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. 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.
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 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. This means signals much less than this also have distortion. Now this may not be occuring in the first RF stages but it likely could be. 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. See the ARRL review on the SDR1000. I am sure better performance will come, but at what price is a question. 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 It only retails for $85,000! Jim WA0LYK --- In digitalradio@yahoogroups.com, "Jose A. Amador" <[EMAIL PROTECTED]> wrote: > > jgorman01 wrote: > > > I have done the same thing to calibrate my vfo's. But remember, when > > you are right on frequency, there is nothing to indicate that there > > is another signal there. And, I'll be honest, I've never seen my > > s-meter add the two signals together which would indicate that the > > powers are being added in the receiver. I have had to reduce the vfo > > signal to keep it from overriding WWV which is kind of my point. In > > fact, I just did this using my RF generator. WWV at 5 Mhz is about 10 > > over S9. The generator is at about S5 with no antenna connected and > > the lead just resting on top of the transceiver. When I switch the > > generator on, the S-meter moves not a bit. You would expect it to > > jump considerably if the RF signals were being added together. > > > > Jim WA0LYK > > With such a large ratio (34 dB based on 6 dB per S unit) you won't be > able to PERCEIVE anything. But theory, nevertheless, applies. > > S meters are logarithmic indicators, precisely because they have a too > wide ratio of signal strenghts to display, say, ideally 94 dB from S0 to > S9+ 40 dB, a 2511886431 (two and a half billon times) power ratio > Being practical, it is really hard to discern a 1/100th movement on the > needle, a 40 dB ratio, being the S meter movement (a D'Arsonval coil > or a mimic of it) a linear responding, VOLTAGE sensitive device. > > To really perceive something, the power ratios have to be > much smaller. A logarithmic indicator COMPRESSES the differences. > A linear indicator would show a larger response, but would have a much > smaller dynamic range. > > Also, it depends on the S meter dynamics. With a fast attack, slow release > AGC loop you will not see any pumping, as the syllabic period of speech on > SSB is longer, and the peak holding AGC dynamics will hide any low > frequency beats. It takes an old style, AM AGC loop to see any pumping, and > a rather small small signals ratio (almost equal signals) to show it. > > Finally, Dr. McGwier gave a reply on something about the radio > characteristics, > advancing that future SDR designs should somehow be able to combat BPL QRM. > > State of the art, conventional radios are nothing else than a more or less > expensive black box in the digital communications chain. Of course, it > has to > have some minimum performance specs, or the chain will break. SDR's should > bring some new tricks to our boxes, as they are the only ones really > capable to > do so, while performing at least equally to hardware defined boxes. > > 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! Groups is subject to: http://docs.yahoo.com/info/terms/
