Hello,
I have toyed with the idea of getting a spectrum analyzer for quite a long time now, but even the cheapest ones on ebay are way outside my budget: Even defective ones are seldom below 700€, working ones start at 1200€ and go up quite fast.
There are a few PCBs out of China inside my budget-range, but those normally start at 500Mhz and go up to 1.5Ghz or higher. Again nothing for me there.
So I decided to build one. If I don't mess up the final exam, I have 5 months of time ahead of me which I can dedicate to fiddling with electronics and this project should keep me occupied for most of that time. Plus it's probably a good learning exercise. And who knows: Maybe I can do math afterall? (The VCO seems to require quite a bit of calculation)
To add a little challenge, I have set the max. Budget (For stuff to buy in addition to what I already have) to 70€. This means that I will have to build most of the device with stuff from my father and grandfather.
The specs so far:
- Frequency Range: Dual Ranges: Audio-Spectrum & 100khz up to 225Mhz
- 2 Input-Modes: 50Ohms for full Frequency Range & High Impedance (Input connected directly to fast Input-Amp) for low Frequencies up to 15Mhz - Switchable with Reed-Relay
- Stepped Attenuator in the Input-Stage (Explanation: I plan to measure DC/DC-Converter and 12V-Rails on a sailing boat with this device to make sure the MPPT and Battery-Chargers - Another, mostly finished project - dont interfere with the Echo-Sounder and other navigation-equipment)
- 2 Stage Superhet-Receiver with Digitizer at the end
- 8/16bit parallel ADC with 2x 2K x 8bit SRAM as sample-memory (I have a ADC0804LCN 8bit ADC in my partbox, so I will start with that one and later put a 16bit ADC in there)
- Digitally controlled Sweep
- Integrated Display (I bought a 3.5Inch Touchscreen last year, which has lost its purpose because the project I wanted to use it for will use a bigger screen) with GUI
- ARM-CPU (Arduino Due)
- Fits into Schroff 19'' 2 Height-Unit Case
- 220V, 12V (Optional, if it doesn't eat too much power: Internal Battery - I have a few Lithium Accumulators available from my not so successful attempt to modify a MHS-5200A Signal Generator)
[li]*UPDATE* Tracking-Generator - Frequency: TBD
[/li][/list]
Theory of operation (Aquisition):
INPUT -> Input-Attenuator/Amplifier -> Mixer with PLL-Controlled VCO (SA612 or 615 if the 612 is too "weak" for a 500Mhz LO / The PLL is a Philips TSA5514) -> 250Mhz Bandpass -> Downconverting Mixer -> 10.7Mhz Crystal Filter -> Variable Gain Amplifier -> Detector/Rectifier -> ADC -> SRAM(AQ)
Theory of operation (Output):
SRAM(AQ) -> Arduino -> Display / Serial Datastream
The whole design will borrow heavily from "The Simple Man's Spectrum Analyzer" by Hans Summers
http://www.hanssummers.com/spectrumanalyser.html, as well as s PC-Bus based Spectrum Analyzer from DKD Instruments described in a PDF I found online (Written by Dan Doberstein and John Cardone - Link:
http://www.n5dux.com/ham/files/pdf/Spectrum%20Analyzer.pdf).
I hope this will be at least a nice exercise and interesting learning experience that results (Best case scenario) in a functioning Spectrum Analyzer for less than 150€.
I will post first schematics sometime next week and will release the Software here too, once I have written it.
Do you guys have any ideas, suggestions, etc.? I'm rather new anything above 20Mhz. I looked over a few service-manuals for Philips Signal Generators and read a few Application Notes regarding HF, but that's basically it.
The first challenge will probably be to build a VCO that is able to reach the 450Mhz required to upconvert the input-signal to the 250Mhz of the first IF.
Btw. T&M-Equipment available: Rigol DS1104Z, Philips PM3320 200Mhz DSO, Fluke 87, Cheap Ebay 2.4Ghz Frequency Counter, 12Mhz Arb-Waveform-Gen.