I've been trying to get into RF electronics, but one thing that is holding me back is lack of test equipment. I have an old analog oscilloscope, but that doesn't help much for things like verifying filter frequency or looking at harmonics. So far I have been keeping most of my experiments to lower frequencies (say less than 10 MHz). This way I can use a breadboard and not have things go way out of spec when I wave my hands nearby. As an example, I've built a 1-MHz LC oscillator and would like to determine if there are significant harmonics present.
I have been considering buying an RTL2832U USB SDR dongle to use as a spectrum analyzer. The basic device doesn't go below 24 MHz, but there are some that support "direct sampling mode" or that contain an up-converter. E.g. https://www.amazon.com/RTL-SDR-Blog-RTL2832U-Software-Defined/dp/B0129EBDS2/ and https://www.amazon.com/Usmile-100KHz-1-7GHz-UpConverter-Receiver-radio/dp/B01422FJMK/
From what I've read, the direct sampling mode is prone to noise and requires a filter to be useful. If I understand correctly this is because the tuner chip is bypassed, allowing signals above the Nyquist frequency to alias in and potentially overload the receiver. For use as a directly-connected (through an appropriate attenuator) spectrum analyzer, would this likely still be an issue?
If I were to choose one with an upconverter, should I worry about it creating spurious harmonics? What about the SDR chip itself? I realize that performance isn't going to be great for such a cheap module, but I'm hoping things like frequency-dependent sensitivity can be canceled out in software.