Vacuum chamber for optical spectroscopy part 3

Okay, so I've got all the parts for the vacuum chamber and assembled the whole thing.

I've started by assembling the window. It is a 3mm quartz plate, sealed with two rubber o-rings (that will be later replaced with fluoride rubber ones)

After that I've screwed down the windows part to the chamber body, sealing this joint with another o-ring. To the vacuum flange an vacuum valve was fitted (Leybold full metal manual valve).

So assembly is done, now for testing it we attached the chamber to pumping station consisting of a rough vacuum pump and turbomolecular drag pump. Pumping station is attached via an metal flexible hose. The pump was left for ~1h to evacuate the chamber.

After one hour of pumping we achieved pressure equal to 1,6 *10e-5 mbar. It is quite good result for that pump. On a normal cryostat pumped for an hour we achieve 5,4*10e-5 mbar, so the results are comparable (the cryostat is slightly bigger). The only one thing to test is the vacuum tight sealing. I'll take a look at the chamber after some time to see if it's still under vacuum.

Some new stuff just arrived

So, I've got some fresh stuff from the workshop. Still in as-received state, but this is a big step for now. Just see how blue-prints turn into reality:

PCB for the ammeter arrived! Not yet populated, as I'm still waiting for the ICs (should be here next week). I hope that everything will fit, as some drills are smaller than expected

Sample holder is also ready. It turned out very nice. Maybe not exactly as in the render, but the crucial parts are ok. Also it is blackened, to limit light scattering from it.

Also the vacuum chamber parts are done, but I still have to put it together - expect some pictures soon.

Sample holder

Some viewers say that I'm getting lazy. To prove them wrong, recently I've drawn a sample holder, that I need to fix my samples in my measurement setup. When everything will be finished I post some pictures of the whole thing, but now - only a drawing from Autodesk Inventor :).

This thing will be used to fix an microscope slide (3" x 1") in a 3 axis stage from Thorlabs. The stage is motorized using linear motorized actuators, also from Thorlabs. This will allow me to automatize a big part of my measurements. Stay tuned.

Reduction ring

I love showing nicely rendered pictures of my projects :3 so here is a small, yet very important thing - an ring with threads, for thread reduction. Outer ring is SM1, and inner is M18. It will allow me to mount a prism in my spectroscope, and this means that I will hove to show it to you as soon as I finish constructing it.

Making these one made me learn how to configure a new thread in Autodesk Inventor, because the SM1 thread is not defined in the software. I anyone would be interested I could make a simple tutorial for that.

Spectrometer will consist of a prism, mounted via this adapter, an an iDus CCD camera. This device is a 1024x256 px camera, designed for use as an spectrometer. I will post some specs later on, when I will show the whole device.

Amiga new PSU and some other new stuff.

I've managed to get some more free time and finished converting the Dell PSU to use it with my Amiga. I've added a simple switch to connect PS_ON to GND and a green LED on PWR_OK to signal if the PSU is working. Amiga is working decently on that, although I have not testet it yet under full load. I guess it shouldn't be a problem. The PSU can supply up to 15A on +5V and 12A on +12V lines. More than enough for my setup (with everything on).

At the moment my Amiga looks like that: 

Not very good indeed. I plan to put it together and mount the metal shielding (at least underneath the motherboard) as soon as I finish all the planned mods. On the left you can  the Protar A500HD, connected to the Amiga via side expansion slot (aka Zorro I). Unfortunately I've failed to run any RAM on it. I've tested like 30 pieces of SIMM memory (EDO and FPM, different size and timing) and none of them showed anything in CLI after AVAIL-ing the available memory. It seems that one realy need the Protar RAM sticks, that differ somehow from normal sticks. So this part of my plans is still a big fail. 

On the other hand I've got a new external FDD, that is lying under the Amiga, as seen in the photo above.

As you can see the trapdor expansion port is empty. It is because I've just received an KCS Power PC Board bought on Amibay.com:

I plan to install it as soon as I finish the 1MB Chip + 0,5MB Fast mod. I will have to solder a small pcb for generating control signals for the additional RAM in the expansion slot, as the Amiga will natively adress the 1MB of RAM that is soldered on the motherboard. KCS Power PC should add additional 0,5MB of Fast RAM and a RAM disk (?) or something ;). We'll se how it works, when it will work. At the moment putting the card in does not make any difference in anything, as the JP7A jumper is cut.

So, there are some plans for the future, concerning this region of my interests:

• Finish the mod for using 1MB Chip on-board and 0,5MB Fast in the trapdoor (I'll be testing it firstly with other expansions).
• Mount the metal shielding of the Amiga.
• Try to run any RAM on the Protar.
• Test and run the KCS Power PC.

Night work on Amiga hardware.

As you might remember, recently I've fried my Amiga PSU. Although I would like to repair it someday, at this moment I've decided to use an ATX PSU from a PC. I bought a PSU from a Dell GX280, because it's nice and slim and soldered the cable from the original Amiga PSU into the Dell one. I have to add a switch and some nice LED to have it complete and test it with my Amiga.

