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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
observing...
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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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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