[hpsdr] Analog Devices note on protecting AD

[John Laur]

The process gain from doubling the samplerate is ~3dB SNR in absense
of other factors; roughly equivalent to 1/2 bit of ADC resolution. SNR
= 6.02 * [Bits] + 1.77 + 10 * log( [Oversampling ratio] )

You would be hard pressed to find a real-world HF signal where this
would make a difference though; it is my opinion that both of the
receiver chains have exceeded the application requirement by enough
margin that the only way to effectively differentiate them is to make
measurements that have little similarity to real world situations.
Copying CW at 15dB below the atmospheric noise is about the human
limit. maybe 26dB down for the weakest JT modes. If we can get the
atmospheric noise into the ADC at 30dB above its minimum sensitivity
without the preamp/driver noise overwhelming it, there is simply not
much else that is required.

There is a lot of low hanging fruit to focus on other than the receiver specs.

73, John K5IT

On Thu, May 21, 2015 at 1:43 PM, Terry Fox <tfox at knology.net> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> The other thing is that the Flex 6700 & 6500 have twice the sampling rate as
> HPSDR hardware (IIRC).  The additional gain from the added decimation
> (assuming apples-to-apples ending rates) might account for a little better
> performance.
> 73, Terry, N4TLF (formally WB4JFI)
>
>
> -----Original Message----- From: Helmut
> Sent: Thursday, May 21, 2015 2:09 PM
> To: lstoskopf at cox.net ; hpsdr at lists.openhpsdr.org
> Subject: Re: [hpsdr] Analog Devices note on protecting AD
>
>
> ***** High Performance Software Defined Radio Discussion List *****
>
> Don't worry. The 16 Bit LTC is even slightly better than the 16 Bit AD and
> shows a bit higher SFDR. You have to compare the ENOB (effective number of
> bits).
> I believe ADC protection of the HPSDR board is quite sufficient. BTW there
> was a nice article in MWRF titled 'Limiters Protect ADCs without Adding
> Harmonics'.
>
> 73, DC6NY
>
> -----Ursprüngliche Nachricht-----
> Von: Hpsdr [mailto:hpsdr-bounces at lists.openhpsdr.org] Im Auftrag von
> lstoskopf at cox.net
> Gesendet: Donnerstag, 21. Mai 2015 18:55
> An: hpsdr at lists.openhpsdr.org
> Betreff: [hpsdr] Analog Devices note on protecting AD
>
> ***** High Performance Software Defined Radio Discussion List *****
>
> analog dialog Volume 49 – April 2015
>
> Had a great time at Hamvention and enjoyed visiting at the booth and the
> banquet.  Just wish that Scotty had handed me my new board for the HF2 and
> RF1!
>
> Had an interesting discussion with a proud Flex owner on how much superior
> their 16 bit AD chip with front end amplifier feedback is compared to the
> one we are using.  Having popped the AD in my QS1R from a few miles away
> lightning strike I see at least that value in having a cheaper device with
> some protection in front.  So I added one of the Clifton Labs exterior amps
> thinking that that would add protection.  Popped another AD.  Guessing that
> the spec sheet indicates a small diode drop PLUS VCC safety margin.  Had the
> preamp on and not the QS1R so the preamp possibly hit the AD with full VCC
> output.  Now I turn on the RX before the preamp.
>
> Anyway, the above article is of interest.  As for the superiority of the
> Flex Analog device to ours????? Are 16 bits 16 bits?
>
> Last year at the RSGB convention I had a short chat with Peter Chadwick
> about his old article on the topic of "are our receivers good enough?"  I
> have a complete HB transceiver built with the Plessey 1600 series devices
> and great crystal filters using devices for which he was an application
> engineer.  It hasn't been on the air for 30 years.  I need to drag it out
> and see how it still stacks up.
>
> N0UU
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