While searching through eBay listings I came across a step attenuator that was listed aa 0-50 dB in 10 dB steps attenuator, and good for DC to 18 GHz. It was recently priced so I decided to spring for it. When it arrived I started to research the origin. It turned out to be from an old HP 8555 spectrum analyzer. Most likely a unit no longer working, but the attenuator was advertised as tested. I found a users/service manual that described the operation and the voltages needed to activate the solenoids. It looked to be simple as it was using 12V solenoids. The attenuator has three solenoids that are used to enable/disable 3 different attenuators in series. After putting the attenuator on the test bench I confirmed that all 3 was working and providing 10, 20 and 40 dB attenuation respectively. So the range is 0-70 dB and not as the specs for the HP 8555 analyzer listed (0-50 db). In the spectrum analyzer the attenuator is placed between the N connector on the front panel and the mixer and can handle +33 dBm or 2W maximum. Good for many microwave applications.
With the information about how the attenuator is operated and the knowledge that all 3 stages was working as expected I started designing the control unit. The first step was to decide how to generate the voltage needed for each of the three solenoids. In order to engage the solenoid the two terminals needed a +12V pulse for 150 ms and in order to switch it back the polarization of the pulse should change ro -12V for 150 ms. This can be achieved with an H-bridge. This is basically 4 transistors configured to have 2 inputs and two outputs. This will allow a positive voltage when the input is high and low and a negative output when the inputs are low and high. In addition there would be no voltage across the outputs if both inputs are low. H-bridges are commonly known to drive DC motors, allowing them to turn in both directions depending on the voltage being positive or negative. Amazon is selling a package of 4 H-bridge modules based on the L298N chip for just $10. It would not make sense to try to build one from scratch at those prices. The module provides 5V input logic and 12V output, exactly what's needed to use an Arduino Nano as the controller.
With the parts ordered I designed a 3D printed enclosure that could be glued on to the side of the attenuator, covering the terminals for the solenoids and providing push buttons to change the attenuation up and down and a display to show the current value. Since the solenoids are latching it's possible to connect the power, set the desired attenuation and then disconnect the power. That way there will be no noise from the Arduino board while the attenuator is in use. I also made the code to set the default attenuation to 70 dB when the unit is powered up. This is needed to set the highest attenuation but also because it's not possible to read the current value/state of the each attenuator. Bringing them to a default value makes sense.
The finished attenuator and control box all assembled.
The inside of the control unit showing the two H-bridge boards on the left and the Arduino Nano on the right. I placed the Arduino nano on an breakout board that allow for 12V power input and easy access to the pins needed to control the H-bridges and the two input buttons.
And finally the unit with power applied. The small display is a 128*32 pixel OLED display with a I2C interface. When power is applied it will show my call sign and then set the attenuator to 70 dB.
Here are links to the Arduino sketch and the 3D stl file.
In the previous post I described how to make a HAM Radio beakon that could work with both CW and FM modulation. As I was testing the results of that I noticed an issue with the FM modulation as it would also modulate the carrier. That was not the intention, but it did function as an FM beacon. In this updated design I have utilized two Selector Blocks to switch the morse code signal either directly to the HackRF One or to go through the FM modulation stages to provide a constant tone.
I also updated the GUI to have a slider to set the CW speed between 5 and 20 words per minute with a default of 12 and added an input box to change the message used to generate the CW code. The screenshot below was created from running the Python code after manually adding the additional frequencies.
The two Python files are available for download: Beacon.py and Beacon_epy_block_0.py as well as the flow graph: Beacon.grc.
I have made 3 different versions of the HAM Radio beacon project using GNU Radio and my HackRF One SDR hardware. The first version, described in the previous previous post and covered 5 HAM bands and used a QT GUI to select the beacon frequency. The second version eliminated the GUI and automatically switched between the 5 frequencies in the VHF and UHF bands. Since there was one additional HAM band I edited the Python file directly to add the 6th band. The 3rd version was similar to the second version but cover the 5 bands from 23cm to 3cm. Since 3cm or 10GHz is above the max frequency the HackRF One can operate on this would have to rely on a harmonic of a frequency below 6GHz.
Over the weekend the PNW Microwave group met to discuss SDR projects and work on other beacons and I got a few suggestion for improvements, especially to the version with a GUI as that will allow for more interactive control of the beacon.
The first suggestion was to increase the output power. The HackRF One is controlled by a "Soapy" GNU Radio block that provides input for center frequency, bandwidth, CGA gain and to turn the output amplifier on/off. In my original version I had left the amplifier off. Turning it on would make it easier to receive the harmonics on 10 GHz. A final version would need some filtering to avoid transmitting on the base frequency, that would be outside of the HAM bands. It will also be necessary to add a bit more power to make the beacon useful, but more about that in a future post.
Another suggestion was to allow the user to switch between CW and FM modulation. The beacon is design to send morse code and when listening to the beacon on an FM receiver it is difficult to hear the actual morse code if the beacon is transmitting a carrier. Implementing a system to generate a FM modulated signal and keep the carrier on while turning the modulation on and off would make it possible to use a FM receiver.
Today I started to implement the additional functionality. There was a few experiments to find the correct blocks to use and to ensure the selection of modulation would make the system work for both CW and FM. The flow graph is shown on the image below.
You can download the GNU Companion flowgraph file here.
The GUI provides fixed variables for audio, RF sampling rate, drive for the CW mode and the message that will be translated into morse code. There are currently no QUI elements to changes these interactively. The GUI does provide elements for changing the frequency, switching between CW and FM and to change the frequency used for the tone in FM modulation. The basic functionality uses a custom Python block named "Beacon Block". This is the code that can transform a string of characters into morse code. The output of this block is fed into two multipliers. Since the output is switching between 0 and 1 the effect will be to have the multiplier turning the signals on and off. The output of the multipliers are then connected to the selector, controlled by the selected modulation. The selected signal will then be fed into the Soapy HackRF sink and also to a FFT Frequency display to visually show the output.
The image below is taken from the version where I edited the Python file to provide beacon frequencies for all the HAM bands from 60 MHz to 10 GHz. The frequencies used is .080 on 6m and 33cm, .280 on all other bands to 23cm and .380 on all bands above 23cm. That way the frequencies all fall within the allocated beacon sub bands according to ARRL's band plan.
Finally the two Python files that were generated by GNU Radio Companion, and edited to allow more than frequencies can be downloaded from these links: Beacon.py and Beacon_epy_block_0.py. Beacon.py is the main file that includes the code to the custom block to handle the translation of characters to morse code. The two files should be located in the same directory.
