RF Chain Construction

This is the very first signal that I tested using the Spectrum Analyzer. It is a simple Colpitts crystal oscillator (running at 3686.4KHz) with a one-section pi filter. The first peak is the fundamental at 3686.4KHz. The second peak is the second harmonic, down about 12 or 13 dB from the fundamental (if memory serves me, this is about what a single section pi filter is expected to do). The time base span is set to around 20MHz, so you see the fundamental and second through fifth harmonics (on my Tectronics 2336 scope, I can get the full 70MHz range on the scope at once).

One thing that you don't actually see in this photo, because the center frequency has been tuned up just a little bit, is the "zero" point. If your SA is working properly, you will have a signal (I think I've calculated it to be somewhere between -10dBm and -20dBm) at 0 MHz (e.g. "DC"). I believe this is the fundamental of the first local oscillator flowing through the RF chain, but it is EXTREMELY useful, because if you can find the Zero point, you can always figure out where you are from there. Note also that there appears to be a "reflection" of the spectrum "below" or to the left of the Zero point, depending on how your "center" tune is set up.