A friend of mine built one of those XR2211 demodulators...it did not work very well on the real HF world. He left it aside and Hamcom with just a 741 squarer performed better than that.
I built something inspired on the AN-93, but not a copy....wonderful ! It had two active filters, full wave demodulators, a comparator, a rather good baseband low pass filter (150 hz at -3 dB) and a Schmitt trigger after that, inspired in an old KAM design. Big ears, it was an elephant! It became the best I had used so far, and even forced it to work on 300 bd packet, using BPQAX25....it did better than the KPC-2 with its AM7910. The baseband could have been narrowed down to 22 Hz and switched (22 and 150 Hz), but never did, as it worked fairly well and I was a bit tired of the soldering iron and decided to just enjoy it. The modulator was a programmable divider with 74163 or 74193 using a marine band xtal that I had in bunch of "rocks". It should have been 2640 kHz, but a 2638 xtal did de job, tweaked a bit with a trimmer. The counters were keyed from the serial port with a 4049 and two resistors, changing the preload and thus the division ratio...it was some ten years ago....and the sine wave shaper was a 4018 Johnson ring counter and a sine weighed resistor network. After the low pass filter, it gave textbook FSK'd sinewaves. I also used it as an external TU with Hamcom....and did very well... Never got a case...it sits in the bottom of a drawer since I got my PTC-II.... Jose, CO2JA Bill McLaughlin wrote: > > > Hi Rick, > > I built one, was horrid! Worked fine on the bench but not so good on > the air...went back to the KSR-33 with a TU using passive torroid > filters...back when you could get a jolt from an open TTY loop...had > a Nuvistor preamp too... state of the art (then)...fun times. > > And yes, the selective fading was an issue....still is; one of the > reasons 300 baud packet away from the MUF suffers so... > > Be well, 73 > > Bill N9DSJ > > --- In digitalradio@yahoogroups.com > <mailto:digitalradio%40yahoogroups.com>, KV9U <[EMAIL PROTECTED]> wrote: > > > > Back around the early 1980's, QST published a TU (Terminal Unit, > what we > > used to use to interface hardware such as a Model 15 teleprinter to > a > > rig) and it used the then recently developed XR chips. One to > produce > > the AFSK tones and one to decode the tones as a PLL. > > > > This design was actually called a "State of the Art" device in the > title > > of the construction article. This was one of the first PC boards > that I > > ever made and built it from scratch. I sort of worked, but the > problem > > was that the tone decoder could only decode one tone and if you had > the > > slightest QSB across the tones, the minute you lost it, you lost > the > > print until it came back again. > > > > It was very hard for me to accept that the ARRL would make such a > major > > misrepresentation of this device. After getting help from long time > RTTY > > operators, they explained that this was not only not state of the > art, > > but was actually a really bad design for HF and could not possibly > > compete with much older TU's. And they were right. I borrowed an > old > > tube TU and found it so much better performing since it used both > tones > > with a comparator, etc. > > > > 73, > > > > Rick, KV9U > > > > > > > > Ralph Mowery wrote: > > > > > ...from watching many hours of RTTY with only 170 hz between > > > > > >the tones, I have seen one tone fade and come back in > > >several seconds, then the other tone will do the same > > >thing. This was observed watching an oscilliscope > > >hooked to the mark and space filters of an ST-6 > > >demodulator. > > > > > >__________________________________________________ > > >Do You Yahoo!? > > >Tired of spam? Yahoo! Mail has the best spam protection around > > >http://mail.yahoo.com <http://mail.yahoo.com>
