ElCheapo-ish Frequency Domain tools: SDR! (or maybe not?) - Page 1
Forum section: electronics / testgear

author: pipe2null
author_rank: Regular Contributor
timestamp: December 15, 2019, 10:11:44 am
content: Please Note: This topic is intended to be about using (or not using) cheap-ish SDRs for general purpose spectrum analysis, RF signal generation, general-purpose (NOT application specific) dev/test/debug/hacking HW and FW of devices that communicate wirelessly, and whatever other practical use ElCheapo-ish SDRs have as (low-grade) test equipment in a basic, economical, home lab setting. It is from the perspective of: Can't afford "the right" equipment for RF but need modestly decent, general purpose functionality on a budget. Basic capabilities are better than none at all. The technical terms "ElCheapo-ish" and "Cheap-ish" are intended as "cheap in comparison to standard lab equipment but not necessarily the absolute bottom price". This includes SDR devices up to around a few hundred ($US), but if a $20 device can provide the needed functionality, awesome. This topic is about SDRs as Test Equipment, thus I am posting the thread here. There are various threads on SDRs, but I haven't seen one for general purpose lab use: they are either application specific or appropriately located on the "RF, Microwave, and Ham Radio" board. If I missed something, please tell me.I am still "beginner-ish" since it has been decades since I actively used EE (and "atrophied" is a gross understatement), so I'm re-learning everything as I go.First off, many thanks to Eeevblog for... Well, lots of reasons, especially helping me finalize my first scope purchase decision: a "field upgraded" MSO5000. And Dave's #168 for a basic lab setup checklist, which I have mostly accomplished with a few cut corners. The spirit of Dave's recommendations to help people (like myself) get started building an economical but decently effective home lab brings us to the Topic at Hand: ElCheapo spectrum analysis w/transmission capability. Or: Using (Or perhaps DO NOT use???) Software Defined Radio's as economical but decently effective "test" equipment to upgrade your home lab for RF work/tinkering.Background for why I started this topic plus a bonus rant (Feel Free To Skip):So I was trying to use the FM radio feature on my smartphone by rigging up a DIY antenna to a 3.5mm plug so I could use my $$$ Bluetooth headphones, but apparently that configuration is not supported. Checking online for apps to connect to online streaming of those broadcast stations led me to the conclusion that "mad cash grabbing" is getting worse everyday, some apps requiring an initial fee plus a monthly subscription just to listen to commercial radio stations that are already broadcasting tons of commercials... I'll stop my rant there. So, I decided to search for USB FM radio receivers. That was a very BAD idea, because I discovered SDR receivers. And SDR TRANS-ceivers. AND Dave's "EEVblog #1178 - Build a $10 DIY EMC Probe". And now that I know those capabilities exist, I HAVE TO HAVE ONE (well, tentatively, multiple, thus this topic).So, what started out as trying to listen to local commercial radio has turned into a plausible excuse to add capability to my lab. You might consider this a cry for help, begging a referral to the TEA group, but "I don't have a problem, D*MNIT! I can quit at any time!"On to the actual topic for discussion. Here are some of my personal (beginner-ish) use-cases:My immediate (but not limited to) SDR use as a spectrum analyzer:I'm going to build one of Dave's "$10 DIY EMC Probe"s, so I need something to connect it to.As a re-learning project, I recently built an arduino controlled multi-channel PWM to drive my workbench's LED lighting. The power supply I used for the project had exactly the needed dimensions to fit, and it is a fan-less design so it is completely silent. But it gets hot. I thought it prudent to monitor it's temperature while inside the workbench's partially vented enclosure, so I used thermal-conductive tape to mount 1-wire temperature sensors on the PS case. Unfortunately, the PS has really bad EMI, and 1-wire communication gets corrupted periodically. When using a bit of wire between the tip and ground of my scope probe, the emissions from the PS can be easily observed (and triggered) on the scope holding the probe over 1 meter away. At some point when I'm a bit more confident in my re-acquired skill level, I want to use the $10 EMC Probe plus spectrum analyzer to zero in on the offensive part, calculate values based on actual measurements, and fix the design flaws in the cheapo 24V power supply. The switching frequency is in the kHz range, but the ringing (forgive if incorrect term) is into the MHz range, at least according to my MSO5000. Granted, I probably do not need a spectrum analyzer for this specific task (the scope has FFT in addition to everything else), but using an EMC probe to zero in on and measure the offensive component without any conductive contact (while connected to mains voltage) would be pretty handy.GP finding devices/components for any observed EMI... Etc: General spectrum analyzer use...My immediate (but not limited to) SDR use as a signal generator:As a learning exercise, I want to measure the actual bandwidth of my 350MHz --- Errrr.... I mean "70MHz" --- MSO5000 using an SDR transceiver as an RF generator to perform a frequency sweep (up to and beyond 1GHz). I believe I should also use a separate (or 2nd channel) SDR receiver as a spectrum analyzer to monitor the output of the transceiver and compensate any error in scope BW measurements that come from whatever shortcomings the ElCheapo transceiver/receivers have. I'll probably ask for advice on the physical setup of this experiment without overloading the transmitter when directly connecting to the scope, proper termination, etc, but that is off-topic.My immediate (but not limited to) SDR use as an RF transceiver for GP device dev/test/hack:2.4GHz: I have a grab bag of Arduino