I have previously built a Colpitts oscillator whose output contained significant harmonic content. Instead of trying to improve the oscillator circuit, I have opted to build a band-pass filter to mitigate these harmonics from the ocillator output.

A local oscillator for use in 40 meter amateur radio bands, as well as the 7.2-7.3 MHz shortwave broadcast band is designed and built. The oscillator uses two variable capacitors for coarse and fine frequency adjustment. A common-base BJT amplifer is used to provide amplification. Feedback is provided through a capacitive voltage divider, making it a Colpitts oscillator. Frequency range is chosen based on a 455 kHz intermediate frequency downmix.

I have the long term goal of designing and building an amateur radio transceiver. In order to do this, I have realized that it would be beneficial to have both a network analyzer and a spectrum analyzer on hand to test parts of the design.

For a network analyzer, I purchased a NanoVNA H4. The NanoVNA is a budget network analyzer that can be used for antenna analysis, measuring filter responses, measuring amplifier gain, and measuring transmission lines. I paid $100 shipped for my NanoVNA H4 from R&L Electronics. For the spectrum analyzer, I purchased a TinySA. It cost me $57 shipped from AliExpress.

A concern that I have when using these devices is that I may connect them to too strong of a signal and damage them. The NanoVNA has a maximum input of +20 dBm and the TinySA will handle +10 dBm. It does not take much to accidentally generate a too powerful of a signal and to blow up the input stages of either device. These are relatively cheap devices, but I still don't want to destroy them. To protect these devices, I have decided to build a set of attenuators.

A frequency-shift keying (FSK) modulator and demodulator is outlined, designed, simulated, verified, and ultimately executed on FPGA hardware. The design utilizes direct digital synthesis (DDS) and uses several different finite impulse response (FIR) filter designs.