[hpsdr] Neat front end from Phil

Mon Jul 31 15:12:23 PDT 2006

Ray,

I forgot the draft layout. Here it is. The size is about 44mm x 42mm :
<http://kotisivu.dnainternet.net/ahti/sdr-1000/newmixbrd4.pdf>. If
this works, I will leave it permanently inside my SDR-1000.

73, Ahti OH2RZ

On 31/07/06, Ray Anderson <ray.anderson at xilinx.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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