EEVblog #343 - Spectrum Analyser Tracking Generator Tutorial - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: August 30, 2012, 10:52:19 am
content: Dave.

author: JuiceKing
author_rank: Regular Contributor
timestamp: August 30, 2012, 11:39:02 am
content: Very neat demo! Thumbs-Up!Some time back, I did a similar experiment with a sweep generator and oscilloscope and was puzzled by the high-frequency response anomalies you explain so well here.Thanks!!For your blog, Dave, it might be interesting at some point to explain the practical limitations of this "low-end" Rigol vs. an old HP/Agilent that you might find at a flea market.

author: EEVblog
author_rank: Unknown
timestamp: August 30, 2012, 11:50:54 am
content: Quote from: JuiceKing on August 30, 2012, 11:39:02 amDamn, that reminds me, I forgot to include a shot of the inductor datasheet showing the self resonant point.Dave.

author: Unknown
author_rank: Unknown
timestamp: August 30, 2012, 12:46:05 pm
content: Quote...and yet we don't have a smooth tracking analyzer on a measurement instrument.If they were that good we ought to have Ghz RF in our toasters by now. What am I missing?I mean imagine if a civil engineer said "Anything below 100 floors is basically ground level" and at the same time all his buildings were crooked and wobbly?

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 30, 2012, 01:33:40 pm
content: Quote from: HAL-42b on August 30, 2012, 12:46:05 pmMicrowave at 2.4 GHz close enough

author: olsenn
author_rank: Frequent Contributor
timestamp: August 30, 2012, 01:46:48 pm
content: QuoteCorrect me if I'm wrong, but I think DC power dissipation in a resistive load is good enough to heat toast. As for microwave ovens, they don't rely on the same circuitry that you typically see in low power RF transmitters for instance. Look up "magnetron" on google to find out more about how this is done.

author: Unknown
author_rank: Unknown
timestamp: August 30, 2012, 02:34:00 pm
content: Quote from: olsenn on August 30, 2012, 01:46:48 pmI was alluding to the fact that as technology matures and gets cheaper it finds itself on most absurd places, as absurd as wifi on a toaster for example. So if they can't put a linear tracking gen on a 1 GHzmeasurement instrument (not a toaster mind you) then their technology is not mature and their "1GHz = DC" simply isn't true. Thats why a 13GHz scope costs 140.000 bucks and not 400.

author: jahonen
author_rank: Super Contributor
timestamp: August 30, 2012, 02:43:19 pm
content: Dave,I think it would have been easier to set the desired start and stop frequencies for this kind of stuff by using start freq and stop freq settings in the frequency menu, instead of the span menu (center/span method).Regards,Janne

author: hpux735
author_rank: Contributor
timestamp: August 30, 2012, 03:35:30 pm
content: Quote from: HAL-42b on August 30, 2012, 02:34:00 pmI fully agree. I think that phrase is pretty annoying.About the high frequency response: I don't know how much you can reasonably expect to achieve with that topology. As 1GHz is not DC, you have to think about isolation. Having the input and output terminals of the filter un-shielded from one another means that the high frequency response is likely through the air. In addition, the waviness of the response is likely caused by various propagation modes. The metal can is acting as an RF cavity.

author: free_electron
author_rank: Super Contributor
timestamp: August 30, 2012, 04:48:00 pm
content: The rf output amplitude is ot flat. The ampmcan only deliver so much power into the system.This kind of measurements needs to be done by hooking up the dut through a power splitter , then taking a baseline measurement the get the output behavior of the tracking generator and storing that in memory. Then doing a measurement. This gives you the true in/out ratio.That'swhy the. Heapo spectrum analusers like those 500$ attens on ebay are crap. You can't store the reference curve. So anything you measure is wrong as it is impacted by losses in the machine and wiring.I normaly use an active probe. Hok up tracking gen to system under test.Probe on input, store cal data , then probe on Output. That way even your cabling and probe is compensated.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 30, 2012, 07:37:57 pm
content: Maybe I'm crazy and/or blind but I think it looks like the signal input and output are shorted through that big copper plane. Or is there a slit in it that I'm not seeing?Regardless, a big "I know what you're thinking" would have been to put the unit under test without any coil at all and see how much of that RF goes through.

