PSK-20 nearly complete and testing begins

This past weekend I was able to finish the basic PSK-20 kit (sans the 5.068 MHz oscillator circuit). Overall, this was a very easy build. It’s not a simple one-evening project, but it can definitely be completed in a 2-3 evenings of work with 2-3 hours each evening devoted to the task.

Steve W3AHL, a member of our local club, mentioned to me a few weeks ago at our regular monthly meeting that he had a ton of lab-grade test equipment at his house. Immediately, a light went off in my head, and I made a mental note to contact him whenever I was preparing to do the final checkouts on the PSK-20 project.

Yesterday, I headed down South to Chatham County to visit him (about a 15 minute drive from my home in Carrboro). We spent the better part of 3 hours with his spectrum analyzer and oscilloscope trying to figure out the answers to the following questions:

1. What is the exact frequency of my Si570 oscillator?
2. How far can I pull the 9 MHz LO down?
3. What additional adjustments are needed in order to use the Si570-based signal generator as the second LO?

The answer to the first question was fairly easy to determine. After applying power to the Si570 frequency generator kit and allowing it to sit for 20 minutes or so, the output frequency was 9,999,992 Hz. Its nominal frequency is 10 MHz, so that’s not bad. Saving this value to memory space “000″ in the menu allows the kit to generate more accurate frequencies than if you work with the assumption that the nominal frequency is equal to the actual frequency.

The answer to the second question is important because that will determine if the crystal I have will allow me to operate on USB rather than the LSB (which the kit is designed to operate on). This odd configuration may be overcome in some software packages. At the end of the day, I’d like to be able to operate on USB (the traditional sideband on 20m); however, if this ends up being a costly choice (dollars-and-cents-wise) I may bypass it. Measuring the frequency of the 9 MHz LO while adjusting the trimmer capacitor C38 helped me determine that the lowest frequency of the oscillator is 9.021 MHz. This is definitely not as low as I’d prefer for the generation of the USB signal. Inrad sells USB crystals for the frequency of 8998.5 kHz, but their cost is $12 plus whatever shipping and handling may be involved. Not exactly the least expensive solution. So, the solution for now is to accept LSB as my sideband of operation on 20m digital modes until such point that I feel like spending what will amount to at least $20 to obtain this new crystal.

The final question to be answered of the day was how to incorporate the Si570 frequency synthesizer into the kit as the second LO. Previously, I had thought about using high-side injection at 23 MHz (instead of 5 MHz) “just for fun” and because other folks on QRP-L had suggested it might be a cleaner signal. After speaking with Steve KZ1X I decided that it might be best to stick with the low-side injection method since using high-side injection may require additional changes to the circuit which I may not want to tackle at this point in my learning.

Going with this thought, Steve W3AHL fired up his oscilloscope and began measuring the voltage output of the synthesizer and comparing it to the voltage required by the SA612AN Gilbert cell mixer. It turns out that the peak-to-peak voltage is much higher than what is needed/tolerated by the SA612AN. Steve W3AHL helped design a 10:1 Pi attenuator using commonly found resistors from Radio Shack. In a perfect world (at least, according to the calculator he used), I would have a pair of 96 ohm resistors and a single 71 ohm resistor in the circuit; however, Radio Shack’s selection of resistors is limited to 100 ohm (for the 96 ohm) and 68 ohm (for the 71 ohm). Not a bad compromise at all!

The neat thing about this attenuator circuit is that it will fit on the PC board very easily. Recall that I did not install any of the 5.068 MHz oscillator parts. This has left open C43, C44, R47, R48, and R49 (as well as a few other pads). The Clock + (in the circuit) and Clock – (to ground) from the Si570 kit were fed using RG-174 coax at C44 and a wire jumper was added at C43 to make the connection between C44 and R47. The Pi attenuator was created by adding one 100 ohm resistor at R47 and at R49 and one 68 ohm resistor at R48. I haven’t had a chance to test this yet, but it should provide the correct amount of voltage for the SA612AN (200-300 mV). It still may be the case that I need to add a LP filter to create a sine wave from the square wave the Si570 creates, but I’m going to see what happens when I power it all up before I go that route.

Our regular monthly club meeting is this evening. I’m planning on bringing the kit to the meeting for a little intermediate show-and-tell and eventually giving a full presentation of my project to the club in February or March.