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Open Source HW RF Signal Generator

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I believe many of you guys have followed my attempt to build a spectrum analyzer that ultimately fizzled out because of Feature-Creep and system-complexity that drove cost and development-time up. In the end I bought a R3131A Spectrum Analyzer on ebay and despite it having some quirks, I'm pretty happy with it so far.

That being said, I now still miss one device here: A Signal-Gen capable of producing predictable results at frequencies higher than 6MHz (Cheap DDS-Generators are only good to that frequency, even though they say on the front they're capable of 24MHz).

The specs of this thing (which are FINAL now!) are the following:

- 10kHz to 3GHz Frequency Span (to match my Spectrum Analyzer)
- -120 to +13dBm digital Amplitude control (This does not need to be flat all the way up to 3GHz, I'd be ok with some amplitude-drop above 2GHz)
- Capability of the main Signal-Chain to send the signal out to an external Module and receive the signal from an external Module (This module could add different forms of modulation to the signal)
- Internal TCXO and ability to take an external reference-clock (from a GPSDO, for example)
- Use of Vero-Board where possible (High frequency signal-path is on custom-made PCB)
- Fit inside a 19inch case

To achieve this, my first idea was to use the ADF4351 Synthesizer-Module I bought for the Spectrum-Analyzer project. It works from 35MHz up to 4.4GHz.
For lower frequencies I'm using a DDS-Module based on the AD9851-Chip. I have already built a simple Network-Analyzer with that module and it worked pretty nice.

Unfortunately, the ADF4351 can only produce a square-wave. So I want to use several LC-Filters to form that square into a sine, which are switched in and out by MASWSS0115TR-3000 switches.
There are several difficulties associated with this approach, so I'm open to different ideas, like frequency mixing, if they can produce a better sine-wave.

The next difficulty is to keep the amplitude constant across the frequency-band. For frequencies up to 60MHz, a JFET-Attenuator or even a variable gain OpAmp is the way to go. For frequencies higher than that, a PIN-Diode Attenuator, controlled by an ALC-Circuit, takes care of precise attenuation of the signal that is then sent through a step-attenuator made out of 10 and 15dB Attenuator-Chips by M/A-COM. This Attenuator provides up to 110dB of attenuation in 10dB-Steps.

I hope, this time I'm not running out of  patience and/or time and actually finish this project :)

For a quicker Overview, now that the "Mark III Prototype" is ready, I have written down the Specs that I'm aiming for at the moment below.

- AC Coupled RF Output: 9kHz - 6.8 Ghz Sine Wave (Filtered Square-Wave from ADF4355/ADF4356 or ADF5355/ADF5356)
- Optional 13.6 GHz AC-Coupled Square-Wave Output (if ADF5355 / ADF5356 is fitted)
- Optional DC Coupled Output: DC - 400 MHz Arbitrary Waveform from AD9957 DDS-Chip + 10M04SCE144 (Intel Max10) FPGA and 16bit Databus SDRAM (Sample-Rate for data-stream from FPGA: At least 125 MS/s - Depends on how fast I can get data out of the SDRAM and how fast the FPGA runs - I'll be using the "C8"-Grade. The I7 or A7-Grade may allow for higher speeds. The AD9957 allows up to 250MS/s at its parallel data-port.)
- Output Power: -90 dBm or lower to +10 dBm (up to 3 GHz) / 0 dBm (up to 6.8 GHz)
- Amplitude Control: 1 dB Steps or better
- Spectral Purity: -20 dBc for first Harmonic or better at 0 dBm Output-Power (Previous prototype based on modules yielded -30 dBc and better for most frequencies)
- Modulation of Output-Signal (IF generated by AD9957+FPGA and up-converted by mixing it with the LO from the HF Signal Source)
- Touch-Display + Keyboard Control Interface (Optional)
- USB Control
- PLL to lock Master Clock to external 10 MHz Reference (Master Oscillator: PL500-17 with 20 MHz Crystal)
- 2x 10 MHz Sine-Wave Reference Outputs (Master Oscillator / 2 with 74xxx74-Chip and buffered by PL133-37)
- STM32F407 CPU
- FT230XS for debugging and in case I can't get the virtual COM-Port to work :)

- Price: 350 € or lower in material per Unit (Partially assembled Boards, components, metal for case, step attenuator-module, etc. - Components bought in single quantity, minimum order for boards).

Link for the GitHub: https://github.com/SaabFAN/ORFSG---OpenSourceRFSignalGenerator

Quick update:
Based on the Suggestions in the "ADF4351 Sinewave"-Thread, this is the current block-diagram for the LPF-Switching. You can also see the Automatic level control in this picture

Just a thought, but something like an AD9912 clocking at 1GHz will get you near DC to maybe 400MHz sine in one hit, and the 9912 has a convenient fast comparator to turn the sine wave output into a square wave, it also programs over SPI, and has built in chirp and limited AWG functionality....

Then the square waves harmonics can be selected out with a filter to get the higher bands, maybe gets you to a GHz or so in 3 or 4 sets of BPFs. Don't forget that a wide tuning range VCO (Possibly YIG) can be phase locked to the output of the DDS, getting you DDS tuning at GHz speeds. 

Relatively low noise 1GHz VCXO are available from Crystek, and PLL chips for locking to the 10MHz reference are an AD/TI sort of thing, low pass filters are a minicircuits thing if you don't want to roll your own.

I would note that a programmable 140dB attenuator at microwave will be more challenging from a screening and leakage perspective then you would ever think possible, this one part is going to take much experimentation, I would personally build (Or buy surplus) a 0-60dB attenuators and then stock a few 30dB in line pads to screw onto the front, less annoying that way.

Personally, I would buy a second hand unit from someone like HP/R&S and have done with it, life is too short, but that is because I have interests that use such sources, but am not really interested in designing them.

Regards, Dan.

The AD9912 is interesting, but I haven't found a module with it yet (I'm not comfortable with spending 50+€ on a Chip that I'm going to have trouble soldering).
There is a module available with the AD9910 though. Worth a look - Would remove the need of LowPass-Filters at lower frequencies (switching to the ADF4351 at 400MHz would remove at least 2 tuneable Filters and leave only fixed filters. The price-tag of 65€ for all the features it has is ok.

It is definitely is something I'm going to add in the future, but for the moment I'd like to keep expenses down to a minimum.
Which is why I'm going to attempt to build the LowPass-Filters myself (except for the 750MHz one, for which I already have the appropriate MiniCircuits-Part).

Using some old used equipment would be ideal, but unless a unit capable of getting as high as 2GHz drops below 250€, I'll try to make the device myself. :)

I wouldn't use seperate filters but integrate them on the PCB. You can design the filters using SVC filter designer ( http://www.tonnesoftware.com/svcfilter.html ). For up to -say- 800MHz you can built them using surface mounted 0603 parts. Just make sure each capacitor has 1 or more vias to the ground plane to make the filter work. For >800MHz microstripline filters are the way to go. I have created a program which can convert the component values found by SVC filter designer into an (elliptic) microstripline filter (see http://nctdev.nl/page_pg%3d17.htm ). PM me if you want me to pitch in a license for this project. Either way you can download it and play with it a little. IIRC there is also a Sonnet Lite EM simulator example project bundled in the installer. Sonnet Lite ( http://www.sonnetsoftware.com/products/lite/ ) can be used to simulate microstripline structures in general and may be helpful for this project.


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