[hpsdr] Neat front end from Phil

[Bob McGwier]

Phil:


Many op amps suffer from the "high noise at low gain" phenomenon.   We 
will hurt the system performance (overload the A/D's) if we require the 
very high gain needed to get low noise out of the SSM2019.    We will 
hurt the MDS if we put an attenuator after the detector and then 
immediately amplify it 40 dB to get low noise.

On the LT1128,  its problem is that its gain bandwidth (40 MHz) will 
hurt six meter performance.  Maybe we don't care?

Bob
N4HY



Phil Harman wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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
>
>
>
>
>
>   


-- 
Robert W. McGwier, Ph.D.
Center for Communications Research
805 Bunn Drive
Princeton, NJ 08540
(609)-924-4600
(sig required by employer)



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