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 observing...
On Wednesday, July 30, 2014 4:22 PM, Marcus D. Leech <mleech@ripnet.com> wrote:
On 07/30/2014 04:14 PM, ematlis@yahoo.com 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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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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