[hpsdr] some comments

[KA2WEU at aol.com]

About the Dynamic Range of  Receivers 


The radio's dynamic range for single tone  signals is affected by the 
automatic gain control range (about 120 dB,  independant of ADC's dynamic range, even 
possible with an 8-bit  ADC). 
It has nothing to do with the instantaneous dynamic  range, which is defining 
the radio's capabilty to discern a weak signal (near  the noise floor) in the 
presence of a much larger signal. The maximum possible  ratio of the two 
signal's levels is called instantaneous dynamic  range). 
The overall instantaneous dynamic range of the receiver  is not limited by 
the nyquist dynamic range of the 16-Bit ADC (about 95 dB). DSP  is decimating 
the ADC sampling rate from 100 kS/sec down to rates appropriate to  the actual 
CW / SSB signal bandwidths of 300 / 3000 Hz (meaning  Fs= 781 / 12.5 kS/sec). 
Noise coming from ADC (including the  quantization noise) is reduced by just 
the ratio of ADC's nyquist bandwidth (=50  kHz) to the signals bandwidth, thus 
giving a corresponding gain in dynamic range  of  

Together  we are getting a total dynamic rangeADC of about 117 dB [CW] and 
107  dB [SSB];. 
[Note, that the instantaneous dynamic is not only limited  by ADC' noise 
behaviour, but also by the synthesizer's phase noise  characteristic; the lower 
the signal bandwidth, the more impact the synthesizer  will have on the dynamic] 
The behavior of a real radio exposed to large signals  spreaded over several 
overcrowded broadcast bands is characterized best with a  measure for a multi 
tone dynamic range like the two  tone intermodulation free dynamic range. This 
is indirectly affected by  the radio's noise figure and the well known two 
tone intermodulation's intercept  point (of second and third order), IP2 / IP3. 
Two tone intermodulation distortions are measured for the  two unwanted 
signals being signal out of band or being both in band to separate  the behaviour 
of the radio's RF parts on the one hand and that of the IF/AF  parts on the 
other hand. 
Care must be taken to distinguish between the  intermodulation distortions of 
the IF path and the  intermodulation / harmonic distortions of the audio path 
 ! 
In band intermodulation behaviour may be influenced by  the automatic gain 
control's state and thus be nonlinearly dependent on input  level.
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