Re: [Discuss-gnuradio] frontend4702 schematic and layout

[Eric Blossom]

On Mon, Feb 23, 2004 at 03:36:21PM -0800, Matt Ettus wrote:
> Quoting cfk <address@hidden>:
> 
> > Dear All;
> >
> > I have a preliminary schematic and layout on my web site and have added the
> > links to the wiki. See the recent changes uner RfFrontEnd for the files at
> > http://home.pacbell.net/cfk/frontend4702.pdf and its cousins.
> 
> Great work.  A couple of comments:
> 
> 
> I don't know enough about I2C, but I don't think R5 and R6 are necessary since
> we'll have the appropriate 2.2K Rs on the main board.

R5 and R6 are not necessary.


> I'm not sure of the purpose of the Q2 and Q3 structures you put around the 
> DAC. 
> They'll allow you to put out more current, but they reduce your already-low
> 3.3V output from the MAX518 to 2.6V.  The AGC needs 4V according to the
> datasheet.  Is there a 5V version of the MAX518?

The MAX518 needs a +5V supply.  Matt and are currently discussing the
right way to do this.  In the mean while, you should probably use the
+5V from the connector.  Ooops.... using +5 will mean that you won't
meet the SDA/SCL VIH_lo spec (0.7 VDD == 3.5V).  Hmmm, looks like
this needs a different solution.

Maybe you could run a single supply op-amp configured with a
non-inverting gain of 2?  Put a small linear regulator on board to
generate, say 9V, from your 33V supply.  Then use one of the aux
ADC outputs available on the connector as the control voltage for the
input of the op-amp.

I'd also add some bulk bypassing for the +5V.  We don't want your
oscillator feeding back to the other boards.

A bit of commentary on power supply matters:

We're trying to avoid having complicated power supply requirements.
The motherboard needs 3.3V and 1.5V.  The daughterboards Matt's
working on will run OK on 3.3V also.  We'd like to get by with a
single 2.5A 5V universal switching wall wart.  We could run a higher
input voltage, but then we'd just be burning more in the linear
regulators on the motherboard.  To generate regulated +5V on a
daughterboard, low-drop-out regulators require at least 5.4V on
their input.  Finding a 6V output, universal input, low cost
switcher is not-easy.  We're also looking at just using a +5V output
universal input switcher, and counting on folks to use the supply that
we provide.  That is, there's no head room for a +5V LDO, so the wall
modules really needs to be at +5V with reasonably low noise.  If you
plug in something above +5V, you cook any daughterboards expecting 5V
+/- 10%.  Not a pretty scenario.  Samples are on the way ;-)

Eric