author: jahonen
author_rank: Super Contributor
timestamp: August 30, 2012, 07:43:27 pm
content: I think that the ripples come from some kind of mismatch. The box is way too small to have any internal resonances at such low frequencies. Anyway, I bodged it on a copper clad board, and got a slightly different results (with 910 nH inductor and 220 pF caps) using a R&S FSV-7 with TG option, using a log frequency scale:It is clearly a 3rd order filter:Regards,Janne

author: JackOfVA
author_rank: Unknown
timestamp: August 31, 2012, 12:13:51 am
content: You can also improve the noise floor by reducing the resolution bandwidth (RBW). Since the tracking generator will presumably match the spectrum analyzer frequency closely, you can reduce the RBW from 1 MHz to say 1 KHz - the result should be a major improvement in noise floor without using the preamplifier.Let me add some Australian content ... there's a great study available athttp://www.dsto.defence.gov.au/publications/2597/DSTO-TN-0531.pdftitled "Construction Techniques for LC Highpass and Lowpass Filters used in the 1 MHz to 1 GHz Frequency Range."It's well worth reading for anyone interested in frequency filters and thanks to Dave for paying the taxes that funded the work.I should also add that the "one hung low" coaxial adapters Dave used (N to BNC) are not to be trusted for anything close to precision work at UHF. And, you have to watch the coax cable and BNC connectors as well. At HF, you can get by with most anything for cables and connectors, but as you move up in frequency, cheap cable with poor braid coverage and nickle plated Chinese adapters must be avoided. (Amphenol silver plated brass connectors are readily available and adequate for most casual work, and if you are really into precision measurements, then some very pricey products are available.) I'll also note that there's a lot of coax cable out of China that may be marked RG-58, implying 50 ohms impedance, but it is far from that. I measured a length of "Digiwave" RG-58 cable for a friend and found it close to 100 ohms impedance. Details athttp://www.cliftonlaboratories.com/the_curious_case_of.htm

author: JackOfVA
author_rank: Unknown
timestamp: August 31, 2012, 12:35:40 am
content: Many subtleties have to be considered when making a swept measurement. Agilent has an excellent application note covering error compensation in VNAs, available athttp://literature.agilent.com/litweb/pdf/5965-7709E.pdfA tracking generator and spectrum analyzer does not make these sort of error correction measurements, other than the simplest type. A true VNA will have a bridge arrangement with a sample of the generated signal being used as the reference for a comparative level measurement.Some improvement with the tracking generator and spectrum analyzer might be found by adding say 6 or even 10 dB attenuators to the TG output and spectrum analyzer input. This will help the return loss seen by both ends of the filter. (Probably the spectrum analyzer input return loss isn't too bad, but I would suspect the TG output return loss.) Mini-Circuits has a useful line of "Adaptenuators" that combines an attenuator and a between-series adapter. For example, a pair of NM-BF-10 might be used in Dave's setup. These are 10 dB attenuators, rated to 2 GHz with a male N on one end and a female BNC on the other.

author: Zad
author_rank: Super Contributor
timestamp: August 31, 2012, 04:41:29 am
content: Each of the peaks on the HF end of the response seems to be 75MHz apart. Maybe that corresponds with the electrical length of the coaxial cable? I would definitely try another cable on it, if only to check that the peaks didn't (or did) move.

author: sagdahl
author_rank: Contributor
timestamp: August 31, 2012, 03:52:15 pm
content: Dave, brilliant tutorial.I have never used a spectrum analyser but now I want one.Just to play around with and measure filters.Rigol should really give you a percentage of all their sales due to the blog.I have already bought an oscilloscope. Guess the logistics can be difficult./Roger

