[hpsdr] Boat Anchors..PreAmps

[lstoskopf at cox.net]

I must say that a very smart guy involved in SDR once told me that ANYTHING you put in front of our quality AD or QSD will add some non-linearity.  Ferrites, capacitors, etc.  But, having blown two ADs by out of the blue lightning (one several miles away and one closer) when directly coupled to the antenna makes me willing to accept that minimal distortion.  Actually the second strike had a X2 pre-amplifier in front, but the AD was turned on.  Remember that the maximum input voltage is usually listed as some base amount plus Vd.  They may be safer turned on than off.

N0UU

---- Larry Gadallah <lgadallah at gmail.com> wrote: 
> I was reading Adam, VA7OJ's excellent reports on his NPR testing of
> various receivers including Mercury (see
> http://www.ab4oj.com/sdr/hpsdr/hpsdr_npr_va7grr.pdf) and I was
> intrigued by this comment:
> 
> "The theoretical maximum NPR for a 16-bit ADC is 85.4 dB, as compared
> to 74.01 dB for a 14-bit ADC.
> Thus, the HPSDR receiver exhibits a significant deviation from the
> theoretical maximum value (much
> more so than the Perseus, whose highest NPR value in my August 2011
> test was 75 dB at 5340 kHz.)
> 
> The RF preamplifier in the Mercury receiver is in the RF signal path
> at all times.  A 20 dB pad is switched
> in at the preamp input for the “Preamp out” function. Apparently, the
> noise loading provokes sufficient
> IMD in the preamp to degrade the NPR as much as 12 dB below the
> theoretical maximum value. (The
> optimum noise loading value decreases by 19 dB with the attenuator
> out, which reflects the inserted 20
> dB attenuation pretty closely. )"
> 
> I'm not sure I grok the meaning of "noise loading" in that comment,
> but I'm assuming that he is implying that the attenuator contributes
> enough noise to trigger some nonlinear operation of the preamp? This
> leads to the question: Is the current design of leaving the preamp in
> the circuit at all times and switching in/out an attenuator preventing
> Mercury from approaching the theoretical NPR capabilities of the
> LTC2208? Would it be better to completely switch the preamp in/out of
> circuit as needed?
> 
> Cheers,
> 
> On 12 April 2011 22:33, Lester Veenstra <lester at veenstras.com> wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > Quite correct, can be anywhere before the A/D. At RF or at baseband.  I suspect that for most of our applications, manual (end user controlled) will suffice. But having the FPGA calculate the number of clips would be useful (until reset) for the user to access.
> >
> >
> >
> > Lester B Veenstra  MØYCM K1YCM
> 
> -- 
> Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
> PGP Sig: B5F9 C4A8 8517 82AC 16B6  02B6 0645 69F0 1F29 A512


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