What causes unwanted spurious signals in my receiver and how do I trace them to their source?

Unwanted spurious signals in an RF receiver are caused by nonlinear products generated within the receiver's own signal chain or by external interference coupling into the receiver. The internal sources include: LO harmonics mixing with signals (the mixer's local oscillator produces harmonics at 2x, 3x, 4x the LO frequency, each of which can mix with any input signal to produce spurious IF outputs at f_spur = m x f_RF +/- n x f_LO where m and n are integers), intermodulation products from strong input signals (two strong signals at f1 and f2 create IM products at 2f1-f2, 2f2-f1, etc., which may fall in the IF passband), ADC clock harmonics mixing with signals (the ADC sampling clock and its harmonics create mixing products similar to a physical mixer), power supply noise modulating amplifier gain (power supply ripple amplitude-modulates the RF signal, creating sidebands at the ripple frequency from each signal), PLL reference spurs (the PLL produces discrete spurs at offsets equal to the reference frequency and its multiples from the LO carrier), and self-mixing of strong signals (a strong signal leaking from the LNA output back to the mixer LO port creates a spurious DC or near-DC product). Tracing the source: (1) catalog all observed spur frequencies and amplitudes, (2) compute which m x f_RF + n x f_LO combinations could produce each spur (a spur chart calculation), (3) vary the LO or RF frequency and observe how each spur moves (an internal mixing spur moves predictably with LO/RF changes while an external interferer stays fixed), and (4) selectively disable or bypass stages to isolate the source.