I also ask you to assume: what is the delay time of the digital filter we get after the signal heterodyning? To any frequency convenient for you.
Heterodyning is just a multiplication, so its delay is rather negligible.
An analog 10th order Butterworth lowpass with 1.5kHz cut-off (corresponding to a 3kHz BW bandpass) has about 0.7-1.3ms group delay, in the 150Hz...1.5kHz range (i.e. for the baseband signal which is modulated onto the carrier of your band). See Figure 8.15 in
https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter8.pdf. The diagrams are normalized to 1Hz cut-off. An analog Butterworth is a minimum phase filter, AFAIK.
In the digital domain you can design a filter having almost the same impulse response as this analog filter.
There will be some extra delays in the DSP chain, but IMO the filter itself still dominates the overall group delay.
Regarding the required characteristics. It may not be achieved.
Why not? It is a matter of the design, though. A high-order filter is a must, in order that you can achieve it.
You can never ever achieve it with a
first order filter, though, regardless how hard you try.
But this characteristic is really inherent in quartz filters.
IMO nobody here denied that quartz filters can achieve that. But again, then they need to have a sufficient number of poles, so I assume that your filters contain rather a handfull of crystals each, and not just a single one?
Btw, Google found
these quartz filters (up to 16 poles). Yet another proof that they are obviously feasible and do exist. I've no idea what's their price, and even less idea what they would cost when tailored for custom frequencies/bandwidths. What do your filters cost? What is actually the price per filter you need to beat with an alternative solution? Is it worth the efforts?
Is the output stringently analog, and still modulated? What happen actually with the signal after the filter? For instance, a subsequent demodulator could possibly be directly integrated into a DSP chain.