compatible modules for Bluetooth BLE/WiFi/nRF that I will be messing around with for a few different projects. Granted, there are much better solutions for test equipment than an SDR (a simple and cheap BLE dongle for instance), but non-application-specific RF spectrum monitoring (and limited transmission capability) can get useful info (theoretically) quickly. Just being able to quickly see that the devices are transmitting SOMEthing is useful. Being able to decode packets on the fly (or send them) for arbitrary protocols at arbitrary frequencies would certainly be nice. Not sure how much or how little of that is already supported in GNU Radio...433.92MHz: One example of a simple commercial device I want to hack around with is the Acurite temp/humidity wireless sensor. After I reverse engineer the communication I might use a cheap SDR to monitor/log readings on my PC (See RNE06044TX at fcc.io)."X" kHz/MHz/GHz: And there are so many other devices, protocols, utilized frequencies, etc. (at some point I want to mess with RFID) that using application specific test equipment for each and every one of them is not practical nor reasonably affordable for non-commercial tinkering. So, some type of SDR transciever with a wide RF range SEEMS like a good idea, at least to get some basic observations on what is going on... Then you can do basic tinkering first, prior to deciding if you want to invest in application specific test/dev equipment, or more expensive general purpose "standard" lab equipment for RF.My immediate (but not limited to) SDR use as an RF receiver:Much of the same use-cases for an SDR transceiver, but obviously Rx only.Monitoring (and compensating for) ElCheapo SDR transmitters/transceivers.Finallysome actual questions:I am attempting to figure out, for the general use case that includes all of the above categories, plus any categories suggested by other posters:Can ElCheepo-ish SDRs be utilized as economical but decently effective "test" equipment to upgrade your home lab for general purpose RF work/tinkering?The answer to this is "yes, BUT...". I understand bits and pieces of the partial yes, butI am looking for feedback from y'all on the "BUT..." part.As a spectrum analyzer:Of course SDR receivers can do this, but how accurate are the measurements? What "must have" features are missing that all standard lab-grade spectrum analyzers have?As an RF signal generator:Of course SDR transceivers can do this, but how accurate is the signal? What hardware (if any) do you need to connect to the antenna output to use it as a more general purpose signal source (without unintentionally broadcasting at a much higher signal strength than the SDR is purposely limited to)?As a general purpose receiver or transceiver for dev/test/hack/monitoring of wireless devices:Of course SDRs can do this, some have been designed explicitly for this purpose, like HackRF for instance, but this topic is about "general purpose" home lab use. Since I haven't purchased an SDR yet, I am not yet familiar with GNU Radio, nor any other related software. YET. What I intend to mean by "general purpose" for this bullet point is: You would probably buy application-specific equipment like a BLE dongle to mess with BLE, but how much BLE dongle "application-specific lab equipment" functionality can be achieved with the same "general purpose" SDR that you can also use for controlling an RC car, reverse engineering an Acurite temp/humidity wireless sensor, or measuring the bandwidth of your oscilloscope?What are the most important SDR specs to look for, given that most economical SDRs I am aware of are not really intended for use as "lab equipment"?There are a few spec categories that I think make sense, but I'll leave this question to people who know much better than myself...Which SDR, or combination of multiple SDRs, will best cover the most home lab use cases, as economical but decently effective "test" equipment?If I am spouting "crazy talk", please call me out on it. I'll finish this giant post up with a few thoughts on this question and a very brief list of a handful of SDR models I've heard of/checked specs on.For general purpose use, it seems to me that having a transceiver as the primary "lab equipment" covers most usage scenarios, but it is a good idea to have at least one additional unit even if for no other purpose than to monitor the output of the transceiver and possibly use that 2nd unit to tune and improve the overall accuracy of the 1st unit's output, especially if you are attempting to use it as an RF signal generator. In addition, nearly all the economical SDR transceivers I am aware of have a minimum frequency range generally around 1-60MHz (LimeSDR at 100kHz), which makes sense for the purposes those devices have been designed, but it seems a bit high to me if you are trying to do general purpose spectrum analysis which I'm pretty sure usually goes all the way down to DC. Since there are cheap-ish Rx-only SDRs that claim to be sensitive down to 1kHz, let's assume that the 2nd SDR unit is Rx-only to save a bit of money and cover more spectrum between the 2 units.For cheap-ish Rx-only SDRs:$120-$280: SDRplay RSP1A/RSPdx/or Duo: All are kinda 14bit ADC, 1kHz-2GHz, 10MHz visible, but the Duo has a 2nd independent tuner.$20: Many cheapO SDRs. Seems to be mostly 8bit (or 7 effective bit) ADCs, much higher minimum frequency or requires up conversion, some have breaks in the supported frequency range... I am not aware of any of the cheapO SDRs that have better ADCs or a wider/lower frequency range.For cheap-ish Tx/Rx SDRs:The models that I keep seeing pop up are HackRF, LimeSDR, FreeSRP, and PlutoSDR (aka ADALM-PLUTO). Ettus/USRP is way outside the cheap-ish price range.Please let me know any thoughts you may have regarding my questions above, or if you know of other SDRs in the cheap-ish $ range that might be suitable as economical but decently effective, general purpose RF test equipment.Thanks!(Yea... This post turned into another epic. Sorry about that...)

