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
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
EDIT:
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
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
- 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