I'm in a hurry a little, because I'm getting a little unpatient with my Protar A500HD - a SCSI controller with Fast RAM. Althought it was working correctly on a ghetto version of the PSU (cables from the Amiga PSU sticked into an AT power supply connector) but the RAM was not working. This might be an issue with power supply quality - the AT PSU I'm using is not in the best condition possible and gives voltages that are not exactly correct, or with the RAM itself. Now I got over 20 different SIMMs to test them and a fancy new PSU - I hope that both will work and I'll manage to get the 8MB FastRAM :-).

No pictures today, but I will post some as soon as I get the ATX-Amiga Power Supply complete and my Amiga running (with 8MB FastRAM, I hope).

Precise ammeter part 2 1/2

I've found a better op-amp for I/V conversion - OPA129. This one has similar parameters to the LMC6081, but allows for using +-15V power supply, and swings up to 13V output. This means that for 10Mohm R1 resistor I can swing up to 130nA, and measure as low as 200pA (due to the fact that offset voltage of OPA129 is 2mV).

To lower the offset voltage at the OPA129 output I've used an datasheet-suggested offset correction, that consists of R5, R6, R7, R8 and C9. It allows to trimm the offset using a potentiometer R8. If this will allow to nullify the offset, I guess it should allow me to measure the current and order of magnitude lower, down to ~50pA, or even less - depending on how effective the offset correction will be.
The PCB design is not varying much from the previous one. I've only adjusted several things to match the requirements of the company that is making the PCB. The order was sent to them so in ~2 weeks I should have the board in my hands. I also ordered some elements for the board, including the most important ones: R1 and R3. As R1 I choose 10M 1% Beyschlag resistor with low temperature coefficient - only 50ppm. R3 is 20k 0,1% 15ppm resistor.
As I calculated Vout = Iin*R1*(R4)/(R3), as R4 consists of a resistor and a potentiometer I assume it will be exact 200k. With the said resistors I will get Vout=Iin*10^10, this means 1 volt per 100pA. R1 error is 10^9*1% = 100k and R3 error is 20R. Without temperature effects (when the circuit will be maintained at 25*C) measurement error equals to Iin*1,1*10e6. This is six orders of magnitude lower than the measurement, which is Iin*10e10.
As I want to limit the bandwidth at around 10kHz I'll have to calculate the capacitance of C1 and C8. C1 calculated is 1,6pF and C8 is 79,6pF. Closest values are 1.5pF and 82pF, and such capacitors I bought to use in the final device.
I also ordered both op-amps, so I'll have to wait only for all the parts to come and then I can solder the circuit and do some tests, so stay tuned.

Precise ammeter part 2

There has been some work done with the design of the picoammeter (I guess it can be called so). There were some slight changes in the schematic and PCB design. I guess that the first stage is nearing to an end.
I've removed R2 from the non-inverting input of the IC1, as there is no need for using it in a symmetrical supply design, and added capacitor in the feedback loop of the IC2A, to filter out any high frequency noise that might be generated in the first stage.

Most of the tracks were thickened up to 0,032 inch where possible and last, but not least, I've added a guard ring around the input of the IC1, what is, I guess, crucial for measuring lows currents (at least ALL application notes and datasheets say so and I do believe in that).

Bottom layer of the PCB

Top layer of the PCB

 The board will be made at a professional workshop, with metalization and gold plating.

Bottom view render of the PCB

Top view render of the PCB

Finishing the schematic and PCB design I did some calculations. In order to finish the project stage I've calculated the elements I'm going to use. Most important part - R1 - will be precise 1GΩ ± 1% or 10MΩ. Unfortunately I have not found (yet?) any ± 0,1% resistors with such big resistance, but I guess this will not be a big issue and we can deal with 1% error. For this resistance I will get 1mV/pA. The offset voltage of the LMC6081 is in order of 1mV, so I guess that minimum measurable current will be in order of tens of pA (counting in the noise etc). Maximum current is limited by maximum voltage output of the op-amp, +5V powered is around 4,7V which gives 4,7nA for R1=1GΩ maximum current for the given supply voltage. If I exchange the resistor with 10MΩ the range will shift to 1nA - 470nA. Unfortunately there is no 100MΩ resistors at my main supplier. Will have to evaluate this more.

The second part, consisting of the OPA2227 should work with gain around 10. With 20kΩ input impedance this means R4 around 200kΩ, which is very reasonable value, if you ask me. The main problem with this part is also output swing. As I'm planning to use the same supply as for the IC1, the output will be similarly limited, as the OPA2227 does not have rail-to-rail output. I can always use higher power supply voltage, but this will complicate the design much.

The dynamic part of the calculations is to be made, as I don't know at which frequency I should limit the bandwidth. Looking at datasheet for the LMC6081 I'm guessing somewhere around 1kHz. This should give reasonable settling time of the measurement and slightly lower the noise.

I would like to thank Mr. Michał Penkowski, from Gdansk Technical University for lots of information and help about such measurements.

Still lots of work to be done!