| 摘要 |
A Radio Frequency (RF)-band tuner stage is combined with a quadrature downconverter stage in a single shielded enclosure as an RF-to-baseband pulse amplitude modulated tuner suitable for receiving RF-band signals from an LNB or the like and converting the signals directly to signals in a desired digital format. The bandwidths within the two stages are optimized for digital PAM demodulation, such as PSK or QAM, and certain functions are shared, such as automatic gain control and carrier tracking information. Electronically switchable attenuators and voltage-variable gain controlled amplifiers, in connection with a low-phase-noise local oscillator employing a microstrip resonator, provide for over 70 dB of dynamic range. The IF frequency and bandwidth are selected so that voltage-variable tunable bandpass filters of conventional design may be used to obtain over 40 dB of radio frequency image rejection necessary for reception of PAM signals. The IF signal from a tuner stage is passed to the quadrature downconverter stage within the same enclosure, where the signal is split, and an IF local oscillator signal is injected into double balanced mixers to mix with the IF output of the downconverter stage. The output of the mixers is the desired baseband I and Q data signals for demodulation, which are fed directly to a demodulator stage, which may also be a part of the same unit.
|
