Now that we have power coming into the board, let’s build something that’ll use it. We’ll start with the variable frequency oscillator (VFO), the thing that lets you tune the radio to different frequencies. I’m not going to take you through oscillator theories or design in any depth–there are many good sources of information available that can teach you more than I could. My favorite is the ARRL Handbook (any recent year will do).
The VFO is a parallel-tuned Colpitts oscillator. Virtually every analog oscillator design consists of inductance, capacitance, and feedback, and this one is no exception. On the schematic, the VFO is in the lower left quadrant of the page and consists of the following parts:
- C49, C50, C52, C53, C59, C60, C66, C67, C69, and the tuning capacitor
- D6, D7
- R37, R39, R40, R42, and the fine-tuning potentiometer
Additionally, the following parts form a buffer on the output of the oscillator to keep it from being affected by downstream parts of the circuit:
- C41 (not shown on the schematic, but connected between the leg of Q15 that goes to 5V and ground), C43
I find it entertaining to compare theory with reality whenever I can, and one thing I wanted to do was to try to compute the expected VFO oscillator frequency based on the component values. We know that the design frequency is 5 MHz – 5.3 MHz or so. Let’s see what the component values tell us.
The primary (21 windings) on T5 makes up the inductance for the VFO. 21 turns on a T50-7 core will give you about 1.9 uH of inductance (you can verify that for yourself using the ARRL Handbook or any number of other sources. The capacitors that make up the capacitance in the VFO combine as follows to find the net capacitance:
- The parallel combination of C67 (39 pF) and the tuning capacitor (max 150 pF) combine for a net 189 pF.
- The 189 pF from the previous step combines in series with C59 (150 pF) for a net 84 pF.
- The 84 pF from the previous step combines in parallel with C52 (39 pF) and C49 (150 pF) for a net 273 pF.
- The 273 pF from the previous step combines in parallel with the series combination of C50 (680 pF), C60 (680 pF), and C69 (680 pF). The series combination nets 227 pF, giving us a total (with the 273 pF from the previous step) of 500 pF.
The net 500 pF in the oscillator circuit doesn’t include a small bit of additional capacitance resulting from the use of D6 in the circuit. D6 is a zener diode used as an additional source of capacitance by reverse-biasing it. A reverse-biased zener diode doesn’t allow current to pass through it and adds additional capacitance that varies with the bias voltage. The fine tuning potentiometer is used to vary the bias voltage on the zener diode, allowing small adjustments in the VFO frequency to be made. In the calculation above, we neglected to include the capacitance of the zener diode, which is okay for our purposes.
Now that we know the net capacitance and inductance of the VFO circuit, we can calculate the predicted frequency of oscillation using the formula below (again, you can find this in the ARRL Handbook):
f = 1/(2 pi * sqrt(LC))
Substituting our values of capacitance and inductance from above gives us about 5.16 MHz, which is pretty close to the expected 5 MHz and good enough for our purposes.
Q15, R37, R40, R42, D7, and C53 are the parts that make up the amplifier used to provide feedback to the oscillator. While they don’t impact the oscillation frequency significantly, they are integral to the successful operation of the oscillator by inserting positive feedback, counteracting any losses in the circuit. Without this amplifier, the oscillator would not oscillate.
Q12, C41, and R28 serve as a buffer between the oscillator and the rest of the circuit. The output of the oscillator is taken from the secondary winding on T5 and is fed through this buffer to the analog switch U4. U4 is used to switch the VFO signal between the two SA612 mixers (U1 and U2) when switching between transmit and receive (more on that in a future installment), and Steve, KD1JV (the rig designer) found that the buffer was needed in order to prevent the switch from impacting the oscillation frequency.
Okay, now let’s build the circuit. Install the parts listed above, except for the tuning capacitor and fine tuning pot. Again, make sure you’ve observed the correct polarity for the two diodes and the transistors. None of the capacitors in this part of the circuit are polarized, so don’t worry about them. Follow carefully the directions for winding the transformer T5. Note that the parts list says the kit contains #24 and #28 magnet wire for the toroids, but mine only contained #26. No matter–you can wind the toroids with #26 with no issues.
(One note here–every kit I’ve built has come with instructions that say to use a soldering iron and a blob of solder to melt the enamel of the ends of the magnet wire and tin the leads. I’ve never had much success with that technique–perhaps because I lack patience. My preferred approach is to scrape the enamel off the ends of the wires t provide bare copper to solder to the board. Whichever route you go, make sure that you remove enough enamel to make good electrical contact with the pads on the board.) Here’s what the finished board looks like:
Here’s a close-up of the oscillator section on the board:
In order to test the oscillator, we’ll need to connect the tuning capacitor and fine tuning pot. As I mentioned in a previous installment, I like to use 0.1″ header pins on the board to make these connections easier (and removable). The three holes for the fine tuning pot line up nicely with the 0.1″ header pin spacing, but the two holes for the tuning cap do not. For the tuning cap, I simply broke off two single pins and installed them individually.
Make sure to use the instructions in the construction manual on p. 12 to get things wired and connected correctly.
Now it’s time to test the VFO. Connect the tuning cap and fine tuning pot to the board. If you built the digital dial, we’ll use that as our frequency counter. Connect the digital dial to a power supply and jumper a connection between its SIG IN pad and the pad labeled VFO on the Survivor board (right next to U4). Connect power to the board and short out the ON/OFF pad to allow the board to get power. With the tuning cap and fine tuning pot both turned fully counterclockwise, the digital dial should read something close to 5 MHz. Later, when construction is complete, we’ll adjust the turns on T5 if needed to tweak the VFO frequency. For now, you can experiment to see what happens if you spread apart the turns on T5 or squeeze them together. As you turn the tuning cap, you should see the frequency change by 300-350 kHz. The fine tuning cap has a less-pronounced impact but will still cause the frequency to change when adjusted.
If you didn’t buy and build the digital dial, you can of course use some other frequency counter. If you don’t have a frequency counter, you can use a general-coverage receiver to detect the VFO signal. Connect a jumper wire to the VFO pad on the board to serve as an antenna, and connect another jumper to the antenna of your receiver, and tune your receiver to the 5 MHz range and see if you can find the carrier that the VFO is broadcasting. Zero-beat that carrier to determine the frequency of the VFO.
Next: The BFO