Directional Wattmeter Project: Questions About Mixed-Signal Best Practices  - Page 1
Forum section: electronics / projects

author: ignilux
author_rank: Unknown
timestamp: August 22, 2019, 10:36:34 pm
content: Freshly-minted ham here (just passed Technician and General!); I'm working on a directional wattmeter for use in the upper MF to lower VHF bands, say, 1 - 50 MHz. The majority of the time the meter will be used to monitor the envelope power of a single sideband transmission-- about 3 kHz bandwidth. I have the analog front end and digital processing and output designed, but where the two meet I feel significantly less confident. My primary concern is keeping the digital switching noise and other RFI/EMI away from the analog front end. I've done a ton of reading both here on the forum and in various manufacturers' application notes, and my head is spinning. There are a few things that are consistent and easy to grasp, like dismissing the idea of separate digital and analog grounds, and instead focusing on a solid, low-as-possible impedance ground plane and paying attention to return paths. Keeping loop areas small.I've included a block diagram below to help illustrate my concerns. The red box around the coupler signifies that it will be in its own diecast aluminum box. Everything else is blue, because it's in the same enclosure. The dashed boxes signify that those blocks are on their own subassembly PCBs, rather than the main board. Peripherals and protocols given where relevant.The main questions:The power supply will be a 12 V plug pack, followed by 6-7 V buck converter, followed by 5 V LDO. In terms of supply voltage filtering, am I able to get enough attenuation of digital switching noise with a damped LC type filter (series inductor/ferrite bead, shunt low ESR cap, shunt higher ESR cap), or am I better off with an entirely separate LDO for the analog supply?Also on the topic of power, what is the best way to avoid a ground loop formed by the two separate coax cables running to the directional coupler, and the loop formed by the chassis connection to ground and the connection to the directional coupler (which will be grounded via coax to the radio)? Common mode choke...? For that matter, should the circuit ground even be connected to chassis ground?Speaking of magnetic filter components, I'm a bit confused on when to use a ferrite bead vs. an inductor. Because they saturate with lower currents, and because their characteristics change with DC bias, it seems that ferrite beads are more useful for signal lines, and inductors for power lines?Looking at the block diagram, you'll see that I have four separate subassembly boards for interfacing to the front panel. Each one will be connected by a short (not more than 6" / 150mm) cable back to the main board. I'm currently planning on ribbon cables with IDC connectors, but I'm open to suggestions. This is an obvious bottleneck for the low impedance ground plane, which makes me wonder if I am best off simply running a bunch of extra ground lines for each connector? The bargraph, for instance, uses three signal lines and one +5V line. Should I just alternate ground/signal/ground/power, etc? In terms of filtering, the highest signal speed is the 400 kHz I2C for the LCD, but the bargraph uses some 74HC glue logic, and I know both of those will have harmonics to be dealt with. Filter and decouple all the things?It'd be really nice if this "just worked", but we all know that is never the case. I'm hoping to layout something robust and adaptable with extra footprints and myriad test points so that I can really dig-in and see what's working or what needs tweaking. Note that this is a one-off for my own personal use, and won't undergo EMI/EMC testing. Please let me know if there's more information that you need, and thanks in advance for anyone who spends any of their time helping me out!

