LB3HC's hightech blog

Amateur radio, RF design, electronics, uC, software, Arduino, AVR, Antennas


BLF188XR HF amp project

I am now working on my latest NXP BLF188XR solid state amp project (spare time project). This new dual transistor package can tolerate approx 70:1 in SWR and can be fed by a 52V DC PSU. The design uses a copper heat spreader approach that is thermally coupled to a forced air cooled alu heatsink with large surface area. The output has dual broadband RF transformers and a current balun for unbalanced feed. The PSU is a power factor corrected switch mode supply that can supply 3KW continously. I will run it considerably below max output to increase MTBF (mean time between failure). The amp will in any case be run below legal limit.

The design features 1KW out on all bands to 6 meters, switched BPF, auto shutdown on overtemp, swr into BPF, swr after BPF and relay sequencing, variable speed fans on psu, low weight.

Below are some pictures of the project.

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Repair of LDG automatic antennatuner

I blasted my LDG antennatuner some time ago. Or …. I thought I blasted it….. It appeared that it was only the resistor in the SWR detector circuit that got burned out. I replaced that resistor and now its ok again.It was easy to repair. However these small LDG tuners dont take more than 100W max. The designers have used ferrite cores, whereas it would have been a much better idea to use carbonyl cores or air core inductors. The latter doesnt get so easily saturated.

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However I must say that the design of the LDG equipment I have seen so far is not very impressive. Why use that BIG chasis when you dont need it? Why use DB9 style connectors on a chassis that is supposed to be watertight? Look at that coax termination there. Both on the board and on the PL259 chassis connector. Why use RG174 teflon coax when you have such crappy terminaions? Perhaps it would be better with no coax at all :) However when the tuner works it works fairly OK. Just dont trust this kind of equipment in a contest or on a dx expedition.


3KW switchmode PSU with PFC for BLF188XR linear amplifier

I am currently on my sparetime working on a solid state linear amplifier with the BLF188XR transistor. For this I need a compact and stable PSU that can easily deliver the current that is required to drive the linear to legal limit out. Even if this PSU can deliver more than that, I plan on running it cool and with little heating.  Below I have posted some pictures of the little bigger  “Anderson Power Pole ” big brother connector. I use pretty fat copper wire to have no voltage drop. The PSU gives 50V out and was purcased as NOS. See another posting for pictures of the quite extensive PSU modifications that I had to do to make it work.

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RF current amperemeter with log detector

I have long had the need to measure antenna current. This can be done by a rf-current transformer and a rectifier made by a shottky diode or fast silicon diode and an integrator driving a mechanical instrument. However the dynamic range is very limited with such a setup.

Therefore I made a new rf-amperemeter design with a log detector chip from Analog Devices driven from a current transformer made out of a split core ferrite material that has response in the HF and lower VHF frequency range.The current trafo is terminated in a few ohms and the RF current conductor (actually the primary) sees only a fraction of an ohm so very little influence is done on the circuit you are measuring. (Apart from capacitive coupling and a slight leakage inductance from the current transformer windings/ferrite combination). The ferrite core is a split type and is epoxied to a clip.

I calibrated the instrument by terminating the generator in a fancooled 100W dummy load and measured the voltage over the load by an oscilloscope. Since it has a log output the dynamic range from milliamp range up to 1,4A RF current. That would peg a mechanical instrument if you would at the same time want to be able to detect a significantly lower RF-current without having to change scales with switches / pot meters etc.

Next I plan on making a OLED display on this design with an arduino controller. Its a sparetime project so lets see how long tiem it takes before I implement that.

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A MIL-SPEC roller inductor for QRO operation

I recently got hold of a motorized roller inductor that is made PROPERLY. This is a MIL-SPEC unit that has been on stock for many years. It features a silver foil that is rolled on to a ceramic former, pretension, a shortcut cylinder that prevents eddy currents and arcing on the unused coil section under QRO operation. Due to that the unused coil is shorted and that the shorting is not a single turn, that the foil is wide, this coil has a very high Q.



Single Sideband (SSB) modulation on the Raspberry PI

F5OEO has recently written some code to transmit a SSB signal using just the hardware in the Broadcom SOC chip on the Raspberry Pi.  You can find more info on this link

I tested the code on my RPI on 6m via a cable connection. It worked OK. If you look at the S-meter you can see that the envelope is constant. This is due to that the RPI has no way of modulating the envelope so the software actually modulates frequency. It is kind of constant envelope SSB.

Below you can see what the signal looks like. (This is received on a SDR via an attenuator). The signal is a bit too wide. However cool test.

If you want to test the code yourself you can check it out over on F5OEO at github

Warning: do not connect the RPI GPIO output running this code to an external antenna without a bandpass or lowpass filter and a valid amateur radio license. Never transmit any signal outside the amateur bands.

Posted: November 13th, 2015 under Amateur radio, Hacks, SDR. Tags: , , , ,
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RF screening modifications to the RTL-SDR dongle

The RTL-SDR 8 bit radio + ADC hardware is giving us an amazingly low cost way of realizing a SDR receiver. However, the unit is made for DVB-T transmissions and not for narrowband and low s/n scenarios. In a DVB-T reception scenario that normally means high signal to noise ratio and wide bandwidth signals so that a slight drift or some internal noise is not so important. When we use this radio for narrowband applications in amateur radio bands, we normally don’t have so high S/N ratio. That coupled with the fact that the RTL-SDR generate a lot of noise and surious emissions necessitates some pretty heavy modifications to get a reasonable performance. The below pictures shows what I did to do a proper modification of a RTL-SDR unit. 1) Internal screening on the RTL-SDR itself 2) External screening 3) Decoupling of the 5V line from the PC to the screened chassis. 4) using a proper RF connector (this one for HF use with a downconverter in front of the RTL hardware)

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RTL-SDR mods you can do in less than an hour (SMA, remove components)

I purchased two RTL-SDR dongles from Deal Extreme earlier. One of them I have converted to a HF SDR with a mixer and proper screened box. Yesterday I modded the other one. It will be used for VHF / UHF receiving in the amateur bands. Therefore I needed a better connector, as the one that is here from the factory is very bad. This took me less than an hour. The mod consisted of :

-Removing the unstandard connector that is used

-Removing the IR RX diode

-Removing the LED Diode

-Scraping off the solder mask on the sorroundings of the old “connector”

-Soldering in a new SMA connector

-Clipping off some plastic on the “chassis” to be able to run the SMA thru

Here are some pics:


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Posted: November 12th, 2015 under Amateur radio, Hacks, SDR. Tags: , , , , , ,
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Very big QRO rolling inductor

I just got myself a new roller inductor. This one is designed on the principle of a silver foil that is rolled away from and onto a ceramic form with guides for the silver form. The ceramic coil forms the coil. There is a large shorting cylinder that the unused silver foil is rolled onto. The effect of this is to significantly increase the Q of the inductor. For high power QRO applications there may either be arcing from the end of the unused part of the coil or heat loss in this part of the coil. How the unused coil is completely shorted with an inner conducting cylinder and the unused part of the coil has no flux thru it.





Roundup of SDR radios


Here is a review of SDR radios currently on the market in the low cost segment:

Posted: May 22nd, 2015 under Amateur radio, SDR. Tags: , , , ,
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