Re: [Discuss-gnuradio] offset at input of LF_RX

[Don Ward]

----- Original Message ----- 
From: "Matt Ettus" <address@hidden>
To: <address@hidden>
Cc: <address@hidden>
Sent: Thursday, June 21, 2007 9:33 PM
Subject: Re: [Discuss-gnuradio] offset at input of LF_RX


[snipped]

> Back to the subject of  what your signal generator is doing, I don't
> know.  If you measure the voltage on the sma connector with nothing
> connected, you will see that it is 0.  If you connect a 1 V source
> through a 50 ohm resistor, you will see that there is 0.5V at the
> connector.  If you connect -1 V through a 50 ohm resistor, you will see
> -0.5V at the connector.

That's not what I get.  With the input open, I measure 108 mV on the SMA 
connector and a DC component from the ADC (detected using the RSSI register, 
to avoid the DC offset filter in the FPGA).  If I short the input to ground, 
I get a negative DC component from the ADC.  To get zero from the ADC I need 
to ground the input through a 50 ohm (approximately) resistance; in this 
case I measure 62 mV at the input to the LFRX board.  My conclusion 
(confirmed by inspection of the schematic) is that the LFRX needs to be 
driven by a source resistance of 50 ohms *at DC* to be correctly biased.

-- Don W.