I think I was not clear enough at my latest message. The equalisation process had to be done in both ways, end-to-end - a loopback test was not acceptable.
Regards, EA3DU -----Mensaje original----- De: Sergio Manrique Almeida [mailto:[EMAIL PROTECTED] Enviado el: jueves, 21 de septiembre de 2006 17:20 Para: 'digitalradio@yahoogroups.com' Asunto: RE: [digitalradio] Re: digital modes and THE RADIO Some time ago telcos used to offer analog leased-data lines services - two-way end-to-end lines, on a terrestrial 300-3400 Hz channel. To be able to accomodate a 9600 bit/s data signal, the lines had to be equalised both in frequency and group delay, according to ITU-T M.1020 Recommendation. So the equalisation was a two stages (and two tones) iterative process: -For equalising in frequency, a sweeping signal was sent - alternating every frequency step (the actual sweep signal) with a 800 Hz tone, which was the reference. Then it took to adjust manually each of the equaliser's cells (I think there was one cell each 200 Hz), as you saw the received spectrum at your testing equipment screen. -For equalising the group delay the process was the same, being the difference the reference tone frequency, which had to be 1800 Hz. As a matter of fact, I think the PSK-modulated carrier (or QAM, or whatever) was at 1800 Hz. Of course, the biggest group delay deviations were at the bandwidth limits, that is, at the channel filter slopes. Those lines had equalizers (TX & RX, frequency and group delay) at each end, usually at the telco data center at each side. That was long ago. Before the Internet and Nx64 kbit/s digital lines. I think amplitude/group delay can be critical issues for amateur radio Digital Voice and wideband data modes. It would be nice to have sound card modes with a kind of adaptive equalizer plugged in. Or at least, a static equalizer to compensate our own SSB transceiver responses both in TX and RX. I measured the frequency response of my HF transceiver (a compact one) at SSB and it was far from flat. I do not have equipment to measure group delay, but perhaps it can be implemented virtually as a sound card application. 73, Sergio, EA3DU [EMAIL PROTECTED] -----Mensaje original----- De: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] En nombre de Mark Miller Enviado el: jueves, 21 de septiembre de 2006 15:51 Para: digitalradio@yahoogroups.com Asunto: Re: [digitalradio] Re: digital modes and THE RADIO Jim, I agree, but we are talking about a test. The test signal would have to have a signal that either sweeps or is shifted in frequency. This would be a test like a two tone test. A two tone test is not a real world test, but it is made with very specific signals and the results are well known. I think we would have to approach group delay the same way. Through equalization most transmitter/receiver anomalies can be corrected. Non-linear effects of the analog circuits cannot be necessarily corrected, but the signals can be designed to reduce their effects. MT63 is a good example. 64 tones that can be run into an amateur radio at full power with third order IM products that are 23 dB down, and not cause excessive bandwidth outside of the necessary bandwidth. I have some spectrum analyzer plots of this, but cannot find them right now, and my spectrum analyzer is in the field. As soon as I get it back I will make some tests. Even PSK31 run at full power really does not cause the IM problems that most people claim. Most of the IM problems happen in the receiver, or in over driven analog sound card and transmitter audio circuits. 73, Mark N5RFX >While they may be sinusoids, they are not steady state. The tones >are switched and their phase may change depending on the modulation. >An example would be the first cycle of a sinusoid applied to >capacitor or an inductor. You will get some distortion. How much >is the question. Phase changes also require a high slew rate >capability in order to not be distorted. Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
