[hpsdr] Neat front end from Phil

[Ray Anderson]

The LT1127 has a noise spec of 4.2nv/rtHz max while the LT1128 is rated
at 1.1nv/rtHz max. Not sure about the current noise. The difference in
noise could be significant in this application.


-Ray	WB6TPU


-----Original Message-----
From: hpsdr-bounces at hpsdr.org [mailto:hpsdr-bounces at hpsdr.org] On Behalf
Of Ahti Aintila
Sent: Monday, July 31, 2006 11:50 AM
To: kd5nwa at cox.net
Cc: hpsdr at hpsdr.org
Subject: Re: [hpsdr] Neat front end from Phil

***** High Performance Software Defined Radio Discussion List *****

Bob,

LT1127 has better gain-bandwidth (65MHz)  than LT1128, and still
available as a package of four opamps. If we accept two packages,
OPA1632 can do upto 180MHz.

On the other hand, as we are integrating the modulation bandwidth
(<200kHz), maybe, the high gain-bandwidth product is not necessary.
In any case, the RC product of the integrating circuit will limit the
bandwidth with either in  the "passive" integrator like the Tayloe
detector or the new "active" integrator, ISD. I'm possibly wrong, but
somebody else in the group may know this better.

73, Ahti OH2RZ

On 31/07/06, KD5NWA <kd5nwa at cox.net> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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
>
> "Life is short and I'm too busy tipping Sacred Cows." Don Seglio
Batuna
>
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