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From: | Matt Ettus |
Subject: | Re: [Discuss-gnuradio] TVRx and BasicRx gain questions |
Date: | Sat, 07 Mar 2009 23:21:34 -0800 |
User-agent: | Thunderbird 2.0.0.19 (X11/20090105) |
Sebastiaan Heunis wrote:
Matt Sorry to bother. Please tell me why I lose 6dB when measuring the signal at pins 17 and 18? I have the RF transformer from Minicircuits that is supposed to match the 200ohm output of the TVRX to a 50ohm so that I can measure it on a spectrum analyser. Because it is matched, I should not have a 6dB loss?
While the transformer does not lose power, you have to understand that the ADC is a voltage input device. A transformer that converts 200 ohms to 50 ohms divides the voltage in half. When we say that the full scale level is 10 dBm, it actually means 2V peak to peak. Once you put it into the transformer, you'll only have 1V peak to peak.
> I believe the input impedance of the ADC is around 220ohms? How does
the BasicRx work? The input is 50ohms, so is there an RF transformer on the BasicRx that matches this to the ADC?
It is a voltage input. We match the input with a 50 ohm terminating resistor.
Matt
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