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Senior Project Status

This week we have a status report due, along with our test plan. I just completed my status report, so here's a summary.
The latest block diagram is at http://www.flickr.com/photos/33604751@N00/501433865/
It's got just a few additions. Most importantly are the buffers between my 10.245MHz oscillator and the two mixers. I used two separate buffers in order to make sure that I didn't get any crossover distortion from the two mixers. These buffers aren't yet designed, but since the voltage swing produced by my oscillator is fairly good. I just need to be able to drive quite a bit more power into the load.
I think that my active filter will end up being a fourth order (40db/decade on each side) butterworth filter. It will probably use a sallen-key filter topology, since the amount of documentation on this is tremendous (I have four books in front of me, all of which discuss sallen-key). I suspect that I will simply do a low-pass followed by a high pass, rather than doing a bandpass filter. This will mean that I need to do two designs, but that's not much of an obstacle. The center frequency is the geometric mean of the two cutoff frequencies, and I want a bandwidth of 50kHz. My calculations show that my lower cuttoff frequency will be 430.686kHz, which we'll go ahead and call 430kHz for calculations. Similarly, the upper comes out to 480.686kHz, which we can round to 481kHz. I might need to shift these frequencies slightly, just to make sure that I don't have too much attenuation at the corners, giving me less than my specified 50kHz bandwidth. I'll hopefully finish the filter design tomorrow morning when I come in to work on things.
I found the standard component values for my passive filter yesterday. I did some simulations, and it looks like it will work really well, though the values I can get will shift things so that 10.7MHz is slightly "down" one of the ripples, rather than at the peak. I don't think this will be an issue.
Rather than doing a full detailed design on things tomorrow, what I think I'll do is draw out the topologies for the filter and the buffers. Once that's done, I can spend the rest of the day laying out my board. If I had known how National Semiconductor sent samples (that is, including development boards), I would certainly have designed my project around using their products. I would just have connected all of the various pieces together using short lengths of SMA cable. As it is, I'm going to lay out my design on a board, and have to get it ordered this week. I'm going to lay out the board from the output of the LNA, which will be an SNA connector, all the way through my active filter. The LO will be on there, along with all of the VCOs and mixers. Both image rejection filters, and the LO buffers will be on the board. The output of the first stage will go to an SMA connector, , which I'll then couple into my VGA. I got a devel board with the VGA, which has spots to put the SMA connectors, so I'm going to just build and test that separately from everything else. Then that will get coupled back onto my board, and fed into the first up-converting mixer. Signals will then flow through the board a bit, and finally end up at another SMA connector, feeding out to my RF power amplifier. If I'm quick and brilliant this week, I MIGHT try to put in some transmission line LC filters before feeding things out to the power amp. I think this would probably help my power output spectrum a LOT, and might even be necessary (uh-oh).
The reason that I'm not going to try to put the VGA onto my board is that I haven't done any development or testing on it yet. I'd really like to do that, and be able to do this as a complete integrated solution, but I feel that I need to get my board laid out and ordered ASAP. Once this is done, and I finalize the specs for my filter, buffers, and the controls on my VCOs, then I'll have some time to mess around with the AGC, and get it working separately from everything else.
Ack, that sure sounds like a LOT of work! I'm coming in early tomorrow to start cranking on this stuff.

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