[hpsdr] Neat front end from Phil

[Phil Harman]

Phil's front end does look very nice and it will be interesting to see the 
performance figures.

Just a cavet when selecting amplifiers for this type of circuit. There are 
actually two noise sources that need to be considered  - the amplifier 
current noise and amplifier voltage noise.

Not taking the current noise  and source impedance into account can lead to 
the wrong conclusions.

Based in voltage noise one would select the SSM2019  over the OPA227 . 
However, if you do the math and include both noise sources and the effect of 
the source impedance then this what happens

      Source Z (ohms) SSM2019 NF (dB) OPA227 NF (dB)
      10 4.2 8.7
      50 1.7 5.4
      100 1.1 4
      150 0.8 3.3
      300 0.5 2.2
      500 0.5 1.6
      1000 0.6 1
      2000 0.9 0.6
      5000 1.7 0.3
      10000 2.7 0.3


As you can see the even though the OPA227 has 3 times the voltage noise of 
the SSM2019 it also has 1/5 the noise current so wins with high Z sources.

At low Z voltage noise dominates so we need an opamp with low voltage noise. 
At high Z current noise dominates so we select one with low current noise.

I suggest that we need to look for an opamp designed to be fed from a low Z, 
like a microphone preamp, for this application.  Also using the device in an 
non-inverting mode can have a significant impact on the system noise figure.

Given the above the LT1128 may perform better than the OPA1632.


73's Phil...VK6APH





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