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Good sources for making a DIY swept front end spectrum analyzer?
Posted by BlownUpCapacitor on 22 Mar, 2024 18:21 -
Hi, I am wondering if anyone knows any good sources for making spectrum analyzers. I looked at my local library and all I could find were some basic electronics 101 books.
I don't necessarily need sources that explain how a spectrum analyzer works, but rather maybe you can point me towards sources that explain the design of the things used in such devices such as voltage controlled oscillators, IF filters, mixers, and so on.
This is for a school project. In freshman year in my HS, we do something called a personal project that lasts about 8 months until sophomore year. We research, think about, and do our project. This could be baking, making a guillotine (yes someone made that in a previous year), or making a robot to help victims of forest fires get to safety. I chose to make an analog swept front end spectrum analyzer. I plan to make this work in a Tektronix 5110 mainframe.
Anyway, I'm just a bit under educated on how certain things work. I am quite familiar with electronics, It's just that I can't find any good sources that explain in great understandable detail on how certain things work. All I know about a VCO is that a varidiode will change it's capacitance based on the voltage applied to it, and we can use that to make a VCO. I'm also a bit confused on how the hell a logarithmic amplifier works.
My goal is to make a spectrum analyzer work with a bandwidth of 0-10MHz.
My basic understanding of a swept front end spectrum analyzer is that it is just a superhederodyne AM receiver with wider bandwidth and can tune across a number of frequencies really fast. -
There is a VCO in the CMOS 40XX series that is good for a swept VCO. Very linear. Sadly, I forget the number. You'll need a good balanced mixer. making the front end track the local oscillator + or - your I.F. frequency is a bit of a trick. Going 0 to 10MHz may be far more challenging than going from perhaps 5MHz to 10MHz where you are only going one octave and not several decades. Coming off your balanced mixer you may want a crystal filter to keep some of the junk out of your I.F. strip. Log amplifiers have been around since the radars of the WWII. They usually involve the log function of a small signal diode. We used the non-linear characteristics in the S.T.C. Sensitivity Time Control because the strength of the echo diminishes as the square of the distance so the gain starts out low for close in echos and advances to max gain over time through a non-linear function. Remember in your analyzer that High-Q narrow bandwidth circuits take time to 'ring up' like trying to accelerate a heavy weight on a pendulum and then have the nasty habit of continuing to ring which is the quality factor of the Q, sharp peak, low loss. Point being 'if you sweep to fast across a band of frequencies the I.F. won't have time to properly respond. That is why the old tube type analyzers swept slow and used the same sort of long persistence phosphors like the radar scopes to get a usable readable trace at 1 to 5 sweeps per second. Of course, now it is all digital storage and LCD screens.
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Very time consuming and very complicated. Better alternative is arduino + some RF IC, like si4732-a10 or similar.
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There is a VCO in the CMOS 40XX series that is good for a swept VCO.
Thanks, I'll look into that.Coming off your balanced mixer you may want a crystal filter to keep some of the junk out of your I.F. strip.
Yes, I'm planning to use 3 stages of standard 455kHz IF transformers and a crystal filter to improve selectivity.Point being 'if you sweep to fast across a band of frequencies the I.F. won't have time to properly respond. That is why the old tube type analyzers swept slow and used the same sort of long persistence phosphors like the radar scopes to get a usable readable trace at 1 to 5 sweeps per second. Of course, now it is all digital storage and LCD screens.
Slow sweep speeds won't be an issue for me. I can use a storage CRT scope such as the Tek 7613 or Tek 314.Very time consuming and very complicated. Better alternative is arduino + some RF IC, like si4732-a10 or similar.
I don't want to do something as simple as that. I already know like 4 people who are doing an arduino project where they use specialized pre-built devices, almost like a kit. I like to keep things as discrete and jellybean as possible.
Plus I have 8 months to do this, so plenty of time. -
No one expects you to build this, but you can Learn from the Master:
https://lea.hamradio.si/~s53mv/spectana/sa.html
This was popular:
https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1985/11/page23/index.html
The must have books for a beginner in RF:
Experimental Methods in RF Design, Bob Larkin, Rick Campbell, and Wes Hayward
Has a full Spectrum Analyzer design that is very good and inexpensive. Covers home made crystal filters well, at levels from beginner to advanced. Covers VCO design and Preamp / IF /RF amplifiers from DC to 2 Ghz Its a great book, but it has doubled in price recently, for an 8 month school project it's perfect.. Try to find it used or on inter-library loan. Goes into the theory and design of every receiver circuit element without high order math from a amateurs' radio point of view. Learn what is in that book and have a good start on a career. I think of it as a book that you'd have to pry from my cold, dead, hands...