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: August 31, 2012, 05:41:00 pm
content: I would love to see how the DSA-1030 compares to the DSA-815, in terms of the tracking generator and response of the SA itself. (i.e. Is the 1030 worth the extra $$$?)

author: Andrei
author_rank: Contributor
timestamp: August 31, 2012, 05:55:39 pm
content: Hi All,I am a bit confused as to what that tracking gen does. Does it measure the input to the spectrum analyzer and output it? If that is the case, how does it address the positive feedback issue when you connect it to the input to the input of the spectrum analyzer and now blow up the waveform ?Thanks

author: jahonen
author_rank: Super Contributor
timestamp: August 31, 2012, 06:25:38 pm
content: Quote from: Andrei on August 31, 2012, 05:55:39 pmA tracking generator basically just generates exactly the same frequency what the spectrum analyzer is currently measuring. So the generator is just a generator which is synchronized to the local oscillator of the spectrum analyzer. Some higher-end spectrum analyzers even let to use an external signal generator as a tracking gen (so spectrum analyzer controls the RF gen via GPIB bus or by some other means during the sweep). So there is no feedback of any kind.Regards,Janne

author: JackOfVA
author_rank: Unknown
timestamp: September 01, 2012, 04:55:43 pm
content: I built a version of Dave's filter, 200 pF shunt C and each end and 1.06uH series inductance, with decent results well up into the UHF range.The shunt C consists of two 100pF 1206 C0G capacitors in parallel, mounted 180 degrees apart. (Capacitors are shown in the red circles). Inductor is 17 turns of # 20 AWG wire on a 0.5" (12.7mm) diameter powdered iron core, Micrometals mix 10. Adjusted by squeezing turns to be 1.06 uH at 10 MHz, measured on an HP4192A impedance meter.The two end pieces of scrap PCB have a small diameter hole drilled through them, and an annular ring cut into PCB centered on the hole, using a diamond dust core drill 1/4" diameter (6.35mm). (Single sided stock.) The shunt capacitors bridge the ring gap and the inductor lead passes through the hole and is soldered to the ring pad, along with the BNC connector center conductor.Response is measured with an HP8752B vector network analyzer, a 75 ohm instrument, with MiniCircuit adapter-attenuator matching pads for 50 ohms. The pad loss is calibrated out during the instrument zero loss calibration process.Frequency sweeps from 300 KHz to 1 GHz and 300 KHz to 3 GHz are shown. All are log scale frequency.I should add that in addition to concern over the inductor becoming a capacitor at these frequencies well above its SRF, it's important to prevent the input and output connectors and leads from coupling to each other, either via loop inductance or stray capacitance. Hence the end post BNC connectors and the the PCB "wings" provide protection against unwanted coupling (sometimes called "filter blowby")At 15 MHz, the measured loss is 2.8 dB, pretty close to the expected 3 dB. End capacitance is a bit below the design value and that bumps up the -3 dB corner frequency.The first point of resonance is at 430 MHz (peak) and then further resonances are seen up to 3 GHz.However, it's not that bad a filter, providing at least 50 dB attenuation up to 1.7 GHz. And up to 976 MHz, at least 70 dB attenuation is seen.I'll put it in the mail to Dave next week for his amusement.The end wings and choice of connector are intended to minimize inductive and capacitive coupling between the input and output thereby reducing filter leakage.

author: G7PSK
author_rank: Super Contributor
timestamp: September 01, 2012, 05:35:54 pm
content: For best results you should also use coax that is cut to the correct length for the frequency in use that is why that bounce was the same frequency apart (circa 75 Mhz as some one else noted)

author: ruku
author_rank: Contributor
timestamp: September 01, 2012, 07:57:24 pm
content: Somehow, I knew that when I came to this thread, people would be dying to show off their analog filter design-fu. I'm so excited!Quote from: JackOfVA on September 01, 2012, 04:55:43 pmOh my goodness, please do this. Are you related to Jim Williams per-chance?Could you write up a small tutorial on how you made those design decisions? I'd love to pick your brain.

