----- Original Message ----- From: "Matt Ettus" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]> Cc: <discuss-gnuradio@gnu.org> 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. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