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 15, 2019, 07:13:19 pm
content: Cheap SDRs are so cheap that you should just buy one. Don't analyze the decision to death. The SDR that you do have beats a spectrum analyzer that you don't have, and if you buy a real SA then the $20 you spent on a SDR will be insignificant in comparison to both the cost of the SA and the value of what it taught you. That said, here's what a spectrum analyzer gets you that an SDR doesn't:* Sweep. "Show me everything from 0-xGHz, repeatedly, as fast as possible" is a useful capability for hunting down unknown signals. SDRs have historically sucked at this.* Spur-free dynamic range. A SDR might have 20dB of it, rated liberally, while, a SA might have 65dB of it, rated conservatively. Spurs are ghost signals that don't really exist in the input but do show up on the instrument. They show up lower than the true signal by at least the SFDR, spur-free-dynamic-range. If you have 80dB of dynamic range but only 20dB of spur-free range, you are going to be seeing some pretty serious spurs. Learning to identify them will be instructive, but a hassle. The hassle compounds with every type of further analysis you want to do on your spur-filled spectrum.* Calibration. SDRs make little effort at frequency and level stability, and very little effort at accuracy.SDR transmitters have the same caveats. If you expect to use your SDR transmitter to mitigate the caveats of your SDR receiver or vice versa, you will be disappointed to discover that having the same set of caveats at both ends makes this rather difficult, and that there is a fairly low ceiling on what you can actually mitigate in software (e.g. if it's out of band, your software can't touch it).If you just buy a cheap SDR, you will quickly discover either that it meets your needs (which were really limited to checking signal presence and maybe a decode project or two) or that it is painfully inadequate for your R&D work to such a degree that you are willing to pay $cost-of-SA to make the pain go away.