But NOT dumbed down like the ARRL Handbook, of which OLDER like 1980s-1990s editions are a good read...
Navy NEETS, the full set... Free from the US Navy...
Art of Electronics, Volume one, Winfield Hill.. Probably can find a PDF on-line.
You need this chart eventually:
https://www.minicircuits.com/app/DG03-111.pdf
You need this chart now...
https://www.minicircuits.com/app/AN40-012.pdf
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You will eventually need the circuit in figure 6 of this:
https://interferencetechnology.com/wp-content/uploads/2015/02/WyattArticle.pdf
Also make it with a 1 Mhz TTL Crystal oscillator
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Print out copies and download the images on this page:
https://techlib.com/electronics/detect.htm
You'll need a rudimentary RF voltmeter, the above page is it. Dr. Wenzel made that page for me way back when I was building a SA, like two decades ago. Its accurate.
You'll need attenuators:
https://www.qsl.net/z33t/rf_step_attenuator_eng.html
Eventually you will want a Ebay "DDS" frequency synthesizer for testing and measuring resolution and level.
For RF under 200 Mhz or so, Manhatten Style PC construction is great for beginners:
https://qrpme.com/docs/K7QO%20Manhattan.pdf
Snag some Analog Devices inc RF log Amps and Detectors... Their web site is a treasure trove for a beginner
Like these:, https://www.ebay.com/itm/166352071945?itmmeta=01HSMAM3EZGBGHTD6ZKK80N4PH&hash=item26bb5b2d09:g:rKgAAOSwKsFlFLEN&itmprp=enc%3AAQAJAAAA0EISSxez%2BBTXxKGsQFea8desWappsYYsz%2FnSnqNuzInJ8rr5A7OGWcg3xPL%2FHpJR4dAm3Ezj8EkPOLlidRfntmKKk7I%2FWGWCvFcJ2igsyf%2B6TpSgfCKaS10YeTcw3xzvehi4yjheZn3BaIzCvwXZfz1hoCE7%2BY0F9ssrStos%2Br5CdiobDBIDMEkeyRosjBwnX%2F85%2F5kv1USYFyMoSdK%2BaaL7hUWOVaYX0PMfy6i7v4hcfuro2xEOhwjxUXHP4Ai3pDlPypm9u8Bz%2B13hzrqujMU%3D%7Ctkp%3ABk9SR9S30IrNYw
There are other varients.
Shop Class: Interdigital filters are a key part in High End Spectrum Analyzers with High Side IFs for greater performance. This is a Java calculator that helps you make them:
https://www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php
Can be made with hand saw, file, taps, and drill, but have your parents permission before using power tools and have supervision.
This program, you may need later: http://www.hp.woodshot.com/ One of my favorite tricks with Appcad is to turn hobby shop brass tubing into Solid Coax Cables and RF fittings such as cased diode detectors.
https://www.broadcom.com/info/wireless/appcad Also hosted at Broadcom...
Lastly: www.minicircuits.com
www.rfparts.com
www.amplifiedparts.com (Great Knobs)
https://www.rf-microwave.com/en/home/ (Italy, but ships to US)
www.mcmaster.com = Hardware treasure trove.
Steve
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Sounds like we have an up and coming future stellar Scientist/Engineer in the making
Quite a project for freshman college student, much less a HS student, so hats off for taking this on
Lots of good advice and info sources above.
If you want to get somewhat lost in just a small section of the SA circuitry, look deeply into Log Amps, especially the more modern Successive Detection Types. We've utilized this concept often in IC design, even directly at very high frequencies.
Good luck and please keep us posted on your project!!!
Best, -
BUC, I'm done with updating my post. Good Luck.. Rf takes patience, and a lifetime of learning. Enjoy... I'll keep an eye on the thread.
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Just a quick question that I probably already know the answer to; will these relays shown in the image be good for 10MHz? I would assume they would suffice, but I am discouraged by the complex attenuation system I see in low bandwidth scopes such as my Tek SC502 and Tek 5A18. The 5A18 is only 2 MHz and looks more sophisticated than what I was planning with these relays.
The black relay is some kind of random relay I got out of a microwave oven, and the orange one is from the surplus of relays I used to build my CW transceiver. I'm not a total stranger to RF. And yes I have an amateur license. I bought these off amazon and they're advertised as signal relays. The transceiver works fine at 3.579546MHz and 7.1MHz with "faster?" (is that the correct word?) crystals. SOOOOO I would assume the relays work fine at 10MHz???
