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Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements

From: mleech
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Mon, 25 Aug 2014 10:42:13 -0400
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-8dBm may, or may not, have damaged either of the the two LNA stages on the WBX.


"Modern" WBXen have TVS and limiter diodes in front of both stages to try to ameliorate laboratory accident consequences, but they offer no firm guarantees.


Nearly all of the receivers on USRP daughtercards are designed to be connected to an antenna, which means a low-noise stage "up front".  This means that using them in the lab requires some prudence and caution, such as putting a 30dB attenuator in front of the device before connecting it to a laboratory signal generator.  There are several orders-of-magnitude difference between the average power levels you'd expect from an antenna, and what you'd typically be trying in the lab, so when you're working in the lab, padding the input with 30dB of attenuation is just a really prudent practice.


Modern RF LNA transistors are, for the most part, "precious little princesses".  They have gate regions that are only a few molecules thick, and "high" currents flowing in these areas will cause holes to form in the gate layer, which will quickly affect gain and noise figure, and then lead to complete device failure.  You can put clamping diodes in front (as Ettus now does on its xBX daughtercard series), but such devices cannot protect in all cases, and they compromise ultimate noise figure and gain a little bit.


Thus, ahem, endeth the sermon of the day.  We turn now to the hymn book "Glory unto GaAs".  :) :)





On 2014-08-25 10:23, madengr wrote:

1)  What you are doing is correct.  What's the power of your signal
generator?  I was using -40 dBm, and as Marcus says, that is strong, but
you'll need to have a good SNR since the noise bandwidth is ~250 kHz. 
Several posts ago you stated running -8 dBm into the WBX.  You may have
damaged the WBX, since that 30 dB difference is about the isolation you may
get with a damaged amp.  I suggest sniffing down the chain of the WBX with a
spectrum analyzer, make sure the two amps are working.  Also as Marcus said,
you need to be using the same bandwidth and decimation, or that will change
the calibration factor.

I choose 250 kHz wince I was working with WBFM channels.

2) If the RX gain is changed, then the k must change to compensate.  See my
*.grc I linked in the last post.  I assigned RX gain to a QT slider then
added it to K.  The RX gain can be changed and the RX power stays about the

3)  I'm on travel now so I don't have access to my computer.  You ought to
update to GR 3.7; lot's of nice features, and it also preserves the flow
diagram even if the blocks of changed.


Gayathri Ramasubramanian wrote
Hi Thank you for your note. My questions here are just based on my previous measurements and your last mail. kindly clarify the same. 1) Is it possible that different USRPN210 devices with WBX boards have the different calibration factors. I set the channel gain to 0 and still get : @ 400 MHz : k = -62.8 @ 900 MHz : k = -58.5 @ 1800 MHZ: k = -52.5 I checked the above with 3 USRPN210 +WBX devices and found the K value to be very close to the stated values. Your values seem to be different , Hence the question. My method was that I first ran the flow graph with channel gain as 0 and k as 0. Then found the power value displayed for the 'channel_power_dBm'. I subtracted this value from my input signal value (from signal generator) and found out the difference. This is what I am taking to be the 'k' value. If I use k values (62.5 for 400 MHz) it in the block too, it give the 'channel_power_dBm' value to be very close to the input signal generator value. I repeated this for 3 devices at the 3 frequencies. Is this correct or am I doing something wrong. As my values seem to be almost 30 ~ 33 higher than ones you are getting from your tests. what could be causing this error/ discrepancy. 2) In you mails you say that we have to subtract RX gain from Calibration factor to baseline it for RX gain = 0 dB . Also in one of your previous mails you had sated that you get k = -34.5 wih RX gain of 15. So in view of these two stmts, was your 'k' value of -34.5 for 1800 MHz. This 1800 MHz is the only one satisfying this rule with previous data set: i.e *-34.5 - 15 = 19.5 *( this is close to the 1800 MHz results you have written about in your last email). I just want to check if my understanding is correct and Im doing the math right. 3) Could you kindly paste a picture of your flow graph for spectrum calibration along with the xml file like last time. Using the xml to know which bock is specified is a little tough. Having a visual flow graph along with the xml would be more effective to understand and relate between them. Please clarify above. Look forward to your response. Thanks Gayathri

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