Re: [Discuss-gnuradio] frontend4702 schematic and layout

[Matt Ettus]

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:

Assuming that the output impedance of IFout1/IFout2 is 300 ohms differential,
then R21,22 and 23 cause unnecessary loss.  VINP/VINN have a 200 ohm input
impedance.  With R1 and R2 in series, that becomes 300 ohms.  The 300 ohms is
mirrored across the 1:1 xformer (T1), making it a good match without
R21/R22/R23, although there is some loss.

On the other hand, we need to make sure that IFout1/2 does not put out too high
a voltage, otherwise we'll need more loss.

The same goes for the secondary RF input.

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.

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?

I'd also repeat my pleas for allowing 2 of the 4702's to be hooked up.

> I have had to guess at the placement of the USRP interface connector and the
> mounting holes as I dont quite know where they are yet. When I do know where
> they are and I know that the schematic is satisfactory to use for gnuradio,
> then I can complete it and we can figure out how we wish to fab a few
> frontend daughterboards.

I'll send you the latest board layout.

>
> I am hopeful that this schematic and layout will be helpful in moving
> gnuradio along a little bit.

Thanks for your work.

Matt