W6SI Blog Page (Entry 3) |
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12/9/14 |
New Receiver Architectures Part 1 |
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Just a few months ago, something astonishing has happened. Sherwood Engineering, known for its high performance receiver ratings, crowned a new transceiver to Flex Radio 6700. Bob Sherwood ranks the radios with the highest third order dynamic range, narrow spaced (meaning 2KHz). The top position has been occupied by German made Hilberling Pt-8000A for the past two years and has not moved until recently. This radio is a classic-architected double super heterodyne design, which is astonishing in itself, but that’s another day’s story. |
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(Courtesy: Sherwood Engineering Receiver Test Data) |
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So how did this new Flex Radio system beat the incumbent? Well, this is a new architecture that made the traditional measure of third order intermodulation measurement meaningless. As you can see from the table above, two of the top three radios are SDR radio. And they both cheat in the dynamic range measurement to get the high ranking, but then both have own shortcomings. Enter Flex Radio 6700. What’s different about this radio? Well, it does something amazing that was not possible until quite recently. After the front end filtering, pre-amplification, attenuation, etc., it feeds the signal directly to Analog-to-Digital converter. Its sampling clock is at 245.76MHz. Easily satisfying Nyquist Criteria deep into VHF frequencies. Once the signal is digitized, it can apply any amount of filtering or signal processing. This approach is called Direct Digital Sampling, and made only possible by the state-of-the-art high speed ADC from Analog Devices. Because there is no mixing circuit to saturate anywhere, there is no dynamic range suppression to worry about. Fantastic. Is this the wave of the future? Perhaps. What’s the hold back? Well, for one thing, this ADC device is extremely expensive. How much? If you have to ask, you can’t afford it, and it is reflected on the pricing of the 6700 radio ($7,500). Could that be a short term problem? Maybe. I don’t know. It nonetheless shows a price advantage over some of the highest class $10,000+ rigs. |
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But here is another shortcoming of this implementation. The ADC is 16-bit wide. That means theoretical maximum dynamic range is 96dB (voltage ratio). That is, if the internal noise residue is absolutely zero, which is not possible. More practical range for 16-bit system is like 65 to 70dB. Hence no matter how you measure it, you cannot have a radio with dynamic range exceeding 100dB. But Sherwood’s data defies that assumption. How? Flex Radio folks did it by using variable gain pre-amplifier and attenuator to cover the range, but it does not mean you can receive 0dB signal and 108dB signal at the same time and make out each of them. Think of it like a digital oscilloscopes. They have over Giga samples / sec rate, but typically using 8 or 9-bit ADC. But they cover the range from a few mV to tens of V range, because they have vertical amplifier gain knob on the front. You have to adjust the range to somewhere convenient to observe the signal. Bob Sherwood adds note on his data table: “This data does NOT imply you should generally run the preamp! This is a testing anomaly of a direct sampling receiver”. (Continued to Part 2) |
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6/26/2016: Leif Asbrink / SM5BSZ sent me a comment, regarding this writing. de W6Si / Tak Hello Tak, I came across your blog page: New Receiver Architecture You then write:"Hence no matter how you measure it, you cannot have a radio with dynamic range exceeding 100dB" THIS IS TOTALLY WRONG. The 75 dB dynamic range is signal power divided by the total noise power in a bandwidth of 125 MHz. Noise is proportional to the measurement bandwidth so when the digital signal is filtered to a bandwidth of 500 Hz, the noise power is reduced by a factor of 250000 which corresponds to 54 dB. That means that the dynamic range of an Have a look at this page: There is a problem on the part 2 of the blog (Entry #7): This video gives a nice illustration on sampling clock jitter (caused by poorly filtering on the oscillator.) https://www.youtube.com/watch?v=_30GIKI60Ug The data in table 1 here: Your texts need to be rewritten, you are misleading others. 73 Leif |
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Above is a reflection of my thought and only mine. But if you have any questions, feedback and/or suggestions, please send me an e-mail. I might reply on this page (sorry, no guarantee) |
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Copyright 2014, Tak Asami / W6Si & OSO Technology |