author: JackOfVA
author_rank: Unknown
timestamp: September 01, 2012, 08:29:44 pm
content: Quote from: ruku on September 01, 2012, 07:57:24 pmMy only connection with Jim Williams is that we both attended Wayne State University -- he dropped out after a year of non-technical study and then went later to MIT, whist I completed my EE degree at WSU. Never met him, though.As far as the filter goes, if you ever take a lumped component VHF/UHF low pass filter apart, you will find that it is built with multiple bulkhead shields strung out through the filter length. (The filter is housed in a long cylindrical or rectangular RF tight box. Between each bulkhead is the inductor and on each bulkhead are the shunt capacitors.)The shunt capacitors are often made from disks of brass or copper and Teflon or mica dielectric. The bulkhead forms the ground side and there's a hole through the bulkhead for the inductor wire to pass and to be soldered or mechanically attached to the shunt capacitor disk.The series inductors are air wound from heavy gauge wire, preferably silver plated.The idea is that the bulkheads shield each filter section (inductor) from interacting with each other and provide a low impedance path for the ground return path from the shunt current.In adapting this configuration for a 15 MHz filter, some changes have to be made.1. The shunt capacitors are much easier to make for a one-off filter by stock surface mount parts. 0805 might be better from an electrical performance prospective, but are a bit too small for a quick kludge. 1206, C0G types are about the right size mechanically.In order to more uniformly distribute the ground return current through the bulkhead, it's better to use several capacitors in parallel, radially spaced around the pad. But there's not a lot of room for the BNC connector posts so 2 or maybe 3 at most capacitors can be used at each bulkhead.It might have been better to drill holes for the BNC posts and mount it on the non-copper side so that more flexibility in capacitor placement and number was possible. That didn't occur to me until after I had built the prototype.2. An air wound solenoid style 1uH inductor is starting to become a bit large - so this suggests a toroid, either with a ur=1 phenolic core or a powdered iron core with a VHF rated material. MicroMetals Mix 10 is a reasonable choice.A toroid is not perfectly self-shielding, but it's noticeably better in that regard than a solenoid form.Regardless, at some frequency the distributed capacitance of the inductor is going to be sufficient that the self-resonant frequency will be exceeded. At that point, the filter looks like a capacitive voltage divider. You may still get some filtering action but it won't be what is expected. And once you are up in the GHz range, the inductor may even start looking like an antenna and will radiate. Those factors suggest you should use the minimum wire length possible. But that is associated with a higher permeability core which will usually have other problems as the frequency increases. So, it's a matter of where you decide to take the hit. And, the shunt capacitors start to look like tuned LC circuits when the inductance of the bulkheads is considered.Regardless, at some frequency the lumped element filter will no longer perform like a filter. If you really have to have effective filtering from DC to light, it probably will be necessary to use more than one filter in series - perhaps in the same physical package - each optimized for a different frequency range.Still, this one turned out better than I thought it would.Jack K8ZOA

author: T4P
author_rank: Super Contributor
timestamp: September 02, 2012, 06:34:16 am
content: Gen1 Cable Modem (back in 2006 i think?)Gen2 Cable Modem (2010)The first one runs at about 800MHz* and the 2nd one at 1GHzThe 1GHz was introduced as a new system and the old model phased out then they looked for a more reliable system although it can be easily done on through hole and single sided boards

