[Elecraft] Analog vs. Digital Front Ends

[Joe Subich, W4TV]

On 9/16/2015 12:43 AM, Alan wrote:
> So how long before ADC technology catches up to the K3? If Moore's
> law applied (doubling of performance every couple years) it wouldn't
> be long. Unfortunately Moore's law applies mainly to digital
> circuitry but the key parts of an ADC are analog (the "A" in "ADC").
> Unless there is a big theoretical breakthrough in ADC architecture, I
> think we still have some years to wait.

The question really boils down to when will we see affordable high
voltage ADCs.  The problem is that every signal in the input passband
is in series and the [peak] voltage adds as a vector sum.  So if one
S9+63dB (-10 dBm) signal causes the ADC to overflow (consistent with
the noise loading limit in the AB4OJ/VA7OJ tests of the Flex-6000
series) the ADC limit is 0.1V (peak) at the antenna.  Spread that 0.1V
across multiple [equal strength] signals and you see the following:

      N      "S"
----------------------------------
      1     S9 +63 dB (-10 dBm)
      3     S9 +53 dB
     10     S9 +43 dB
     32     S9 +33 dB
    100     S9 +23 dB
    316     S9 +13 dB
   ~450     S9 +10 dB
   1000     S9  +3 dB
  ~1400     S9  +0 dB (-73 dBm)

Since it is the instantaneous peaks that cause problems, increasing the 
number of signals decreases the frequency of the ADC overflows.  There
is certainly analysis that can be done to compute the probability of a
peak given a specific number of signals and frequency distribution but
my best guess is that the number of signals involved will be somewhere
between 10 and 100.  Of course, if one of those signals is very strong
- a 1.5 KW neighbor 0.5 miles away or a broadcast station that puts
a few volts on any wire in the air - it does not take many (if any)
additional S9+10 dB signals to push the system over the edge.

73,

   ... Joe, W4TV