Spectrum analyzer build concept - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 10, 2016, 10:08:56 pm
content: Hello again!I was browsing the ebays earlier, and found a giant market of these little RF eval-style boards, little LNA's, mixers, filters, detectors, etc. with some SMA's on a bit of pcb, that costs next to nothing. 5-6 dollars with free shipping, typically.Here's an example:http://www.ebay.com/itm/RF-Wide-Band-Amplifier-30dB-High-Gain-Low-Noise-LNA-Amplifier-0-1-2000MHz-/172398024811?hash=item2823b9186b:g:kZQAAOSw5cNYHDKGSo I'm going to lay out the groundwork for a spectrum analyzer made mostly from these modules. Here's the block diagram for what I'm going for. obviously with this construction, changing the order of amps, filters and whatnot is easy.It's gonna be a ca. 1-20 MHz. I'm modest. These modules typically don't go lower than 1MHz, but up to around 500, so plenty of dev room for later.I've attached the schematic, it's pretty much standard. Mixing with a swept frequency from an RF VCO driven by a ramp generator and then through a log detector.The biggest issue is the VCO (VTO? what's most RF-y to say?) - Can't find that as a module, unless i want a band in the UHF region, so I guess that'll have to be made as a little varactor based oscillator.I gather the ramp generator has to be fairly stable too, and I still haven't gotten my head around how to set a smaller freq. range. Amplitude would lower the max freq. and dc offset could raise the lower, but it'd be fidget-y to have to settings, that affect the other, like that.What do you think? Is this a terrible idea? I'm not aiming for a super high-end instrument, anything that could show me some harmonics and maybe crude filter response, with a tracking gen. would be pleasant.Thanks in advance.

author: Bicurico
author_rank: Super Contributor
timestamp: November 10, 2016, 11:39:24 pm
content: Hi,You may want to visit my blog... (http://vma-satellite.blogspot.pt/)I have been spending huge amounts of time programming a new software for the Chinese NWT500 clones, sold as NWT4000 (with two ADF4350/ADF4351 synthesizers for simultaneous Input/Output) or as SMA Simple Spectrum Analyser/Signal Generator (with just one ADF4350/ADF4351 for alternate Input/Output).These devices are cheap (around 60 Euro for the cheap ones and up to 280 Euro for the latest enhanced NWT6000 model).You can get a good idea of which components are being used and I can tell you that the results are pretty amazing for such a cheap device.I hope this helps in your quest!Regards,Vitor

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 11, 2016, 02:27:34 pm
content: Looks very nice, but I think I'll stick with cheap modules for now. I've also realized I may have botched my SA theory. I was planning to go for a "direct conversion" ish where the LO is in the same band as the signals to be analyzed - I'm seeing a fair bit of evidence you have to upconvert, to get an IF above your desired band. If that's the case, it might be as simple as to swap the 20 Mhz VCO i'd planned with a VHF VCO, readily available:http://www.ebay.com/itm/80-110MHZ-RF-Radio-Frequency-VCO-VTO-Voltage-Tunable-Oscillator-RF-Signal-Source-/331760189529?hash=item4d3e72c059:g:IkUAAOSweW5VZIRw

