--- In digitalradio@yahoogroups.com, "Jose A. Amador" <[EMAIL PROTECTED]> wrote: > > jgorman01 wrote: > > > S-meters are not just logarithmic indicators, they also indicate the > > gain reduction being applied in the RF/IF chain. As I said in a > > previous post, it is an indicator of the reduction in gain, i.e. how > > much of an attenuator is being inserted. By inserting this attenuator > > you are not just inserting an S5 level of reduction, but an S9+10 dB > > level of attenuation. > > I cannot follow the reasoning of expressing attenuations in S units. >
I did not mean to confuse anyone. I merely wanted to indicate that AGC reduces the gain of a receiver and is comparable to inserting an attenuator. > >Therefore the smaller signal is reduced by a > > much larger amount than its absolute level would need. This means it > > doesn't come out of the audio amp at an S5 level but at something > > much less. > > The initial purpose of AGC is providing LISTENING COMFORT in domestic > radios. > > The RSSI indicator (S meter) is an extra in communications receivers. > The earlier > receivers were not even calibrated in S units, but had some arbitrary > calibration. > > AGC aims at having some constant maximum loudness, so you can tune > between several > stations without holding the tuning knob in one hand and the volume > control on the other. > > Of course, it may have solved more than a single problem.... > It has solved more than a single problem. By using AGC, the dynamic range of each amplification stage can be reduced thereby saving money. I dug out the schematic of my Icom 761, which has been considered a fairly decent receiver, and the AGC is applied to a PIN diode attenuator prior to the first RF amp (not the preamp) and all 3 stages of IF amps. I didn't follow it any further since this illustrates what I was saying. > > SDR's still have to deal with the real analog world at some point. RF > > preamps and amplifiers that have a large dynamic range are not easy > > to design and build. That is why AGC is applied to them, to limit the > > range they have to handle. > > As far as I know, high performance radios do not use AGC before the > first selectiivity > element (roofing filter or whatever), as the dual AGC loop radios have > been demonstrated > to protect dynamic range more than the audibility of the desired signal. > A large signal > actuating a wideband AGC on the front end is not a good solution from > the PRACTICAL, > real world communications standpoint. > > If you really wan't to gauge a radio, use it in a contest, preferably, > on a large one. Shortcomings > will come afloat by themselves. The Icom 761 doesn't have a roofing filter and it's dynamic range probably doesn't compare to a high dollar receiver that does. > > > As an experiment turn off your AGC and > > see what level of signal it takes to overload at least some of the > > stages in your receiver. I can HEAR audible distortion on S9 signals. > > It depends on the gain distribution or S meter calibration among > different radios. > > > This means signals much less than this also have distortion. > > I cannot follow this. > > SMALLER signals are also distorted? Common sense indicates that larger > signals > certainly would.... All I am saying here is that if I can HEAR distortion with my ears on a given level signal, I am sure that there is significant distortion on even lower level signals that my ears simply can't discern. > > > Now this may not be occuring in the first RF stages but it likely > could be. > > Generally, it may happen from the last IF amplifier towards the front > end. A good > gain distribution would achieve it. Otherwise, it is a real bad design, > a steal... As I pointed out about my Icom 761, there is AGC applied throughout the different receiver stages. I wouldn't call it "real bad design", just an engineering decision to provide acceptable performance at a minimum cost. > > > SDR's may very well be an answer to cheaper high performance > > receivers, but so far the measurements I have seen don't show a > > dramatic improvement, for example, even half again the dynamic range > > of current decent analog receivers. > > I cannot associate cheap with bad so straightforwardly. > > I see one of the merits of the amateur SDR designs is affordability. For > the people > who can afford a PC (not everyone can everywhere), it can provide > better filters and > demodulators, and improvements that can be enjoyed as soon as you > upgrade the > software. It is not so easy to do with a hardware radio, unless you > really know what > you are doing. The basic performance is defined by the front end, and I > can assure > it may not be perfect, but is a good design. I have not bought it, but I > have done some > homebrewing with the parts I've got, and it works well. > > There is another project, the HPDSR, but it seems to me it is not that > affordable nowadays. > Looks like a next generation project to me, since the limitations of the > available soundcards > would not be the bottleneck. > > > See the ARRL review on the > > SDR1000. I am sure better performance will come, but at what price is > > a question. > > The price of a 1 GHz PC plus a good soundcard plus the front end > hardware. Depends on > the specific market, but in the US it can be real cheap. > > I read a post here that in the US, $300 is pocket money. Really is it? > I wish $300 was pocket change. I am no longer working, and living on a fixed income means a lot of savings for $300. > > Here is a reference I found about a high performance system. > > > "The Model 7640's FPGA serves as its control and status engine, and > > is supported by 512MB of DDR SDRAM for buffering functions, such as > > data capture and delay. The transceiver digitizes HF (high frequency) > > or IF (intermediate frequency) input signals using a pair of 14-bit, > > 105 MHz A/D converters, and generates output signals with two 16-bit, > > 500 MHz D/A converters." See it at > > http://www.linuxdevices.com/news/NS3911104852.html > > <http://www.linuxdevices.com/news/NS3911104852.html> > > > > It only retails for $85,000! > > Let's be realistic. It may be very high performing, but is not affordable. > > I am afraid the $ 500 hammer story repeats again. Take it or > leave it.... I only wanted to point out that performance DOES come at a cost. Perhaps that is what I should have said to begin with! A simple description of a complex problem, Jose. > > Jose, CO2JA > > > > > > > __________________________________________ > > XIII Convención Científica de Ingeniería y Arquitectura > 28/noviembre al 1/diciembre de 2006 > Cujae, Ciudad de la Habana, Cuba > http://www.cujae.edu.cu/eventos/convencion > 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! 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