> how do you quadrature modulate without Hilbert filters? > Perhaps I'm using the wrong term - the operation in question is just the multiplication of a signal by e^jwn. Or, equivalently, multiplying the real part by cos(wn) and the imaginary part by sin(wn) - a pair of "quadrature oscillators."
> i think you can calculate how much energy exists at the negative > frequencies and that comes out to be an error image signal that also gets > modulated up or down along with the image you want. > Right, to the extent that the LPF fails to completely block the negative frequencies, they remain as error images and show up in the output. However it seems easier to track this in the Weaver case, since this error is given directly by the suppression characteristic of the LPF, rather than via in-band phase cancellation as in the Hartley/Hilbert case. My thinking is that the Weaver modulator gets a direct benefit from oversampling, since the error images get moved further and further out into the stopband of the LPFs (easing their design constraints), whereas the Hartley approach does not, since it is stuck trying to maintain a phase relationship across the signal band, no matter how much it is oversampled. > i'm looking at the diagram and i think that's how my old Heathkit HW-100 > did it where the image rejection was done with > piezo-electric-crystal-lattice filters which are BPFs (not LPFs) with > bad-ass selectivity on both sides. > This sound like a third, related method (apparently called the Bandpass Method). Weaver is kind of the same underlying idea, but it down-modulates the signal first so that the filter can be done at baseband (as an LPF) instead of doing it directly on the high frequency signal (then you need a BPF with tight bandwidth and high center frequency, which is expensive). The downside is an extra modulator is needed. E
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