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Using Low-cost Electronic Modules By Jim Rowe Self-Contained 6GHz Digital Attenuator This digitally-programmable module can attenuate signals from 1MHz to 6GHz by 0 to 31.75dB in 0.25dB steps. You control it using five small pushbutton switches, while a tiny OLED screen shows the current setting. I recently became aware of this digital attenuator that has a frequency range extending all the way to 6GHz, and 0.25dB attenuation steps from 0dB to 31.75dB. It seemed quite impressive, so I decided to review it. It is available from a few different sellers online, such as the satisfy­e lectronics Store on AliExpress, at: https://vi.aliexpress.com/ item/1005006358745427.html You may be able to find the same module on other platforms like eBay and Amazon. If you want to buy the same one, make sure it matches the photos shown here. Some of the identifying features are the OLED screen in the middle of the module and the row of five tactile buttons along the bottom. I paid AU$27 for it plus $9 postage for a total of $36 (about £18.50). New module The new module is likely available from several suppliers on the web, but I ordered the one shown in the photos from Banggood, catalog code 1648810. Currently, it’s priced at $51.80 plus $6.70 for shipping to Australia. Like the earlier 3.8GHz module, it’s almost certainly made in China. The new module measures 56 x 40 x 16mm overall, not counting the SMA connectors at each end for RF input and output. The digital attenuator section is on a small PCB fitted down inside a 56 x 40 x 10mm CNC machined aluminium block which forms the module’s ‘case’. The rest of the module’s circuitry is mounted on a second PCB measuring 56 x 40mm, which forms the top of the case. The OLED panel is mounted on the top of this PCB in the centre, along with the micro-USB power socket, the mini slider power switch and a tiny SMD power LED. Then along the PCB front are the five small pushbutton switches Practical Electronics | December | 2024 used to select the attenuation setting. Presumably, the rest of the controller circuitry is mounted on the underside of this PCB. The UHF attenuator chip is probably the Analog Devices HMC1119, a ‘big brother’ to the HMC472 used in the aforementioned 3.8GHz attenuator. According to the Analog Devices data sheet, the HMC1119 has a range of 100MHz to 6.0GHz and seven control bits, giving a setting range of 0 to 31.75dB in 0.25dB steps. It has a specified insertion loss of 1.3dB at 2.0GHz, drooping to around 1.5dB at 3.5GHz and a whisker below 2.0dB at 6GHz. Pretty impressive! As with the 3.8GHz attenuator, I couldn’t find a full circuit for the new module, so I could only work out a basic block diagram for it, shown in Fig.1. The RF1 input and RF2 output pins of the HMC1119 chip are coupled to the SMA input and output connectors via capacitors. Apart from various bypass capacitors, that makes up all of the actual attenuator section. Below is the control section, based on a microcontroller (possibly an STM32F103C8T6, like the one used in the 3.8GHz attenuator). Operation The microcontroller (MCU) controls the attenuation settings of the HMC1119 via the seven programming lines, while the user determines the attenuation setting using the five small pushbutton switches S1-S5. To make this easy, the MCU displays the current attenuation setting on the OLED screen, controlled using a standard I2C serial interface. When power is first applied, the MCU sets the attenuation to 00.00dB. Fig.1: a simplified version of what we expect the block diagram the 6GHz attenuator to look like, as there is no full circuit diagram available. 33 To change this, you first press S3 (the OK button) and then press S1 (<) or S5 (>) until the display is flashing the setting digit you want to change. Then you can press either S2 (+) or S4 (-) to change the value of this digit. To change other digits, use either S1 or S5 to move to them, then use S2 or S4 to change their value. Then if you press S3 again, this will be the new setting. It’s pretty straightforward, and although the tiny pushbuttons used for S1-S5 seem to be the same as those used on the 3.8GHz module, the additional two buttons seem to allow the setting to be changed more reliably. Perhaps the firmware in the MCU has also been improved to make it less susceptible to contact bounce. I have also shown a USB-serial interface chip in Fig.1. This chip may or may not be in the 6GHz module; I’ve shown it purely because it was present in the 3.8GHz module. It’s possible that, in this case, the data lines from the micro-USB connector go directly to two pins of the MCU, but they certainly are routed somewhere on the PCB. Either way, it would allow the attenuation setting to be programmed from an external PC, as well as from its own ‘keyboard’. So the micro-USB socket is not just to feed power to the module, but also for external control. As with the 3.8GHz attenuator, there’s virtually no information provided on doing this external control, but I found a very cryptic suggestion in the ‘Customer Q&As’ section of the Banggood info on the module: “Go to github.com/emptemp/ att6000_control for Python code.” I’m not familiar with the Python programming language, so I sought help from other S ilicon C hip staff. They advised me that all the ‘att6000’ Python code seemed to do was send serial text commands in the format “wv0XXYY<LF>”, where the XXYY characters indicate the desired attenuation setting XX.YY. In