Electronics > RF, Microwave, Ham Radio

Electrically tunable crystal band pass filters

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gf:

--- 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).

EDIT: Sorry, was too good to be true. Had a typo in the calculation. I'll re-calculate and update the message when I find some free time.
False Alarm.

mawyatt:

--- Quote from: rhb on June 08, 2023, 09:46:12 pm ---..... We are the lineage that originated DSP. .....

--- End quote ---

That is questionable since DSP concepts were developed for telephone line digital communications back in 40s and 50s at Bell Labs, and later became known as MODEMs that were part of the US air defense system called SAGE in 50s, and AT&T offered commercial versions of these in the 50s. Pagers were also in use in the 50s.

Had the honor of working along side Dr William Acker, a brilliant DSP guru that was behind the world leading high thru-put Group Data Modems at Honeywell in the 60s & 70s with his key patents in DSP based channel equalizers, later became Paradyne and eventually acquired by AT&T.

So Digital Signal Processing has been around for a long long time, likely preceding the transistor, although not generally known in the early days specifically as DSP, just another useful means of signal processing.

Best,

rhb:

--- Quote from: mawyatt on June 09, 2023, 02:31:13 pm ---
--- Quote from: rhb on June 08, 2023, 09:46:12 pm ---..... We are the lineage that originated DSP. .....

--- End quote ---

That is questionable since DSP concepts were developed for telephone line digital communications back in 40s and 50s at Bell Labs, and later became known as MODEMs that were part of the US air defense system called SAGE in 50s, and AT&T offered commercial versions of these in the 50s. Pagers were also in use in the 50s.

Had the honor of working along side Dr William Acker, a brilliant DSP guru that was behind the world leading high thru-put Group Data Modems at Honeywell in the 60s & 70s with his key patents in DSP based channel equalizers, later became Paradyne and eventually acquired by AT&T.

So Digital Signal Processing has been around for a long long time, likely preceding the transistor, although not generally known in the early days specifically as DSP, just another useful means of signal processing.

Best,

--- End quote ---

Norbert Weiner developed the foundations of DSP on contract to the Navy in 1940.  It was known as "the yellow peril" during the war because of the classified color and the difficulty of the mathematics. It was published in 1949 as "The Smoothing, Interpolation and Extrapolation of Stationary Time Series"

The oil industry immediately jumped on it and funded the Geophysical Analysis Group with 8 students.  Enders Robinson and Sven Treitel wrote a series of papers on DSP which appeared in Geophysics that were reprinted  as the Robinison-Treitel  Reader and widely distributed by seismic service companies.

Here's a more detailed story from the man who did the first deconvolution by hand on paper after digitizing the traces by hand in the summer of 1952.
 
https://library.seg.org/doi/10.1190/1.2000287


My PhD supervisor at Austin was a member of GAG, Milo Backus.  So yes, reflection seismologists originated DSP.  We were the only users that could work with a 125 Hz Nyquist.

mawyatt:
Interesting read, was not aware of GAG involvement!!

Altho should have been since a former colleague (Prof at Cornell) father, Dr Peter Molnar at Colorado, won the Crafoord Prize in Geoscience in 2014 :clap:

Thanks for the info :-+

Edit: BTW the colleague at Cornell, Dr Al Molnar, he's the academic emphasis behind the Polyphase Mixer, Mixer-First, N-Path Mixer, whatever you want to call it. If you check the IEEE papers mentioned, you'll find he's one of the original authors.

Best

rhb:
I have by any definition a *large* DSP library.  One of the the things I find *really* annoying is the general failure by the EE community to acknowledge what they owe the oil industry.

TI didn't start by building modern high speed wide word ADCs.  They started by building 250 Sa/s 16 bit ADCs which they sold for princely sums when they started building them in the 60's.

It's an interesting counter example to industry exploiting defense research.  Though Weiner's assignment was to figure out where to point an antiaircraft gun.  So it's defense exploiting industry exploiting defense.  And there's no another layer of industry exploiting defense.

FWIW I created an example of a pair of overlapping filters today using a 1 Hz resolution frequency domain with a 10 MHz Nyquist array and trapezoidal filters.

Have Fun!
Reg

Edit:  The norm today for an offshore survey is 5-6 sets of parallel lines with stations on 6.25 m or less spacing x 100-200 m (to keep the dozen 10-20 km long cables they are towing from tangling).  It takes several months to acquire the 10-12 TB of data and typically 6-9 months to process it.  Total bill for the project will run around $30-50 million.  These are done when it's time to decide where to place the wells.

The imaging step, the digital equivalent of holography, compute is  7-10 days on 10-20 thousand cores.

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