Amateur radio, RF design, electronics, uC, software, Arduino, AVR, Antennas
The Red Pitaya SDR board is based on the Xilinx Zync SOC and has 14 bit external A/D converters. However, for SDR usage on the HF bands from 0.1-30 MHz (and for that matter up to 50 MHz) the Red Pitaya is a bit “deaf” in the stock configuration. I have made a broadband amplifier that has a fairly high gain and very good IIP3 properties. Below I have posed some pictures of the prototype amplifier.
This is the prototype amplifier. I inserted a ferrite ring on the input lead to roll off the VHF / UHF sensitivity to reduce problems with nearby broadcasters etc. There is a also a PI network attenuator on the ouput and I have inserted a couple of beads in that as well to roll of the outpu response when frequency increases. The other components in the lower part is a input pi attenuator I used when I did some VNA frequency response measurements. This as well as the RCA plus is not used (RCA plugs are surprisingly good for low level RF signal routing in the HF bands and nice to use in the lab)
Here I used a more professional attenuator with a large attenuation range and flat response to determine the proper attenuation level after the preamp into the Red Pitaya. Reducing gain after the first amplifier has very little effect on the noise figure. Reducing it before the first amplifier directly adds to the noise figure.
For upcoming SDR projects I have acquired a Red Pitaya board. This hardware is the first low cost RX / TX capable SDR hw to come onto the market that is open source and can match the Ettus Research USRP periperhal. It has a combined CPU and FPGA signal chain with two channels 14 bit 125 MSPS A/D and D/A. It also has a Dual core ARM Cortex A9+ FPGA (Xilinx Zynq 7010 system on chip). Only a few years ago this caliber of hardware had to be custom designed and was typically used in radar antijamming systems, radar signature classification systems, ultrasound, sonar and in high end vibration analysis tools (as examples). The ARM CPU on board can run Linux and it has GNU-Radio support. For fast data transfer there is a GBE interface to other host systems. With a a RTOS on the ARM core or a zero copy IP stack under Linux it should be possible to approach fairly close to 1 Gbit/sek transfer rates to host systems (if needed).
I purchased a couple of Anytone AT-5189 4m FM radios at a flea market. As the radio is of Chinese origin I was interested in seeing how the chinese engineers is coming along regarding design, waterproofing / IP degree, PCB layout, internal shielding, component selection and general workmanship.
Summary: the radio is surprisingly well designed. A cast alu chassis is made with milled grooves for O rings, professional layed out PCB, proper ground vias and compartmentization, no stray cabling inside. The integrated chips used in the design are LMX1511 synthesizer, MC3311 compander, M62364 DA, KIA278 LDO, RDO7MVS18 driver. Well known integrated circuits.
Alex VE3NEA has coded the cool Voice Shaper software (see more here http://www.dxatlas.com/VShaper/ ) This software implements the equivalent of RF speech clipping in software. Unfortunately, the VE3NEA software does not support modern sound sources like USB headsets. It only accepts an old fashioned analog input from a sound card (at least on my machine). I was able to get around that by using Virtual Audio Cable repeater function plus a couple of virtual cables to route the audio back to Audacity for recording of the result down to mp3. Here is the setup.
You can hear the effect of the speech clipper by playing the MP3 audio file recorded purely from software below:
Pretty hefty punch!
Below you can watch a video illustrating the effect of a speech clipper on the signal to noise ratio on SSB. You can get as much as 6dB (too be verified) of increase in S/N ratio from a non compressed SSB signal due to that the average power level is raised significantly.
I have been working on a trackball based controller for my HDSDR SDR project lately. This is a small R&D project that is run on my spare time where the goal is to determine if it is possible to use a trackball as a VFO for software defined radio (SDR) in contests. The project started out based on a demand for a more ergonomic way to operate a mult receiver in a contest environment that is less fatiguing during 48hours duration of a major contest like CQWW or CQWPX. The goal is that it should be possible to operate all radio functions you need from one hand only: VFO, speed of vfo, band, mode, filter width, volume, gain. I have modified a Marconi trackball and the controller is a Trinket Pro controller (Arduino)
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.
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.
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.
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.
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.