I'm starting to build up the gear for the kitbiz to get off the ground, and the first large investment just arrived at the shack. Thanks to some blog posts from Eldon WA0UWH, I found AmScope.com, who sells all manner of microscopes for low prices. I ended up finding a scope that was very similar to the one that I worked with at Tek; the SM-4T. It's a 7x-45x trinocular stereo zoom scope with a double-arm boom. I also ordered a 80-LED ring light with a variable brightness control. The beast barely fits on my tiny construction bench, but I'll have to deal with the cramped space for now.
Here's some of my impressions after assembling and using it. First off, the base is massively heavy; which is great for stability but no fun when transporting it. I thought the poor UPS drive was going to get a hernia getting it off of his truck. The fit-and-finish is pretty good. Not quite the same as the high-quality brand name microscope at Tek, but I felt like it was good for the price I paid. The manual is terrible. I'm still not sure that they sent me the right manual, since the diagrams in the once I received show a completely different model. But the back of the manual does show the parts for my microscope, so who knows, maybe you just get some generic manual for all of the different models. The Engrish factor is pretty high, as you might expect. Fortunately, I was still able to easily assemble the microscope by referencing the photos on the AmScope website.
The image quality is quite good, although the field of view is a bit smaller than my Tek microscope. The interpupillary distance and diopter setting is fully adjustable. Once I got them set correctly, the microscope was a pleasure to use. The 80-LED light did a good job of providing bright white illumination for the circuit boards that I examined. My first impression is that I've received a good value for my money. AmScope claims that their production line is the same one that manufactures microscope for Zeiss, Leica, Nikon, and Olympus. Based on the quality, that certainly seems like a credible claim. It's a bit spendy for the average homebrewer to purchase, but if you are serious about doing a lot of SMT work, I think that these microscopes would be an excellent choice for the shack.
Today I finished construction on the circuitry for the RMS-to-DC converter described on the October 1992 QST article by W0IYH on measuring receiver performance. This is an article that I found on the EMRFD CD when I was searching for information on measuring the noise figure of amplifiers and receivers. I didn't find the article directly, but was directed to it because it was referenced in another paper (on the EMRFD CD) by W0IYH on homebrew noise sources. Thankfully, the authors were kind enough to include both papers on the CD, since the October '92 article gave a lot of the background needed to fully grasp the noise source article. The detector is a fairly simple circuit, since all of the hard work is being done by the Analog Devices AD636JH RMS-to-DC converter. This device measures the true RMS value of DC and AC signals (up to 200 mVrms full scale) by converting the input signal RMS value to an equivalent DC voltage. The rest of the circuit basically consists of op-amps providing plenty of buffering and one stage of gain. The gain stage is present so that you can use a 10:1 scope probe on the input.
As you can see in the photo to the right, there are some external controls for the circuit. At the far left is a pot which controls the signal level from the first buffer into the second buffer (which preceeds the gain stage). This allows you to accurately calibrate the detector to an on-board 200 mV reference. The rotary switch to the upper-right of the pot is the input coupling selector. You can choose from DC, AC, or GND, just like an oscilloscope. In the middle of the photo, you can see the 10:1 Tektronix scope probe that I'm using with this detector. Finally, in the upper-right corner is a rotary switch that allows you to switch in three different levels of signal attenuation: 0, 3, and 10 dB. The 3 dB and 10 dB levels are set with the little blue trim pots to the right of the probe tip. By having this attenuation built-in to the detector, the process of making NF and S/N measurements is a lot simpler.
Unseen in these photos is the simple dual polarity power supply that I built for this detector. Wall current is fed into a 120 V:12 V transformer, which then is recitified by a 1N4007 and regulated by a 10 V zener for both the positive and negative rails. I've still got to get the thing in it's enclosure, but of course I had to smoke test it first before I went any further. After trimming the offsets on the gain amp and the RMS-to-DC converter, the circuit performed pretty much exactly as I expected. No great surprises, fortunately. I've still got a bit of copper clad to trim off of the top edge of the board, then I should be able to get the whole thing mounted in the enclosure and ready to go by this weekend.