author: 0culus
author_rank: Super Contributor
timestamp: December 15, 2019, 07:27:53 pm
content: My opinion is that if you are doing serious RF work, you need a spectrum analyzer. SDRs are, as another forum member put it, basically the cheap toy oscilloscope of the frequency domain world. You can spend a lot of money on some high end SDRs (Ettus for instance), but to be honest, that's $1200 US that could fund a spectrum analyzer.Personally, I threw my hat into the boat anchor segment, and I invested in HP 8566 and 8568 analyzers. They are heavy but if it's in good working order, things like phase noise performance are going to be superior to the lower cost options on the market today from Rigol and Siglent, mainly if you are doing narrowband carrier stuff (which is mostly what I'm interested in). These analyzers were decades ahead of their time when they were introduced. However, if you are wanting to work outside the realm of narrowband carriers, modern options from the Chinese OEMs are fairly appealing. They will typically be able to at least cover the 2.4 GHz ISM band and usually are equipped with an onboard tracking generator too, which is a super useful feature. I believe Siglent doesn't charge for the tracking generator either (@tautech can correct me if I'm wrong!)

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 15, 2019, 07:46:30 pm
content: Yep. It is not economical to work around the deficiencies of a SDR. Your application can either ignore the deficiencies entirely -- in which case SDRs are great -- or you can't, and you should bite the bullet and buy a SA. If you already know that you want to do R&D rather than just tinker, you will not be able to ignore the deficiencies of a SDR, and you should bite the bullet and buy a SA.

author: tautech
author_rank: Super Contributor
timestamp: December 15, 2019, 08:01:10 pm
content: Quote from: 0culus on December 15, 2019, 07:27:53 pmYou are quite correct, SSA3000X and the upgraded SSA3000X+ now with touch display, keyboard and mouse support and modulation analysis to match the SA/VNA combo SVA1000X range features all have TG as standard fare at no additional cost.Cheapest models; 2.1 GHz SSA3031X start at $ 1395.

author: pipe2null
author_rank: Regular Contributor
timestamp: December 16, 2019, 01:23:10 am
content: Thanks for the feedback!Is my understanding reasonably on point?:To put it in time domain terms: Using a cheap-O $20 SDR or a cheap-ish $100-$400 SDR (ignoring Tx capability) as a general purpose spectrum analyzer is on par with using a $15 DMM or a $70 DMM when you should really be using an oscilloscope. It is by no means a perfect comparison, I'm just trying to get a rough gauge on the magnitude of difference.Buying a cheap-ish SDR instead of a cheap-O SDR strictly for SA use might provide a tiny marginal increase in measured accuracy, but also multiplies the cost 5-20x. Buying a high end SDR might have similar accuracy and dependability as a standard SA, but the high end cost is similar to SA's but without some of the features that dedicated/standard SAs have.Attempting to mitigate error and inaccuracies by using multiple SDRs is just as likely to multiply problems as it is to provide an incremental benefit, but it is 100% probable to multiple the cost.If my above understanding is reasonably correct as far as spectrum analysis is concerned, then I think you've answered my questions fully, Thanks! Since it's only $20, it's kind of a no brainer that I'll pick one up, or perhaps a cheap-ish one just to play/tinker/learn Tx as well... But I'll expect: frequency and level (and phase shift?) inaccuracies, lots of phantoms to waste time investigating, and a very low probability of detecting/capturing intermittent signals. Oh, and I'll also expect significant headaches and pain if attempting to use any inaccurate/erroneous measurements in projects.Future budgeting-wise, when I do take the plunge on a real SA, at some point I will likely need to work withprotocol "X" that uses 2.4GHz. What minimum spec should I keep in mind for an SA?For the moment I'll have to settle on being Pinocchio a little while longer.For the sake of completeness, using a cheap-ish SDR as an RF signal generator: You've made it pretty clear to expect poor and potentially unusable results. That said,is my idea of using an SDR transmitter to measure the bandwidth of my scope worth pursuing, strictly as a learning exercise?I'm curious if there are any sellers of old equipment that throw in a free sailor's hat with purchase?

