Akademisk Radioklubb


Month: September 2017 (page 1 of 2)


Samfundet is currently closed down in preparation for UKA. In between the sound checks, rehearsals and other activities that occur in this hectic period, we managed to sneak in some contacts as LM100UKA in the CQ WW RTTY contest. We decided to run as Multi-Two to get the most out of the few timeslots when we could operate.

Henrik LB5DH working 20 m on Saturday.

One of the challenges we face when operating in contests from Samfundet is Electro Magnetic Interference (EMI) to the other groups at Samfundet. Particularly when operating legal limit on the low bands, audio and lighting equipment tend to get unwanted interference. Normally we try to locate the weak spots and seal them with ferrites and filters, but during UKA there is a lot of new equipment so it is hard to get to all the problems in time. The solution is often to cooperate with the groups that are responsible for audio and visuals and coordinate so that we do not transmit high power levels on the low bands while they are working.

Conditions on 20 m was also good on Sunday.

The contest was also our first good test of the Flex 6500 for RTTY. Thanks to the panadapter, finding an open frequency to work was a breeze. It also allowed us to quickly diagnose that we had set the wrong tone spacing (oops). We had a bit of trouble setting up SmartSDR to talk to MMTTY through N1MM+, but in the end we got it working through the help from some good resources online. What was not so great was setting RIT/XIT using mouse and keyboard. Hopefully our Maestro will arrive soon to address this problem.

QSOs per hour. Orange: 15 m, Red: 20 m and Blue: 40 m.

The QSO-rate was largely influenced by when we had a chance to operate in between different sound checks. As the graph shows, this was mostly during the afternoon on Sunday. Conditions were ok, but only one band was significantly open at a given time, typically either 20 m or 40 m.

Score and multipliers per band

We got 657 QSOs and 107 mults over the weekend. A good result considering the operating conditions, both on the air and at Samfundet. Thanks to LB5QG, LB1MH, LB5DH, LB7JG and LA3WUA for their operating efforts.

Repairing broken receiver on a USRP N210 WBX40 daughterboard

Kimmo Kansanen at NTNU recently donated some USRP N210 units they no longer had a use for. We have started to use them for various communications experiments, for example estimating the antenna patterns of our VHF/UHF antennas against the LA2VHF and LA2UHF beacons.

While operating one of the N210s we were sudddenly unable to receive the LA2VHF beacon that had previously been easily decodeable. We also saw that the noise floor had increased by 20 dB. Jens, LB6RH, decided to investigate matters further.

Inside the N210 a WBX-40 daughtercard provides the RF-frontend. The device functions as normal, except for in receive mode. The receiver section of the WBX-40 should be a good place to start looking.

The WBX-40 daughterboard after removing coaxial patches that attach to the front panel of the N210.

To further investigate what may be wrong we started investigating the board for any obvious short circuited connections. We were unable to find any such sources. The next step was to probe around the board with a multimeter to check the voltages being generated from the different voltage regulators on the board. Since Ettus Research provides the schematic for the WBX-40 online this process was greatly simplified.

LB6RH found that the 3.3 V rail on the output of voltage regulator U308 was only at 1.2 V. Suspecting that something was wrong with U308, he removed the component and attached a laboratory power-supply to the 3.3 V output pin. After turning on the supply (with the current limit set low, to avoid frying the circuit) he noticed that there was still a short circuit. To identify which component caused the short we borrowed a FLIR thermal imaging camera to check what components got hot when we turned up the current limit on the power-supply.

Unfortunately we did not get a good picture of the thermal test, but the chip that got hottest was the Low Noise Amplifier (LNA) U313. Since our problems are related to poor reception we thought this might be a likely candidate. After removing U313 we turned the power-supply back on, and saw that there was no longer a short, hurray!

A closeup after removing components U313 and U308.

We ordered new components for U308 and U313 from Digikey, and soldered them back in place.

U308 – Analog Devices Inc. ADP3336ARMZ linear regulator

U313 – Broadcom Limited MGA-82563-TR1G MMIC broadband LNA 

Replacements for U308 and U313 have arrived.

The soldering battlestation. A microscope helps when soldering small parts.














After replacing the two parts we connected everything back and tried powering on the device again.

We are now able to receive LA2VHF again!

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