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[Discuss-gnuradio] increasing output power from lftx with resistor mod g

From: ematlis
Subject: [Discuss-gnuradio] increasing output power from lftx with resistor mod gives saturated output
Date: Wed, 30 Jul 2014 12:38:28 -0700


I have attempted to increase the output power from my Rev 2.2 LFTX as per 
instructions from Matt Ettus on this list dated Aug 3 2013 in which he suggests 
reducing the values of resistors R3 and R5, but I am seeing a clipped output at 
peak values much less than expected.  

Using the ettus.com LFTX schematic as a guide, I left the resistors on the 
board untouched, but soldered wires to either side of R3 and R5 to add 
additional resistors in parallel.  My first attempt was with resistors of value 
220 ohms; I figured this would about halve the value at R3 and R5 which are 
originally 225 ohms.  I do see some increase in output power, but very minimal. 
 Specifically, whereas before I was observing 1 V P-P, or about 2.5 milli-watts 
on an oscilloscope that is 50-ohm DC coupled,  I am now seeing 1.34 V P-P, or 
about 4.6 milli-watts with a 1 kHz sine-wave.  To obtain this without clipping 
I had to reduce the amplitude on the USRP sink block from 1 to about .77, 
othwerwise the tops of the sinusoids flatten anywhere above the 1.34 V P-P 
level.  So I appear to be saturated. 

I am using a USRP1 Rev. 3.  I verified that the LFTX output op-amp in question 
is supplied with +/- 3.3 volts, so I don't see why it is clipping at half of 

I am hoping to use the LFTX in an HF Ham radio application.  I have 
mini-circuits ZHL-1A TX pre-amplifier for this purpose, but was hoping to 
obtain a little more output from the LFTX than I am currently getting to drive 
the ZHL-1A.  Any thoughts?



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