On 07/30/2014 04:14 PM, emat...@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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