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Re: [Discuss-gnuradio] Questions about the 4937
From: |
Eric Blossom |
Subject: |
Re: [Discuss-gnuradio] Questions about the 4937 |
Date: |
Sat, 15 Mar 2003 18:28:55 -0800 |
User-agent: |
Mutt/1.4i |
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