The project log is written backwards. After some initial success, the development will be described. In short, it worked better than expected.
- A PIC16F18345 EVB is configured to generate 400-500kHz using the NCO peripheral
- Sweep is generated by a for loop and updating the NCO registers
- Internal HF osc is configured for 32 MHz. Supply is 5V from USB port.
- Frequency output is on RC0. This is turned on and off for each sweep.
- A trigger and marker signal is generated on RC1. The marker goes low when output is between 453 and 457 kHz.
- The 5V frequency output is attenuated and filtered by a simple R-C network and pot. Not very good, but it did work
- The signal is tied to a vacuum tube radio IF strip. It's coupled into the signal grid of the converter tube.
- IF output is at the detector, where the AVC voltage is typically found.
- IF output is AC coupled into the scope channel.
- The other scope channel is connected to the marker at RC1.
- The scope display is very stable.
- The IF transformers could be adjusted to get a nice peak centered around the marker notch.
The code was done in MPLAB X. It's ugly. I'll clean it up a bit and post it later.