Electronics > RF, Microwave, Ham Radio
Electrically tunable crystal band pass filters
rhb:
--- Quote from: gf on June 09, 2023, 05:23:19 am ---
--- Quote from: T3sl4co1l on June 08, 2023, 09:31:58 pm ---Keep in mind they could be doing further audio processing, emphasis, expanding, noise gating, etc. I kind of doubt that's actually the case, but it's a possible explanation for intelligible (to a human) results when the code is otherwise digitally resolvable. That is, it's not pulling information out of nowhere (bandwidth), it's expanding apparent bandwidth by nonlinear methods.
--- End quote ---
I think of a simple comparator, which recovers rectanglular pulses from the pulse-shaped ones 1)
Pulse shaping on the TX side also plays a role. If the signal is already properly band-limited, you can even send it through a brickwall filter (a little bit wider than the signal's occupied bandwidth), and the brickwall filter won't add additional ringing.
EDIT:
1) For instance, if root raised cosine pulses with alpha=0.5 are turned into rectangular pulses with a comparator at 50% threshold, the resulting edge jitter is still only (roughly) 10% of the symbol width (see timing jitter of the 50% crossings in the attached eye diagram). So I guess the "crisp" rectangular dits recovered by the comparator should be well recognizable (if this speed can be recognized by a human at all, and granted that the signal is not too noisy). At 50 baud (20ms symbol duration) and with alpha=0.5, the occupied bandwith of these RRC-shaped pulses is only +-37.5Hz from the center. Maybe even a smaller alpha (-> smaller BW) is still feasible (I just calculated for 0.5).
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The IC-705 has the following IF BWs to choose from:
AM: 9kHz, 6 kHz & 3 kHz
SSB: 3kHz, 2.3kHz & 1.8kHz
RTTY: 2.4 kHz, 500 kHz & 250 kHz
FM: 15 kHz, 10 kHz & 7 kHz
DV: 15 kHz, 10 kHz & 7 kHz
CW: 1200 Hz, 500 Hz & 250 Hz
The Fc of both filters are individually adjustable in 50 Hz steps. Clearly AM & SSB *must* be linear.
FWIW The BW of a CW signal is 0.35/rise_time. It's actually not the keying rate. But the keying rate does set the maximum usable rise time.
gf:
--- Quote from: rhb on June 10, 2023, 12:11:37 am ---Clearly AM & SSB *must* be linear.
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Potential non-linear processing was rather meant in the baseband, after demodulation. For instance I see now way how linear processing could re-sharpen the pulse edges once they have been blurred and high frequencies have been completely eliminated. However, non-linear processing can do that, re-introducing high frequencies.
--- Quote from: rhb on June 08, 2023, 02:17:36 pm ---I'd always said exactly the same thing until I used the TPBT feature on an Icom 705 to completely reject a stronger signal 30 Hz away on one side and another 50 Hz on the other side on the waterfall display from the station I was copying!
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At which data rate? Still 50 baud (= 20ms symbol duration = 60 wps), or slower?
T3sl4co1l:
--- Quote from: gf on June 10, 2023, 05:43:59 am ---
--- Quote from: rhb on June 10, 2023, 12:11:37 am ---Clearly AM & SSB *must* be linear.
--- End quote ---
Potential non-linear processing was rather meant in the baseband, after demodulation. For instance I see now way how linear processing could re-sharpen the pulse edges once they have been blurred and high frequencies have been completely eliminated. However, non-linear processing can do that, re-introducing high frequencies.
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Quite. And, without measurements, we're merely left to speculate what the actual response is.
As you're [rhb] well aware, DSP can be changed at the flick of a bit; a filter doesn't need to be consistent across modes (or time or space for that matter; we're not restricted to LTI systems here!). It might be easier that way, but it could also be that they went to the trouble of computing coefficients live, instead of literally shifting center frequencies around. Analytic filter design might've been hard back in the day (for certain values of "day"; it's really not that new), but it's entirely understood now, and eminently computable. It could even be that they detect the kind of program matter (not actually very hard in context of full modern computing, but again -- even less likely in an embedded context I would guess), and apply other kinds of filtering (including nonlinear effects as above) to improve legibility.
Tim
mawyatt:
Recall a non-linear device used in very early telephone and radio called a "coher" or something similar. Evidently had almost magical properties based upon lightly compressed carbon particles as in a current/voltage dependent resistor, altho can't remember details (OK I'm old, but not that old!!).
Best,
tggzzz:
--- Quote from: mawyatt on June 10, 2023, 01:04:42 pm ---Recall a non-linear device used in very early telephone and radio called a "coher" or something similar.
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https://en.wikipedia.org/wiki/Coherer
I'm old enough to remember (when I was a kid) reading about them in very old books :)
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