[hpsdr] Odyssey Siren Design

Tue Sep 26 21:42:40 PDT 2006

Hello Steve,

and thank you for your valuable comment.

1. In some TI application note OPA1632 was suggested as a buffer
before ADCs. According to my limited search the speed and noise
specifications look really good, the 1/f noise is lower than most of
the THS4XXX devices. Maybe, I missed some.

2. I wanted to use SPDT switch, as also suggested by Phil C, in order
to use direct I and Q clocking without 1-to-4 decoder as used in the
present SDR-1000 TRX board. Wider sampling pulses with proper
impedance match, according to my understanding, yield lower losses in
the integrating circuit. I admit that my supply voltage (3.3V)  may be
really above the recommended 3.6V maximum with strong signals. By all
means, use SN74LVC2G53 instead. That seems to be a good choice. For
improved dynamic range I suggest 5V supply and ±5V (or more) for
OPA1632.

3. Your "SN74CB3Q33253" must be a typo. Obviously you mean
SN74CB3Q3253? I don't like 90° sampling pulses as mentioned above.
With the 180° pulsing scheme the clock skew should not be a problem
within I and Q channels individually and any skew errors between I and
Q may be software corrected in the calibration process. Anyhow, if you
can find a DPDT switch that would be great.

73, Ahti OH2RZ

On 27/09/06, Steven Bible <n7hpr at tapr.org> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello Ahti,
>
> Joe and I have been studying your circuit for Project Odyssey.  I have a
> couple of questions:
>
> 1. What criteria did you use in selecting the OPA1632?  I've been reading
> the app notes that you recommended, and out of curiousity, searched the TI
> web site for full differential op amps to see what else is available.  I was
> surprised that TI's parametric search did not mention the OPA1632.  There
> were many THS4xxx devices that popped up.  I am sure that low noise is a
> major criteria.  I did not see any of the THS4xxx devices have any better
> noise than the OPA1632.  I was a little concerned that the OPA1632 was an
> expensive part (Digi-Key $3.94 qty 1), but when I saw the prices of the
> THS4xxx devices, it suddenly because less expensive :-)
>
> 2. I was studying the data sheet for the SN74AUC2G53.  I noted that the Vcc
> is to be 2.7V, but your schematic shows you are powering it at 3.3V? (the
> absolute maximum is 3.6V).  2.7V is not a problem I think for this circuit,
> but wanted to know your comments.  I also found the SN74LVC2G53 can work at
> higher voltage if needed.  I have not compared the data sheets yet.  I also
> found that sourcing the SN74AUC2G53 to be a little more difficult than the
> SN74LVC2G53.
>
> 3. I am a little concern with using two separate muxes and the possible
> clock skew between them.  I am sure that using two separate devices makes
> the PCB layout easier.  But I was thinking of using the SN74CB3Q33253 that
> was mentioned on the list (or was it the SoftRock40 list) to minimize my
> thinking of clock skew, but I wanted to get your thoughts.
>
> Thank you very much for sharing your circuit.  We hope to have it built in a
> couple of weeks for testing for Project Odyssey.
>
> 73,
>
> - Steve, N7HPR
>  (n7hpr at tapr.org)
>
>
>
>
> > -----Original Message-----
> > From: hpsdr-bounces at hpsdr.org [mailto:hpsdr-bounces at hpsdr.org]On Behalf
> > Of Ahti Aintila
> > Sent: Thursday, September 21, 2006 1:30 AM
> > To: Joseph Julicher
> > Cc: hpsdr at hpsdr.org
> > Subject: Re: [hpsdr] Odyssey Siren Design
> >
> >
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > Joe,
> >
> > The bias arrangement of the opamp OPA1632 may not be the best
> > regarding the power consumption. Due to the difference between the
> > reference voltages of the signal switches and the opamps there will be
> > some extra current drawn, if Vcom is forced to 2.5V. For safety, I
> > recommend the common reference voltage 1.65V as corrected in the
> > version "b": <http://kotisivu.dnainternet.net/ahti/sdr-1000/2rzisd-b.pdf>
> >
> > 73, Ahti OH2RZ
> >
> > On 20/09/06, Joseph Julicher <n9wxu at mac.com> wrote:
> > > very nice.
> > > That makes complete sense and answers all the issues.  I will use
> > > that in Siren.
> > >
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