School spectrum analyzer project.

[Smokey]

Learn how to make a real circuit board and have them made at JLCPCB or something.  However your project turns out, you will at least be able to make circuit boards at the end of it.

[BlownUpCapacitor]

Learn how to make a real circuit board

Already do! https://youtu.be/02W6lhY8H5M?si=VaUy5YmIQaYIvUBl

Jokes aside, I will. I have already designed a few PCBs on easyeda and ordered from jlcpcb. I get free pcbs from jlcpcb essentially turns out. If you use easyeda you get a $8 coupon everymonth that covers the pcb cost. If I order say a $7 pcb, and use the $8 coupon, all I have to pay for is shipping.

[Sensorcat]

A lot of stuff for a 15 year old eh? My local e-waste center sells this kind of stuff for very very very cheap. The 7904 was $50, the 5110 with all plug-ins as $75, and the BK 4040 was $20. Just some small repairs and cleaning were all that was needed.

OK, your situation is completely different to what I imagined. I don't think anything important is missing. That's good, because building an instrument without an instrument is almost impossible.

So you are 15? It could be that you are the youngest person to ever build a spectrum analyzer. And once you succeed, the only problem is that many people will not be able estimate the achievement.

[BlownUpCapacitor]

Update: Don't expect anything new until June 10th. I have finals coming up and my grades are falling behind. A series of screw-ups and simple mistakes cost me a bit.

[zapta]

Have you considered to use a fast ADC and FFT algorithm, e.g. on a raspberry Pi or a PC?  It may be easier to design and more flexible to use. Just sample a short burst of samples at 20 Mhz, and use a python script to compute the FFT and display the graph.  The python script is trivial once you read the burst of ADC samples.

[BlownUpCapacitor]

Have you considered to use a fast ADC and FFT algorithm, e.g. on a raspberry Pi or a PC?  It may be easier to design and more flexible to use. Just sample a short burst of samples at 20 Mhz, and use a python script to compute the FFT and display the graph.  The python script is trivial once you read the burst of ADC samples.

I could, and that would be easier, however, the school requires that I learn something very significant from this. I think if I do that, I'll learn how to code and use uCs, and some RF design, but not much else in terms of electronics I think.

A swept front end would give me lots of experience in RF and electronics design. Plus I have until January to complete this. That's nearly 6 months. Really the project is due near end of December, as I have to write up a 25 page report on this project, but still almost, or maybe more than half a year's time for this project. So it can't be too simple.

[zapta]

Makes sense. I don't know what the requirements at your school are, just speculating.

If you want to add complexity, once the fft analyzer works, you can add to it a frontend mixer that will shift a narrow band from high frequency to lower frequncy. This will give your analyzer high resolution and at high frequency bands. Probably much better than you can do with mixer only. And then you can record it over time and generate a spectrum over time graphs. It's a good segue to SDR and signal processing.

https://training.dewesoft.com/images/uploads/29/fft_3d_measurement_1587721452.png

[BlownUpCapacitor]

Update: Don't expect anything new until June 10th. I have finals coming up and my grades are falling behind. A series of screw-ups and simple mistakes cost me a bit.

Yea, uhmm, I got bored from studying Chemistry after an hour and decided to create a 3D model of the Tektronix 5000 series plug-in chassis. https://www.tinkercad.com/things/iGbtzBgoZeZ-tektronix-5000-series-plug-in-shassee

I plan to 3D print this and use it to make my project work in a Tek 5110. I just need to figure out how to do that in the first place.

Stoichiometry hurts my brain sometimes.

Anyone know some good pointers for Lewis structures too?

[erikka]

In case you have still time to consider options you could search ebay.com for these modules "rf oscillator voltage controlled"
They come in different frequency ranges.
Using such a module, a mixer, a band pass filter at the high if frequency, a second module and mixer for the down conversion to the low if where you have the resolution filters, your most difficult components become the high if filter and the resolution filter(s)
For the log power detector you can use a AD8307 module, also from ebay
The log power detector doubles as a generic rf power meter that you can use, together with one of the vco modules, to test your if filter.
You can use a 4 opamp ic and some potmeters and other passive components to build a voltage sweeper where you have one potmeter for the center frequency and one for the span.
In this way you can build an sa without microcontroller, using a xy scope for display,  and I know it will work as I did build it.

[BlownUpCapacitor]

So I made the 3D model easier to 3D print: https://www.tinkercad.com/things/82Jht3C4EZv-3d-printable

Also, my finals start tomorrow... and my first one is chemistry. Wish me luck on figuring out Lewis structures, stoichiometry, atomic theory, and literally everything else as a freshman with 45 seconds per question.

Thanks.

Update: I think I got at least a D- 

[Solder_Junkie]

If you want to view up to 500 MHz with a few Hz resolution bandwidth, this simple idea is hard to beat... not really what you are looking for but a fun and very low cost way to check IMD and keying bandwidth of transmitters.

https://www.qsl.net/g4aon/ssa/

SJ

[BlownUpCapacitor]

Does anyone have any tips on creating LC oscillators?

So while experimenting with a super simple variable resolution filter, I finally decided to ask about this pet peeve.

So whenever I make an LC tank circuit for, well, ANYTHING, there is always something wrong. Either the Q of the system is too low, or the inductance of the coil is calculated wrong somehow and the resonance of the tank circuit is off by a few megahertz. And when I move my hand too close, the thing changes its resonance also. Same thing with an oscilloscope probe when I'm trying to tune the thing with a trimmer cap, and when I remove the probe it oscillates at a few megahertz higher AFTER I installed it in such a way that I can not fix it.

So far, the only thing that could possibly bypass the scope probe thing that I've thought of and tried myself is using a different coil to induce an AC magnetic field in the tank circuit and use another to pick up the resonance from another coil that feeds into a buffer to prevent any capacitive changes that would mess up my resonance. But my hand, even 4 inches away messes with it when trying to tune using this method.

Does anyone here hold some tips they can share that would help?

[Solder_Junkie]

And you want to build a spectrum analyser? Making an RF oscillator is pretty basic stuff, your image shows both a crystal and an inductor, and a physically large inductor at that!

Either you want an LC oscillator, or a crystal one, don’t try to combine them.

There are literally hundreds of circuits on the web for crystal oscillators, one of the most reliable types is the Collpits oscillator. If you want a really clean crystal oscillator, there are techniques to do that too. This set of low noise beat frequency oscillators are based on work by Ulrich Rohde ( of Rohde and Schwarz fame):
https://www.qsl.net/g4aon/pdfs/BFO%20v2_3.pdf

SJ

[BlownUpCapacitor]

The main reason why I'm using a tank circuit is because it offers a low impedance to other frequencies compared to a high impedance for it's resonance frequency. This is something a crystal can not provide as it is inverse. The crystal has a low impedance for a single frequency (ignoring spurious resonances) and a high impedance for other frequencies.

I've also gotten around the LC resonance shift problem by shielding the crap out of it using copper clad boards or some other thing. This has worked okay, but it uses more material than I'd like, thus my question of tips. But if it is the only true way, I guess I'll keep doing that.