DuWayne (KV4QB) has done something very cool here. He's taken an Arduino Nano, a cheap AD9850 DDS board, a small screen, and a couple of log detectors, and he has built IN AN ALTOIDS TIN a scalar network analyzer that lets you see the bandpass of a filter. (We posted an earlier version of this here: http://soldersmoke.blogspot.com/2015/01/duwaynes-ad9850-arduino-tft-swr-scanner.html ) Wow. I've been doing this by hand, changing the input freq at 100Hz increments, measuring the output, putting the results into a spread sheet, converting to log (db), creating a graph... DuWayne makes it a lot easier. DuWayne is being encouraged to write up the results, possibly for QRP Quarterly.
Started playing around with the SWR scanner that I had been working on. Waned to see how hard it would be to make a very simple scalar network analyzer out of what I had. Really wanted something small to use for checking bandpass other filters. Hoped to get about 30 db. of range ,which should be enough for most filters. I have a couple of 8307 log detectors, but was afraid that it would be a pain getting it working and shielded in an Altoids tin along with the rest of the circuitry. Went with something even easier than the resistive SWR bridge I already had. Replaced the bridge with two basic diode RF probes, and changed the amplifiers so I could adjust the gain. I use one to measure the direct output of the 9850 DDS module, and the other for the output of the device under test. Kept the same control function as in the SWR scanner. A short push on the encoder button starts a sweep of the selected band. Holding it down for over a second cycles through the bands. Once a scan is done you can use the encoder to scroll through the sweep. I display the frequency and iDUT value in db relative to the output of the DDS module. The USB connector is available and different start and stop frequencies can be entered if needed when working with IF stages.
Well it worked much better than I had expected. After a simple adjustment of the amp gains with the output looped directly to the input, I was getting nearly 50 db with the loopback removed. Just using some standard value resistors, in a pi attenuator I got a very nice looking sweep that was within a couple db of the 40 db i had built it for. Since I only used standard value resistors, I though this was good enough.
Then I used ELSIE to design a 14mhz lowpass filter, again used standard values for L and C that I had on hand . Really happy with the results I got.
Attaching some pictures of the progress so far. As you can see that with what I used to build the test fixtures, I am amazed that they even worked at all.
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