other words, sending the command “wv02375<LF>” should change the attenuator’s setting to -23.75dB. They also informed me that the command should be sent at 115,200 baud, not the 9600 baud that seemed to be used previously. I did try this out, and the results are described below. Performance I measured the performance of the new attenuator module using my Signal Hound USB-SA44B HF-UHF spectrum analyser and its matching USBTG44A tracking generator. Both were controlled by Signal Hound’s Spike software (V3.5.15) in its SNA (scalar network analysis) mode. Since the SA44B and TG44A combina34 The 6GHz digital attenuator from Banggood has an OLED screen and weighs about 57g. tion will only work up to 4.4GHz, I could only check the module over this range. I first used this setup to check the module’s performance at an attenuation setting of 00.00dB to see its insertion loss. This is shown in Fig.2; the measured insertion loss is less than -2.5dB up to about 1.3GHz, then droops down to about -6.0dB at 2.5GHz, then improves to about -2.5dB at 3.0GHz. It then droops to about -4.5dB at 4.0GHz, before moving up again to reach -4.0dB at 4.4GHz, which looks promising for its insertion loss at frequencies up to 6GHz. After this, I did response tests at ‘major’ attenuation steps: -5dB, -10dB, -15dB, -20dB, -25dB and -30dB. These settings were chosen to give a good idea of the module’s overall performance. After examining the results I then checked the response at a number of ‘fine detail’ settings: -1dB, -1.5dB, -2dB, -3dB, -4dB, -7.5dB, -10.75dB, -14dB, -19dB, -28.25dB and -31.75dB. During each of these tests, I saved an image of Spike’s plot of the test results. Then, knowing that there wouldn’t be enough space to reproduce all 18 of the plots separately, I combined all of the plots into a single composite plot, to allow for easier evaluation – see Fig.3. The upper plots in Fig.3 (down to about -20dB) have a shape almost identical to that of the top 00.00dB plot, just separated from it by the chosen attenuation setting. For frequencies above about 1.75GHz, the higher attenuation plots (-20dB and greater) develop an increasing number of bumps and dips. These are very apparent in, for example, the red -25dB plot, the purple -28.25dB plot, the red -30dB plot and the blue -31.75dB plot. All of these four plots show an increasing tendency to have a significant dip between 2.5GHz and 3.1GHz. I suspect that this may be due to small resonances inside the HMC1119 chip and/or its surrounding tracks on the attenuator section’s PCB. There might also be standing waves inside the attenuator box at specific frequencies. These plots tell us that the attenuator’s performance is quite respectable, at least for frequencies up to about 2.2GHz and for settings up to about -20dB. But the errors do increase for frequencies above 2.2GHz and with settings above -20dB. Of course, the attenuator would still have many practical uses at frequencies above 2.2GHz and with settings above -20dB, especially if you were to use Fig.3 to correct for likely errors. Armed with the information mentioned earlier on how to control the device over a serial connection, it didn’t take me long at all to test sending new attenuation settings from my Windows 10 PC, using the TeraTerm serial terminal application. Fig.2: using Signal Hound’s Spike software the 6GHz module could be checked at an attenuation setting of 0dB to measure its insertion loss. Note that the setup used for testing can only measure up to 4.4GHz, so not the full range of the attenuator. Practical Electronics | December | 2024 All I had to do was plug the cable from the attenuator into a USB port, then go into Settings → Devices to find out to which Virtual COM port it had been assigned. Then I started up Tera-Term and set it up to communicate with that port at 115,200 baud, with the 8N1 data format and with only an LF (line feed) at the end of each line. I was then able to change the attenuator’s setting at any time simply by typing in a command like “wv01575” and pressing the Enter key. No problem! The attenuator’s OLED immediately showed the new setting (like “-15.75dB”) and also sent back an “OK” message, to confirm that the command had been received and acted upon. I should perhaps note that there does seem to be provision on the top of the attenuator (just to the left of the OLED) for connecting a separate serial interface, as you can see in the photos. But there’s no information on doing this. I guess that the command interface is the same, but I haven’t tried it, so I can’t say for sure. Conclusions Overall this new attenuator module seems reasonably good value for money when you consider its relatively wide frequency range and low price. I also like its ability to be programmed using the built-in MCU, control buttons and tiny OLED screen, or from a PC via the USB port (and presumably from a separate microcontroller, via the serial port header). My only real gripe is that when I tried to unplug the USB cable from the micro-USB socket after testing it, the socket lifted straight off the PCB. It seemed to have been poorly soldered, and as a result, I had to spend quite a bit of time soldering it back on (under a microscope). I’d have preferred a mini-USB socket, as these seem to be a bit more rugged PE and also attach more securely to the PCB. Fig.3: a graph showing the combined result from a variety of response tests on the attenuator at various settings. Practical Electronics | December | 2024 35