Today, I’ll be continuing my 80m transceiver build that I started in T-Kit 1380 Kit Build: Part 1.
At the end of the last post, the board looked like this:
Today I’ll be moving on to the VFO section of the board. A VFO, or variable frequency oscillator, is the circuit that allows you to tune a radio. This particular VFO is based on a Collpits oscillator, and can tune over a 50-70 kHz range centered on a frequency determined by the component values. The frequency range shown in the image may seem a bit strange. This transceiver can be built to cover that 50-70 kHz range somewhere near 3.5 MHz to about 3.75 MHz. The short explanation is that the frequency we’re interested in is shifted by the frequency of the VFO to an intermediate frequency of 8 MHz, where we can do filtering and amplification at a single fixed frequency. Since a lot of circuit characteristics are frequency-dependent, performance is much better if the components can be selected for just one frequency.
The majority of the components are supplied with the kit, so their values are fixed. One of them, an inductor, I had to wind myself. Since this phase required quite a few components, I decided I’d lay them out before I started.
Rather than start building immediately, I decided to wind the inductor first, so I could get that out of the way. The instructions specified 28 turns of the green #28 enameled wire on the red toroid core. I had to count the turns several times to be sure.
The inductance of the coil is dependent on a lot of things, including the material the core is made of, the diameter of the core, the number of windings, and the spacing between the windings. Later on in the build, I tweak the range covered by the transceiver by adjusting the coil spacing.
From this point on, it was simply a matter of stuffing the board and soldering, as per the instructions. The one thing I would have changed was the process for doing initial testing of the inductor. They have you tack a couple leads to the pads you’re going to use, and then tack the inductor to those. Unless your inductor is wildly off, you’re not going to be rewinding it, so I would have skipped that step and just soldered it in directly at the beginning.
The testing of phase 2 was relatively simple, because I’m using a frequency counter. I just hooked up the frequency counter, and adjusted the spacing of the turns on the coil I mentioned before until the VFO covered the range between 4.470 MHz and 4.391 MHz.
I’ll talk more about it in the next post about the transmit mixer and filter, but that provides an actual range of 3.530 MHz to 3.609 MHz. This includes the QRP CW calling frequency at 3.560 as well as W1AW’s code practice sessions transmitted on 3.5815. It does not include the main CW DX window between 3.500 and 3.525 MHz, but I’m still working on getting my Amateur Extra license, so I’m not authorized for that part of the band anyway.