-- 78.193 Mhz ------78.21 Mhz (Same wavelength, Fine Tuning.)

Channels 3,4, & 5 are set for different green and blue wavelengths, Driver II does not use the Whitelight laser's yellow line as programmed.

Sometimes PCAOM drivers are calibrated for different applications. While it is easy to change wavelengths and fine tune by adjusting the two 16 position switches per channel, RF drive power is another matter. Not many laserists own RF power meters or RF spectrum analyzers. If one has problems obtaining 80-90 percent transfer with their PCAOM, perhaps the previous user adjusted the RF drive, as well as course and fine tune of the frequencies. Too much power can be as bad as too little, one does not want a over driven crystal causing ghost beams.

On the channels that are the same, one can see the slight difference in the fine tuning of the frequency of the selected wavelength. That is easily adjusted using a laser power meter and the driver's dip switches.

While one can slowly turn up the RF attenuator drive and follow the laser output with a laser power meter, one cannot prevent overdrive if the crystal is badly aligned. It is advisable to follow the RF power with a meter to make sure your RF drive is reasonable. A fast oscilloscope and a proper 50 ohm RF dummy load in place of the cell can be used if you have a really good oscilloscope with delta cursors.

I'm glad I checked driver II before grabbing it and using it in the field. Its easy to change wavelengths anywhere, but I would not have taken a RF milliwatt meter out to a show site. Driver II was set up by a professional for a special application. Most of the time drivers are swappable with little adjustment required, but this unit was a exception.

Something to think about if you still use gas lasers or have used your PCAOM with DPSS for blue, green, and violet
modulation.

If you think you have a bad crystal, have some one with RF tools check the configuration. This took 10 minutes to measure both units. Ham radio operators and commercial two way radio shops have the tools, as will some universities. A typical VHF wattmeter for amateur radio use does this task just fine.

Tools used for this test: frequency counter, RF millwatt meter, a 10 dB 2 watt attenuator to reduce the RF output so it would NOT damage the sensitive frequency counter. Neos does install a -20 dB sample port on the drivers, but it requires a SMC RF connector, something few laserists would have.

Steve

[PhantomRage]

Ham radio operators and commercial two way radio shops have the tools, as will some universities. A typical VHF wattmeter for amateur radio use does this task just fine.

Funny, as I am a ham radio guy as well. I have all needed test gear to test/build many RF devices. (need that stuff for all the repeater's I have on the air)

If in Reno Nevada and need some testing done. I am more then willing to help.