| 摘要 |
In the superheterodyne receiver, the aerial 1 is followed by a prefilter 2, a preamplifier 3 and a premixer 4. The signal path then branches to the main mixers 5 and 6, which are in quadrature and the output signals of which are fed as intermediate frequency zero to the corresponding further inputs 5 and 6. These may be low-pass filters and further quadrature stages. The common local oscillator 9 supplies the three mixers 4, 5, 6 with an unmodulated signal of half the reception frequency, the phase upstream of the main mixer 5 being shifted through 90@ by the phase mixer 10. As a result, the oscillator signal cannot reradiate onto the aerial. The same isolating effect is obtained if, as a variant, different local oscillator frequencies are fed to the premixer 4 or possibly further premixers 4 on the one hand and to the main mixers 5, 6 on the other hand. A condition is that these frequencies deviate from the receiving frequency, so that interference voltages can be suppressed in the prefilter. There thus occurs a mixing down to zero in two or more stages. <IMAGE>
|
