Re: [Discuss-gnuradio] Questions about the 4937

[Eric Blossom]

On Sun, Mar 16, 2003 at 12:25:13PM +1100, Alan Gray wrote:
> Just a few questions about the Microtune 4937 module that seems to be 
> in general usage. 

[snip]

> 2) After noting that the eval board from Microtune is rather expensive, 
> it looks like I'll be building my own, I've taken a look at the 
> schematic provided and ran into the following problem: My A/D has an 
> input impedance of 500 ohm, not 50 ohm. I'm not thoroughly conversant 
> with the hardware just yet, but I'm thinking this might be a problem. 
> So, how much trouble and I going to get myself into trying to redesign 
> the schematic to allow for the differing input impedance. Is it as 
> simple as replacing the 50 ohm resistor on the output of the OPA691? 
> (hey, I gotta hope for an easy way out) or will I have to work on 
> something more complex?

You're probably going to be connecting the eval board to the A/D with
50 Ohm coax.  What I'd try first it to just connect a 50 Ohm resistor
between the A/D board's input pin and ground.  This will effectively
turn it into a 50 Ohm terminated input.

Also, after building a couple more boards using the schematic on the
Wiki, it turns out that the input impedance of the IF-AGC and RF-AGC
pins of the 4937 seems to vary quite a bit, and that the MAX518 DAC
can't reliably drive them directly.  You'll need to add emitter
followers to the DAC outputs to lower their output impedance.  I used
2N3904's with a 1K resistor to ground.

Eric