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Re: [Discuss-gnuradio] increasing output power from lftx with resistor m

From: ematlis
Subject: Re: [Discuss-gnuradio] increasing output power from lftx with resistor mod gives saturated output
Date: Wed, 30 Jul 2014 14:01:29 -0700

According to the AD8047 datasheet, the output voltage range is +/- 3 volts into 
a 150 ohm load.  That works out I believe to 30 milli-watts of output.  So it 
should be able to sink sufficient current to do better than what I'm 

On Wednesday, July 30, 2014 4:22 PM, Marcus D. Leech <address@hidden> wrote:
On 07/30/2014 04:14 PM, address@hidden wrote:

> Hello-
> 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 this.
> 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?
> Thanks!
> Eric
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> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
I would look at the datasheet for the driver amp.   But +6dBm is 
pretty-close to the max output of the DAC as I recall, so you might be 
better to just
   put an interstitial amplifier in place.

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