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2019, 01:29:03 am
content: Quote from: pipe2null on December 16, 2019, 01:23:10 amThis 2.1 GHz unit hacked to 3.2 GHz would be perfect for your needs:https://www.eevblog.com/forum/buysellwanted/fs-(uk)-siglent-ssa-3021x-hacked-spectrum-analyzer/Note it has all options hacked too.

author: pipe2null
author_rank: Regular Contributor
timestamp: December 16, 2019, 01:56:04 am
content: @tautech: I'm not sure if you are the angel or the devil sitting on either of my shoulders, whispering advice that is good but also encourages an expenditure that Ishouldconsider outside of my current budget, although current "budget" is unrealistically small by an order of magnitude for any real equipment, so... Heh. Either way, I suspect you are a recruiter for the TEA group. Thanks (I think?) for bringing that item to my attention, I'll check it out.

author: BravoV
author_rank: Super Contributor
timestamp: December 16, 2019, 02:21:49 am
content: Quote from: pipe2null on December 16, 2019, 01:56:04 amMore temptation ...->Used spectrum analyzer buyer's guide

author: pipe2null
author_rank: Regular Contributor
timestamp: December 16, 2019, 07:36:36 am
content: So... After digging around a little bit, I have the impression that the Siglent SSA3021X (and the newer plus version apparently) is the current HackChampion of the spectrum analyzer world, same as Rigol's MSO5000 is for scopes?

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2019, 08:14:50 am
content: Quote from: pipe2null on December 16, 2019, 01:56:04 amTEA Enabler !Quote from: pipe2null on December 16, 2019, 07:36:36 amSure but where do you put the limit on the feature set you want ?When SSA3kX first came out I got the 3.2 GHz model at distributor demo pricing and as it was my first ever SA I was damn nervous about buggering it with novice errors but it's survived albeit not without sounding the input overload buzzer a few times !However since then and until recently the SVA range has more of my attention due to its vastly greater feature set being a VNA as well as a SA with the the same SSA feature set plus the goodies of touch and keyboard/mouse support. The new SSA3kX+ range has now leveled that playing field.Of late the SVA1015X has the VNA option as permanent now and at a not insignificant increased price when previously it was the same price as SSA3021X. Now having the even newer SVA1032X I feel I have a tool for life unless I start chasing the 5 GHz WiFi bands.All these while in my hands have been improved with the help of unnamed forum members;SSA3032XSVA1015XSVA1032Xplus numerous scopes etc.It is hard to know what might be your future needs however IME as my skills and knowledge have grown I'm finding the better gear is helping drive my knowledge further and faster.RF is scaryandfun, enjoy the ride.

author: pipe2null
author_rank: Regular Contributor
timestamp: January 09, 2020, 10:21:56 pm
content: I'm not trying to resurrect this thread, just thought I'd post what I ended up with on the short term and what I'm looking at for the longer term.I bought an RTL-SDR Blog v3 kit ($30 including cheap but effective antennas for starting out), a LimeSDR, a LimeSDR-mini, and a $200ish grab bag of cheap RF components (LNAs/RF filters) and SMA adapters/cables with the intention of making SMA/RP-SMA the "standard connector" whenever possible in my lab. Each of the SDRs have an intended immediate and longer term use for projects, so no, I did not buy a grab bag of SDRs for the sake of having them.Test-Equipment-Wise short term: For the purpose of TEMPORARY/short term make-shift test equipment, I can use any of the SDRs for basic, uncalibrated, "Haunted" with ghosts/etc, spectrum analysis.One thing to note, I also bought a cheapo $15 RF noise generator so I can get a crude but reasonable idea of the frequency response of RF filters, both purchased and DIY. With this combination of tools, I now have at least ultra-basic RF capability in my lab.Test-Equipment-Wise longer term: I am seriously looking at buying a SA, SA-TG, -OR- a VNA. From the various thread I have read, I think a plain old SA would be a waste of money. At this point I'm trying to understand the differences (worth paying for) between a single path VNA that might be within $ range and an SA with built in TG.

author: 0culus
author_rank: Super Contributor
timestamp: January 10, 2020, 12:02:57 am
content: Someone correct me if I'm wrong, but I believe it's usually better to have both a spectrum analyzer and a VNA. The characteristics that make a fine VNA don't necessarily translate to a good SA.

author: tggzzz
author_rank: Super Contributor
timestamp: January 10, 2020, 12:22:14 am
content: For as few experiments with a cheap SDR rx dongle, have a look athttps://entertaininghacks.wordpress.com/tag/sdr/Starting point: doing morewith less, as a fun experiment.