[hpsdr] Neat front end from Phil

[KD5NWA]

Instrumentation amplifiers suffer from the same fate, you need to run them at 
high gain in order that their performance not degrade.

Since you will  most likely be running at a gain of 1 or close to that, has 
anyone considered making a discrete amplifier in order to keep the noise low.

On Monday 31 July 2006 11:14, Bob McGwier wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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

-- 

Cecil
KD5NWA
www.qrpradio.com www.hpsdr.com

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