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Re: [Discuss-gnuradio] frontend4702 schematic and layout

From: cfk
Subject: Re: [Discuss-gnuradio] frontend4702 schematic and layout
Date: Mon, 23 Feb 2004 20:35:06 -0800

Dear Matt:

I got the mechanical drawing and your note. I completely understand and
respect your IP wishes, no problem.

> Assuming that the output impedance of IFout1/IFout2 is 300 ohms
> 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
> 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
> a voltage, otherwise we'll need more loss.

I'm really looking for guidance here as I havent used the Microtune device
before. The reason I put the 5 resistors and 2 capacitors was to match your
schematic on the Wiki. In looking through the Microtune data sheet, I am
guessing it is a 300ohm output impedance based on their test circuit in
section 6. Thats also where I got the 680ohm resistors from. Certainly we
can not stuff (DNI, or DoNotInstall) some of the resistors/capacitors on
ifout1 & ifout2 coming from the 4702 tuner until we figure out its proper
care and feeding. We can also put zero ohm resistors or change to other
values for the same reason.

Perhaps someone else might have an opinion on the newsgroup regarding a
better guess at what the first cut of this daughterboard should have between
the ifout1/ifout2 of the Microtune 4702 and the input to the USRP.

> I don't know enough about I2C, but I don't think R5 and R6 are necessary
> we'll have the appropriate 2.2K Rs on the main board.

Since there are 2.2K pullups on SCL & SDA on the USRP, I'll remove them from
the daughterboard. Your're right in that only one set of pullups is needed
for IIC. I've done several IIC projects over the years.

> I'm not sure of the purpose of the Q2 and Q3 structures you put around the
> They'll allow you to put out more current, but they reduce your
> 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 only reason I put Q2/Q3 was to match your original schematic. I havent
used the MAX518 before. The last time I did an IIC DAC I used the Philips

I went back and looked at the MAX518 data sheet and it is a 5volt part. I am
suspecting that the IIC levels you are using are 3.3V levels coming from the
USRP. I havent tried mixing 5V parts and 3.3V parts on the same IIC
interface before. I suppose I could put a resistor jumper option so we can
run the MAX518 from 3.3V if 5V doesnt IIC properly. Let me draw that up over
the next few days and submit it to you. Please, lets go over this schematic
and layout until you are satisfied that it should work fine. I absolutely do
not mind redoing this until you are happy.

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

I shall be done. I just didnt get it before. Sorry for my denseness. With
two 4702's then we get I/Q for free, right?

For the sake of argument, lets assume the daughter board is in the upper
left slot. The one where the interface connector is J668. On the layout, the
4702 is a through-hole part and shouldn't be over the top of the USRP
interface connector. What I am currently thinking is to rotate the 4702's so
that they come out the left side of the board. I would need to put one above
the USRP interface connector and one below the USRP interface connector.
Each 4702 is a littel over half an inch or about 12.8mm. It is a really
tight fit to put two above the interface connector. The rest of the parts
can basically fit anywhere, with the proviso that the digital stuff needs to
be as far away from the analog input to the USRP as possible.

If I do this, it would mean making the daughterboard a little wider then
55mm to allow the mounting tabs on the ends of the 4702 to have some board
to fold around. I actually measure the 4702 at 61mm and change. There would
then be two of them as you had described. One would feed the A input and the
other would feed the B input.

I think that a single size Rx board would hold two 4702's that way. Perhaps
you can let me know if this causes you any consternation by my being a
little fast and loose with the size of the daughterboard and making it 65mm
hi by 61.75mm wide instead of 65mm hi by 55mm wide.

I'll be working at home tomorrow and will try to fit my mind around the
geometry a little bit more. For now, I need to go sit next to W1FE while she
watches the Chinese news from Taiwan. I dont speak but a few words of
Chinese, being from Pasadena myself, but it is politic to sit together this
time of the evening.

Charles Krinke, WA6LWB

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