The Weaver Modulator
The block diagram of an analog Weaver modulator is shown in fig.1. The input signal spectrum is converted to a zero i.f. band pass signal by means of a quadrature frequency converter and a low frequency  oscillator. The zero i.f. signal is then filtered by two identical low pass filters which suppress the unwanted sideband and converted to the final frequency by a second high frequency quadrature up-converter.

Fig.2 shows the processing steps operated on the spectrum of an audio input signal to obtain the desired IF SSB output. For a radio communication system it is widely accepted to limit the input audio bandwidth to 2400 Hz (from 300 to 2700 Hz). The audio input is a pass band signal with a 1500 Hz center frequency.
With the first mixing one of the two side bands of the input signal is converted to a zero carrier frequency signal (usually called by engineers a complex low pass signal, or zero i.f. signal). To obtain such a zero i.f. signal, the frequency f1 of the l.f. quadrature oscillator should equal that of the input signal, that's to say 1500 Hz.
The other sideband is moved (in absolute) to an higher frequency. It is up to the two low pass filters to reject it, while passing unaltered the zero i.f. sideband to the next conversion stage. The low pass filters should exhibit a low attenuation and ripple in the band -1200/1200 Hz (wanted sideband) and introduce a large attenuation for frequencies, in absolute, greater than 1800 Hz (unwanted sideband).
The second quadrature mixer converts the zero IF single sideband signal to the desired IF frequency f2. Fig.2 shows the generation of an USB signal.  It is sufficient to invert the sign of one of the quadrature oscillator outputs to obtain a spectral inversion and the generation of an LSB signal.
Note that the suppression of the carrier is due to the action of the first two mixers, which are supposed to be perfectly balanced, and to the assumption that the input signal has no DC component.