author: Fank1
author_rank: Regular Contributor
timestamp: November 12, 2016, 12:10:20 am
content: You need a sharp filter somewhere.As is it wont work because you are just amplifying the whole spectrum.The filter bandpass will determine what resolution you get.In addition the attenuator needs to be the first element in the circuit to prevent overloading the amplifiers.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 12, 2016, 03:13:44 pm
content: Gotcha, a resolution bandwidth filter after the mixer. Lets say I went for the 20mhz bandwidth i want, and upconvert it with the 80-100 MHz vco. Should it then be a 80-100 Mhz bpf? narrower? Or should it take the f_lo - f_in, so 60-80 Mhz? Can't seem to wrap my head around that for some reason.You're right about the attenuator, ofcourse.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 12, 2016, 05:13:58 pm
content: Okay, so updated design, see attached. I'm also including a link to the small eval/dev modules I'm finding, might be of interest to some.Ebay modules:30 db LNA:http://www.ebay.com/itm/RF-Wide-Band-Amplifier-30dB-High-Gain-Low-Noise-LNA-Amplifier-0-1-2000MHz-/172398024811?hash=item2823b9186b:g:kZQAAOSw5cNYHDKGMixer:http://www.ebay.com/itm/1PC-0-1-500MHz-AD831-high-frequency-RF-mixer-drive-Amplifier-Module-HF-VHF-UHF-/112073878843?hash=item1a181fad3b:g:aagAAOSwZVlXmVzlDigi attenuator:http://www.ebay.com/itm/1PC-Digital-Attenuator-PE4302-Parallel-Immediate-Mode/142140998106?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao %3D1%26asc%3D40148%26meid%3D002538719ee145e8833d3065f7baec23%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D112073878843Log detector:http://www.ebay.co.uk/itm/AD8310-0-1-440MHz-High-speed-H-frequency-RF-logarithmic-detector-power-meter-/331950538055?hash=item4d49cb3d47:g:oWYAAOSwV0RXvsXq80-120 MHz VCO:http://www.ebay.co.uk/itm/80-110MHZ-RF-Radio-Frequency-VCO-VTO-Voltage-Tunable-Oscillator-RF-Signal-Source/331760189529?_trksid=p2047675.c100013.m1986&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D38661%26meid%3Dea80d74d6e1040528e4952dd03e2acaa%26pid%3D100013%26rk%3D3%26rkt%3D12%26sd%3D222311694955

author: Kalvin
author_rank: Super Contributor
timestamp: November 12, 2016, 05:48:49 pm
content: I was wondering if a direct conversion receiver like the tayloe mixer followed by a low-pass filter (preferable adjustable bandwidth) and a cheap RMS detector/peak detector would make a simple spectrum analyzer. Although the VCO would need to run at four times the desired center frequency, the popular analog switches used in the ham radios are good up to 30+ MHz (120+ MHz clock frequency).

author: Kalvin
author_rank: Super Contributor
timestamp: November 13, 2016, 02:29:39 pm
content: Well, my suggestion of a direct conversion receiver using a tayloe mixer/detector has some problems: The tayloe mixer is a sampling mixer, which means that due to the sampling the harmonics of the carrier frequency will be folded to the baseband and detected as well (It can be shown that the odd harmonics of the carrier frequency are folded to the baseband and detected by the tayloe mixer/detector, see the paper below). For example if the tayloe mixer/detector is tuned to 1MHz, also the signals at 3MHz, 5MHz, 7Mhz ... are detected, which is bad of course. Thus, a more conventional spectrum analyzer design may be more practical in this case. Here is a nice paper "Analysis and comparison of switch-based frequency converters" by M. Soer analyzing the tayloe mixer/detector properties:http://essay.utwente.nl/58276/1/scriptie_Soer.pdf

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 14, 2016, 02:51:27 am
content: Sorry, this is a bit of a sidestep, but it has always confused me;These little RF block eval boards, like the ones I posted links to and pretty much made by all the big RF integrated circuit companies, usually with that "small square pcb with SMA's and corner holes" form factor -How are you supposed to get them in an enclosure?If the enclosure fits nicely the size of the board, it'll be amazing, only you can't get it in for the SMA's - if it's large enough, all sides with coax connectors won't fit out through the holes!Is the only salvation to either scrape off the solder mask from the ground plane, and solder foil over the RF path? Or fold a thin sheet box from zinc or copper and flex it enough to get the connectors through?Desoldering the connectors, and then re-soldering them in place would be terrible with the thermal mass of the enclosure, and just mounting panel connectors in the box and soldering them directly to the board would defeat the porpoise of having board mounted connectors in the first place.

author: steve_w
author_rank: Regular Contributor
timestamp: November 14, 2016, 04:28:24 am
content: ChristofferB,This web page may give you some ideas:http://www.scottyspectrumanalyzer.com/regardsSteve_w