author: vk6zgo
author_rank: Super Contributor
timestamp: September 02, 2012, 06:38:22 am
content: I started to say"By & large,a 15MHz single section LPF is unlikely to be used in a situation where it has to have a lot of loss around 70MHz & above."-----but I realised as I wrote it,that is exactly the situation faced by an Amateur Radio operator on 14MHz or so,when a neighbour is trying to receive Australian CH2 analog TV,which has its vision carrier around 64.25MHz,& sound carriers around 69.75MHz.Remember TV front ends are broad as a bull's backside!Of course,it's not quite so bad,as the 5th harmonic will not have a lot of energy,but it definitely seems like a good idea to have several sections,as is commonly done.As to the bumps & hollows in the TG,if you can correct them,all is well,particularly as you usually only looking at a fairly restricted bandwidth,very seldom at full scan.I had a chance to buy an old Polyskop sweeper at the last Hamfest,but at 64kg ,& $300,I thought better of it.I only would have got all stirred up,messing around with filters,anyway!

author: poptones
author_rank: Frequent Contributor
timestamp: September 02, 2012, 06:39:47 am
content: we both attended Wayne State University...When were you in Detroit?

author: JackOfVA
author_rank: Unknown
timestamp: September 02, 2012, 11:10:12 am
content: Quote from: poptones on September 02, 2012, 06:39:47 amFrom birth through 1976 when I moved to the Washington DC area.

author: JackOfVA
author_rank: Unknown
timestamp: September 02, 2012, 12:31:16 pm
content: A few more filter images and measurements.First is Coilcraft's P7LP filter module. These are inexpensive 7th order low pass filters (under $5 each and free samples are available from Coilcraft) intended for PCB mounting. I made a small adapter board with BNC connectors and assembled a couple dozen P7LPs in various frequency ranges to use as quick and dirty filters for tests and the like. The main drawback to these filters is the ferrite core inductors used and the consequent intermodulation and harmonic distortion if strong signals pass through the filter. I'm sure my filter board could be improved as well, but it's adequate for my purposes. As the ripple in the stop band shows, this is an elliptic design filter.Next is a 24 MHz cutoff low pass filter used by RCA for 100 watt level marine SSB transmitters. It's all through hole parts. Looking at the response, I believe it's a Chebyshev design.Then is an RCA 37 MHz cutoff low pass filter for 250 watt FM two-way transmitters operating in the 30-35 MHz range. No images of the inside, though, as it's soldered shut. The filter response looks to be elliptic.Lastly is the home brew 15 MHz LP filter swept response overlaid with these three filters, 300 KHz - 1 GHz. Not unsurprisingly, the multiple stage filters show sharper transition from pass band to stop band -- that's why we use multiple section filters -- and all show spurious responses to some degree or other as the frequency increases to some multiple of the stop corner point.Although Dave did not mention it, his 15 MHz filter and the one I built to duplicate it is a Butterworth.

author: T4P
author_rank: Super Contributor
timestamp: September 02, 2012, 02:34:21 pm
content: Nice VNA you have there

author: lordnoxx
author_rank: Contributor
timestamp: September 13, 2012, 07:56:07 am
content: Hi Guys, I have a short question about the Rigol DSA 815.From its specs I can read that it has a frequency range from 9kHz to 1.5GHz. But at one or two points in the video Dave talks about a range of 0Hz to 1.5GHz. So whats the truth? Can it really go down to nearly 0Hz? And if so....why does Rigol specify a range starting with 9kHz?I am actually not quite familiar with spectrum analyzers so maybe you guys can help me there. Thanks!

author: EEVblog
author_rank: Unknown
timestamp: September 13, 2012, 11:23:01 am
content: Quote from: lordnoxx on September 13, 2012, 07:56:07 amNo, it can't, 9KHz is the minimum."DC" is just an RF expression that means "low frequency", it's not actually DC.If you want very low frequencies, what you want is a DSA (Dynamic Signal Analyser)Dave.

