Back before the RTL-SDR era, there were a number of writeups on building homebrew spectrum analyzers out of TV tuners. Example: https://www.qsl.net/n9zia/spec/

Back before the RTL-SDR era, there were a number of writeups on building homebrew spectrum analyzers out of TV tuners. Example: https://www.qsl.net/n9zia/spec/w6hph.pdf https://www.qsl.net/n9zia/spec/w6hph.pdf