My plan for this project is to follow this very rough road map. First, research and gain knowledge, start thinking about what I need for this project and if they would work well in it, design something, make a prototype, improve the prototype, make it look nice, design PCBs, get physical PCBs, 3D print a nice housing, assemble, showcase my project next year, done.
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i believe the "Experimental methods in RF Design" spectrum analyzer is the best option.
By adding some modern components you will be able simplify the device.
Add a DDS, Arduino and a Logamp (AD8307) you will be able to have a digital control and use a laptop as the display instead of a oscilloscope. -
A couple of books, repeated from my post to a different thread about SAs:
Two useful reference books about the previous generation of swept-spectrum SAs with digital control and analog processing (ca. 1990) with examples from Tektronix.
(1) M Engelson Modern Spectrum Analyzer Measurements JMS, 1991; Practical uses.
(2) M Engelson Modern Spectrum Analyzer Theory and Applications Artech House, 1984; More mathematical background than (1). -
Look in the Tektronix 'Circuit Concepts' series, too, i.e.:
Spectrum Analyzer Circuits by Morris Engelson (1969)
https://w140.com/tekwiki/images/5/53/062-1055-00.pdf -
see mini-circuts for amps. attn, mixers, VCO for RF/VHF,UHF bands,
https://www.minicircuits.com/
Use attenuators and protection, since mixers and front ends are prone blow out if overloaded.
Jon -
I have a bunch of analyzers and recently bought an MMS but building Scotty's looked like fun, so maybe 5yrs ago, I bought a complete set of extra boards, all done by Osh, from a friend. He built it and really likes it. I doubt i will ever build it, but having the boards is a motivator.
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If you can change the range to about 50-800MHz, you can modify an old TV tuner. http://www.gbppr.net/wireless/appendixF.html#9
Or mod a cheap RTL-SDR. There's software to use a RTL-SDR as a spectrum analyzer, but the latency of the frequency select makes sweeps very slow. If you could have a microcontroller take care of the sweeping and somehow sync the data capture to it, it can work much faster. -
What an ambitious project! I think that's a pretty cool challenge to take on. Good luck!
I would definitely advise you to buy a VCO IC. I built my own analog VCO and that alone took 6 months to get right. You can do it, but you have bigger integration projects to focus on.
There's an SSM chip that will do that for you. I forget the number.
Typically for a VCO you want one volt of input control voltage to double the frequency it outputs. This means an exponential conversion of f_out = 2^(Vin). That's where the expo /log converter comes in. This is the tricky part of a discrete VCO to build. It's a very sensitive circuit, needs temperature compensation, etc.
VCO ICs should have one built in so you can just input a 1V/octave control signal and it takes it from there.
There's a community of DIY music synthesizer folks online. VCOs and amplifiers is what it's all about. If you search for "DIY synth" or "sdiy" you will find a lot of resources including open source schematics for things you could repurpose for your homebrew test equipment.
Check out the "music tech DIY" forum at modwiggler.com. Lots of helpful folks and links to more resources there. -
I actually built the Wes Hayward design about two years ago. I put together a three video series on it on my alter-ego YouTube channel, here's the playlist:
https://www.youtube.com/playlist?list=PLYNQ4erd0Qw_i-QfPq328sHskbDIZSACk
Also Wes still maintains some additional notes and subsequent mods on his design on his website.
https://w7zoi.net/sa-stuff.html
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Update: So I finally got around to actually starting some prototype designs, and I've got down two of the main parts. The mixer and VCO. I still need the detector, log amp, attenuator, LPF, and other stuff; those I'll save for another weekend.
For the mixer, I've tried using a ring diode mixer mentioned in some of the sources provided. But I just couldn't get it to work. I think it's just the very low impedance of my transformers. They only start to be effective at 100MHz from my Tek 191; I think that's just the RF transmitting from one side of the board to the other, and not actually being conducted.
So I opted for a different design. I remembered that my dual tube transceiver project that I copied from LA6NCA used a 6BE6 pentagrid converter tube to mix the LO and the RF in to make a CW tone, and I just so happened to have a couple extra 6BE6s lying around. So I just decided to use that; it worked perfectly. I could have just used a dual-gated MOSFET, but I think using tubes is just much cooler.
Also, about the VCO, it has a range of about 5MHz to 8MHz, so that limits my bandwidth to 3MHz. I wanted 10MHz, but that's proving difficult. Most of the sources I see are using commercially manufactured VCOs; they're also for microwave ranges.
Anyway, just a brief update. It's 11pm right now and I need sleep.
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There is a VCO in the CMOS 40XX series that is good for a swept VCO. Very linear. Sadly, I forget the number.
Is the 74HC4046 what you were thinking of?