author: Dago
author_rank: Frequent Contributor
timestamp: September 14, 2012, 08:02:03 pm
content: I'd be really interested in a review of the signal hound which is a 4.4GHz spectrum analyzer you can attach to your computer. Base model is bit over 900$ and you can even buy a 4.4GHz tracking gen for around 500$. It seems unbelievably good for the price!http://www.signalhound.com/

author: olsenn
author_rank: Frequent Contributor
timestamp: September 14, 2012, 10:18:11 pm
content: QuoteThe problem with the signal hound is that is is extreamly slow. Some people on here mentioned how the DSA815 is slow... but compared to the SignalHound, it's the difference between a cheetah and a turtle! I don't think the noise floor/sensitivity is very good either. The real advantage to the SignalHound is bandwidth.

author: Unknown
author_rank: Unknown
timestamp: August 30, 2015, 01:30:27 pm
content: Quote from: EEVblog on September 13, 2012, 11:23:01 amQ: Why is the minimum 9kHz? What is the limitation?

author: tree
author_rank: Regular Contributor
timestamp: August 30, 2015, 05:01:55 pm
content: Quote from: kc3ase on August 30, 2015, 01:30:27 pmIf you're referring to why it doesn't go down to DC, then:There's a DC decoupling capacitor in series with the input. This prevents DC from being "seen". Only the AC is passed through.If you're wondering why it's 9kHz and not some other non-DC frequency, then the simple answer is that was the bandwidth that was designed in.

author: Christe4nM
author_rank: Unknown
timestamp: August 31, 2015, 01:44:40 am
content: 9kHz is the minimum because FCC has defined that RF energy is EM energy in the range 9kHz to 3000 GHz. (Source: Henry Ott, Electromagnetic Compatibility Engineering, page 7)As a result the industry took that as the standard minimum frequency for spectrum analyzers.

author: SNGLinks
author_rank: Regular Contributor
timestamp: August 31, 2015, 08:09:56 am
content: That's for RF spectrum analysers. There are AF analysers that go way below 9kHz.

author: rf-loop
author_rank: Super Contributor
timestamp: September 01, 2015, 12:54:15 pm
content: If I get this kind of result with what ever spectrum analyzer (with direct coaxial cable from TG out to SA input) I'm highly alarmed and start deep investigation what is going on before I continue to use spectrum for any important measurements.Something is badly. I'm surprised Dave did not stop show and before continue first look what hell is going on. This is NOT at all normal imho.I have long time experience with many kind of professional grade spectrum analyzer in practice and my eyes tell with this Rigol image that least there need suspect some proble. Is it user mistake or spectrum analyzer itself problem can not know with looking just some video. But if it is my equipment I do some real tests for know more.I did one example for just thinking what is going on there. (small box where signal trace continue)(Image is from this Dave's video but perspective corrected some amount)

author: SNGLinks
author_rank: Regular Contributor
timestamp: September 01, 2015, 01:56:58 pm
content: It's 0.5 dB per division so it looks bad. It's within spec.

author: rf-loop
author_rank: Super Contributor
timestamp: September 01, 2015, 02:24:39 pm
content: Quote from: SNGLinks on September 01, 2015, 01:56:58 pmOf course I know there is 0.5dB / div setting, so what, it do not matter at all if there is 0.1dB or 1dB or 10dB/div. But these jumps and small blocks sharp edge level shifts do not look normal (specially because shifted waveform parts match nicely to each others if you remove imagined shifts. As I told, if it is my equipment I will start investigate what it is. (It is easy to test if know what is doing and have right equipments)Also it is not question if it is inside specs or not and also this do not tell anything bout if it is inside specs or not. But, as told, this is not question.It is hard to believe these "steps" are norrmal, even when spectrum is low-end entry level equipment as this is.

author: rf-loop
author_rank: Super Contributor
timestamp: September 01, 2015, 06:19:24 pm
content: Quote from: PeDre on September 01, 2015, 04:44:30 pmThank you. This explain it. (lets think it explain it fully until someone show other)I believe (and hope) there is possible to run it also without cal correction for some user needs where these sharp level steps may disturb some special shape measurements because it do not interpolate between cal points. It is not nice if there is sharp step just in critical point if example doing some filter adjustments.Least it is important that user know his equipment for avoid mistakes.