On 5/19/2012 8:08 PM, wb4...@knology.net wrote:
> Joe, I'm not sure some of your suppositions are correct.
Actually it depends on one's assumption of the number and strength
of the signals applied to the front end. 120 dB of spurious free
dynamic range is trivial if one is measuring it with *two tones*.
However, when one has five or six -33 dBm "local signals" (easy
on 160 or 80 meters during a major contest ... or 40 meters in
Europe with the superpower broadcasters just outside the band)
the required signal handling is many orders of magnitude higher
than just two -23 dBm (S9+60 dB) signals. Instantaneous peak
voltages do not add linearly ... they add exponentially.
Even with decimation the peak voltage to the ADC can not be allowed
to cause an overflow. In addition, one must be concerned about Herr
Nyquist ... and minimum sample rates for the highest operating
frequency.
73,
... Joe, W4TV
On 5/19/2012 8:08 PM, wb4...@knology.net wrote:
> Joe, I'm not sure some of your suppositions are correct.
>
> First of all, the ADC samples at a significantly higher rate than the
> incoming RF (for HF), at 245.76MHz. Then, those samples go through
> decimation inside Digital DownConverter (DDC) in the FPGA, and the
> higher the decimation, the more the processing gain, and therefore added
> "bits". I believe the typical assumption is that you gain the equivalent
> of approx. one-half bit (3dB) of dynamic range every time you decimate
> (divide) the sample rate by two. This is real gain, not some imaginary
> trick or magic. If you start with a 245.76MHz sample rate, and end up
> with 240kHz (example), you could achieve a processing gain of an
> additional five bits, or 30dB. If my quick math is correct. All without
> AGC or other games.
>
> Throw away at least one A/D bit, so 15 bits(ADC) plus five bits(proc
> gain) yields 20 bits, or about 120dB of dynamic range. I must be a
> little off, as I saw one note on the Flex reflector that they think
> 153dB or so of dynamic range. Maybe by lowering the final sample rate
> some more, or not tossing all of one bit of the ADC. I believe Mitola
> suggested that 130dB of dynamic range is adequate for HF, I'm not sure
> if that includes noise from storms, etc. BTW, the new software is called
> SmartSDR.
>
> I have been playing with DDC receivers for a couple of years now, and I
> firmly believe they will be the future. I suggest that thorough research
> of DDC-based receiver design will alter your perception. It did mine.
>
> I love my K3/P3, and do not plan to replace it anytime soon. It just
> works GREAT, and is a nice compact box without requiring a computer to
> use. But, I was very tempted to by a Flex 6500. That is, until I saw the
> $200 per year charge for the software. Technically, very leading edge.
> Marketing, you decide.
>
> Hope this isn't too off-topic.
> 73,
> Terry, WB4JFI
>
>
>
> -----Original Message----- From: Joe Subich, W4TV
> Sent: Saturday, May 19, 2012 7:29 PM
> To: elecraft@mailman.qth.net
> Subject: Re: [Elecraft] OT: Dayton New Equipment Show
>
>
> The Flex 6000/6700 is an "on frequency" direct digital conversion
> radio - that is a 16 bit analog to digital converter operating at
> the "front end" of the radio with no intermediate conversion/IF
> and filtering stages. In addition, all of the DSP is contained
> in the radio (no more PowerSRD). The new software for the Flex
> 6000/6700 (reportedly "PowerRX") is essentially a "glass control
> panel").
>
> A general purpose computer is required for PowerRX but communication
> between the computer and transceiver is via TCP/IP which means the
> computer can be anywhere. Audio input and output will be available
> both on the transceiver and from PowerRX (interesting for remote
> operation scenarios).
>
> The issue will be the severely limited dynamic range of the 16 bit
> ADC. Without effective AGC ahead of the ADC there *will* be overload
> problems on crowded bands (e.g. 160/80/40) where any transceiver must
> deal with many extremely strong ("local") signals while maintaining
> maximum sensitivity. With AGC, the strong local signals will cause
> "blocking" as the gain is reduced to prevent overflow of the ADC.
>
> The new Flex design is certainly "interesting" but may not truly be
> ready for critical "real world" use with the widely varying signal
> levels in amateur service.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 5/19/2012 3:43 PM, Tony Estep wrote:
>> On Sat, May 19, 2012 at 11:53 AM, Edward R.
>> Cole<kl...@acsalaska.net>wrote:
>>
>>> ...website now shows the Flex-6000/6700 newest entry into the SDR
>>> market....
>>
>> ===========
>> Said to be digital from antenna connector to output. Their hardware has
>> long needed updating, so perhaps this will be it. However, Flex's really
>> weak link has always been their software/systems integration, so it
>> remains
>> to be seen if this does anything to mitigate that.
>>
>> Tony KT0NY
>>
>>
>>
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