author: DaJMasta
author_rank: Super Contributor
timestamp: August 22, 2019, 11:44:51 pm
content: I don't think you're likely too far off of "just work" in terms of EMI, but there's certainly stuff you can do to minimize the risk. Part of this is your application, the noise floor is going to be the first thing effected by EMI, but where you probably want to operate is considerably higher power levels, so you don't have to necessarily go all-out for noise reduction since the range it would effect probably isn't the one you'll be interested in.That said:1. I think it would probably be sufficient, an LC filter on the rail going to the analog section could keep a lot of the noise generated by your digital stuff, and the LDO with some extra caps on either side should do well for eliminating switching noise from the supply (though this depends on LDO choice, most should list PSRR rating). A second LDO for the analog section is often a good choice (in parallel with the digital one for a separate rail), but probably isn't necessary.2. I don't think the ground loop would be big trouble, but you may want to try a star ground of sorts - ground the coupler's grounds and analog ground to the same tie point, then the digital to that (maybe at the power supply area), and join the chassis ground to that power entry point ground (or external chassis ground tie point, since the plug pack probably isn't doing it). This works with ground planes as well, you can have a plane for your analog section, a plane for your digital section, and a small break separating them except for where you want them to come together. Worth mentioning that if you're just powered from a plug pack, your device may just be floating unless you have an extra chassis ground point (and if it does float, then its ground will be through the coax connected to it).3. Ferrite beads are single turn inductors through a permeable material, nothing particularly fancy. That means their inductance/filtering capability is a lot less than most inductors, but it also means their current handling/size/cost are better for a given equivalent packaged inductor. Most are rated by impedance at 100MHz, so while they are useful for keeping out upper range signals to prevent mixing products, they won't be filtering much of anything in the band of interest. Since they have a core, the bias current does reduce their filtering capability (like on a power rail), Analog Devices recommends about 20% of their rated DC current as a max for good filtering performance here: for a bit of a deep dive...https://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.htmlRibbon cable will suffice, though parallel digital conductors do like to interact with each other. That said, the edge rates aren't going to be hugely fast, and adding ground conductors lowers the impedance and can physically separate noise producing lines - probably not necessary since none of the cables are mixed signal. If you're really concerned with edges from the digital lines, you can add a small LC (ferrite beads as the L can be good here), or small RC filters to signal lines to slow the edges... but don't go nuts with it or you'll slow the rise time to the point the digital stuff can't deal with it. Otherwise, you can try physically separating the analog section from the digital and signal lines to minimize coupling further. You could also look into shielding the analog frontend, but again I think this isn't required. You may also find that the speed your MCU isn't required, and going with a slower crystal or multiplier setting could be sufficient for your processing requirement but would lessen emissions again (though maybe not as much as one would hope since the edge rate would be about the same as the full clock speed).And it's worth mentioning that your consideration is really only about the analog parts, so while you can do something to try and reduce emissions from digital sections, the digital stuff will probably be pretty happy on its own without much special treatment beyond decoupling caps. Just keeping the analog stuff on its own section of the board without digital lines going through/near it and a little LC filter on the supply line for the analog section will go a long way towards keeping things quiet. If your digital stuff was a lot faster, a lot more sensitive to power supply noise, or driving much higher currents, it would be much more of an issue.

author: ignilux
author_rank: Unknown
timestamp: August 23, 2019, 01:56:13 pm
content: Lots of good stuff here -- thanks!Quote from: DaJMasta on August 22, 2019, 11:44:51 pmTo that end, I'm oversampling by a factor of 16, and will decimate for an ENOB of 10 + 2 = 12.Quote from: DaJMasta on August 22, 2019, 11:44:51 pmYes, I'm finding that most of the time they're more than happy to list their 60+ dB PSRR at 120 Hz, but finding a figure for 500 kHz - 1+ MHz requires tedious sifting through datasheet graphs.Quote from: DaJMasta on August 22, 2019, 11:44:51 pmThat's part of my concern, actually. I'll have two separate coax lines, in addition to incidental grounding through e.g. the LCD bezel making contact with the enclosure.Quote from: DaJMasta on August 22, 2019, 11:44:51 pmWould a ferrite core around the cable itself be an easy "belt and suspenders" solution, avoiding the pain of having space for chip ferrites on the PCB?Quote from: DaJMasta on August 22, 2019, 11:44:51 pmI'll be doing this regardless. The plan is to have the sections organized thusly, to keep ground and signal paths separate without physically splitting the ground plane:---------------------------------| PSU |---------------------------------| | || Digital | Analog || | |----------------------------------Quote from: DaJMasta on August 22, 2019, 11:44:51 pmYeah, I'm trying to bear in mind that the 16 / 20 MHz clock of the MCU is pretty slow, and that I'm likely overthinking this. A big part of it is that I'm trying to reinforce good habitsnow, so that when I design something where it matters it will be much more familiar and comfortable.Anyway, thanks again for your input here. Cheers!

author: patrick1
author_rank: Contributor
timestamp: August 23, 2019, 03:38:28 pm
content: might be abit of a stretch, - but tried ebay ?. analog gear like that comes in new and old, and often for $20

author: ignilux
author_rank: Unknown
timestamp: August 23, 2019, 04:59:23 pm
content: Oh, I could totally buy one for less than I'm going to spend building this. But that's not the point-- I'm building this to have fun, learn something, and have a useful tool to show for it in the end.