In bygone days, S-9 was to be produced by a 100,000 microvolt carrier at the receiver's antenna terminal with the RF gain control at min (attenuation?) and AGC on.
Also, the S unit was to equal 5.5 dB. Rockwell-Colling and a few other manufacturers of amateur radio receivers in the late 1970's had their receiver signal strength meters measured in dB or microvolts. The 12 o'clock position was 100,000 microvolts...I have forgotten what dB it was. Walt/K5YFW -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] Sent: Wednesday, October 04, 2006 12:18 PM To: [email protected] Subject: Re: [digitalradio] Re: BPL-Busting Modes/Techniques 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. >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.... > 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. > 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.... > 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... > 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? > 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.... 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 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/
