RE: [Discuss-gnuradio] TVRx and BasicRx gain questions

[Kees Jeronimus]

Which can easily be seen if you realize that in ideal
Passive transformer no power is added or lost.

Remember : P = (U*U)/R

No let's use it for the transformer:

Pin =  (Uin*Uin)/200 
Pout = (Uout*Uout)/50

Passive no loss transformer:

Pin = Pout

So:

(Uin*Uin)/200 = (Uout*Uout)/50

<=>

(Uout*Uout) = (Uin*Uin) * (50/200) = (Uin*Uin)/4

So:

Uout = Uin/2

There you have the factor 0.5!

Kees (W1/PE1LDW)

-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of Matt Ettus
Sent: Sunday, March 08, 2009 3:22 AM
To: Sebastiaan Heunis
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] TVRx and BasicRx gain questions

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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