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. 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. >
This is wrong. The SDR-1000 has a measured IMD-DR over 100 dB, with an IP3 north of 30 dBm. It does not have a single analog amplifier with agc on it. All agc is done strictly in software. Great use is made of the fact that 200 kHz wide I/Q if's may be captured using modern audio codecs that exhibit 99 dB of dynamic range themselves (or more) and then we proceed to filter and downsample and get increased range by doing that. This of course limits the blocking dynamic range to about 100 dB as well. This will increase as a DIRECT result of getting and using better codecs. This is happening now in support of the SDR-1000 by HPSDR. > 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 SDR-1000, with which I am intimately familiar (having jointly written all of the DSP software in it with AB2KT) was a stack of boards layed out with a free tool. The stack of boards are 3x4 inches because that is what Eagle would do for FREE. It is a "beyond lucky" happenstance that with small component modifications the thing is able to get the numbers mentioned in the ARRL review. I suggest that you have misread it if you do not understand what a complete REVOLUTION the SDR-1000 is. The high dynamic range and the IP3 measured in the ARRL review ARE AT 2 KHZ! Not 20 kHz, not 5 kHz but 2 kHz. And the only reason the measurements are not done closer than that is the ARRL laboratory is incapable of have a sufficiently noise free generator to measure that close. The ARRL review you quote states (AFTER the review for the Orion and IC-7800 came out) that is was about the best receiver ever measured in the ARRL labs. MAJOR technical innovations have been made inside and outside of Flex Radio on the theory and implementation of the what Gerald calls the QSD and what others call the Tayloe detector. I understand its principles of operation completely having completed a detailed transform analysis and have suggested how to greatly improve the circuit. These considerations are being applied by Gerald in his new receiver design and they are being applied by HPSDR in a separate design based on low noise amplifiers I chose and a codec I recommended and an approach that started with Phil Covington, N8VB and spurred my technical analysis after Tayloe said that the essential nature of the beast was an integrator and not an RC network. I do believe people will be shocked at how unbelievably capable these inexpensive receiver components will be. For Flex Radio, they will be applied in expensive radio systems under design for the high end user. In HPSDR, they will be applied in modules clearly aimed at the experiment. We have, absolutely no doubt in my mind, the CORRECT tools to do amazing digital work for HF and reasonably inexpensively should we choose or very expensively if we choose to have a major new rig. > "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! > The HPSDR Mercury and Ozy boards are 135 MHz 16 bit A/D with > 90 dB range (I measured the A/D) and two Cyclone II FPGA's cascaded. After you do digital down converters in the FPGA's, we will have in excess of 100 dB of dynamic range (processing gain) which we can aid with the blocking dynamic range (near/far A/D issue) with ham band filtering. I am going to GUESS at the total cost for this. THIS IS AN EDUCATED GUESS AND TAPR WILL NOT BE HELD RESPONSIBLE OR LIABLE FOR MY GUESS. These boards plug into a backplane designed for the HPSDR project. $40 kit for the Atlas backplane. Mercury is a high speed A/D and FPGA and I am going to GUESS that it will sold, wired and tested for UNDER $300.00. It plugs unto the Atlas backplane. THIS IS A GUESS. Ozymandias is a USB interface and bus master board with another Cyclone II FPGA. Thus we have cascaded FPGA's in this system before entering into the USB port to deliver to the remainder of computation and display and GUI. A lot can be done with this system. I can process all of 20 meters, tell you who us USB, RTTY, CW, PSK31 and do a "PSK31 deluxe" style demodulator on the stronger signals (the digital, not the voice). I GUESS that it will be sold, wire and tested, for UNDER $300.00 There is a modified QSD, of the type I mentioned above, being developed into another plug in board. It will ultimately have in my opinion, >110 dB dynamic range, IP3 north of 40 dBm and it will use the same codec as the thousand dollar sound cards we lifted the idea from (the Akashi AKM 5394A) and this will be laid down in a completely balanced current mode I/Q mixer/IF filter to mix to nearly baseband for software processing. > Jim > WA0LYK > > In my capacity as chairman of the ARRL SDR Working Group, Flex Radio software designer and technical bottle washer (unpaid volunteer), my HPSDR contributor status, AMSAT VP Engineering and TAPR member and supporter, it is frustrating to me personally that what is happening right under amateur radio's nose is so badly misunderstood and insufficiently appreciate. I believe as strongly as I believe I am typing this note that most of you have purchased your last conventional HF transceiver because of this work. What vehicle should I use to scream these roof tops so educated interested people like Jim can understand how much things are changing? 73's Bob N4HY -- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." - Einstein 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! 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