LTDZ-DSP project is an attempt to improve the performance of this inexpensive 35MHz-4400MHz spectrum analyzer and tracking generator using only minimal hardware modifications, and by replacing the original firmware with this new version of the firmware, while maintaining the compatibility with the existing PC GUI applications.
Summary of the project goals are as follows:
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Improve the dynamic range of the scalar network analyzer from 50dB to 80dB.
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Implement digital receiver bandwidth filters with selectable bandwidth, which allows better spectrum resolution in the spectrum analyzer mode.
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Improve the linearity of the RMS/LOG-detector, providing more accurate and wider dynamic range up to 80dB.
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Provide firmware source code which can be used for experimenting with the LTDZ board.
The firmware source code is provided "as-is", without any support, without any guarantee of its correctness, or that the the firmware will perform as expected. In short: Use this firmware at your own risk.
Please note: It may not be possible to read and create a backup for the original firmware, as the boards may be shipped with copy-protection enabled. So, keep in mind that you will most probably lose the original firmware.
Finally, if you do not know how to use ST-Link programming tools, you will most probably end up with a bricked LTDZ board.
LTDZ is an inexpensive 35MHz-4400MHz spectrum analyzer and tracking generator, readily available from various on-line sellers in price range USD $40 - $50.
LTDZ provides three main operating modes:
- Signal generator 35MHz - 4400MHz.
- Spectrum analyzer 35MHz - 4400MHz.
- Scalar network analyzer 35MHz - 4400MHz.
The original LTDZ hardware and firmware provide approximately 50dB of dynamic range in spectrum analyzer operating mode and network analyzer operating mode. After a few and simple hardware modifications and a firmware update, the available dynamic range in the network analyzer mode can increased up to 80dB.
In the spectrum analyzer mode, the original receiver bandwidth filter (RBW-filter) is fixed to 120kHz, which makes it practically impossible to make RF measurements requiring good spectral resolution. The new firmware provides a set of digital filters with variable bandwidths from 3kHz to 120kHz, depending on the step size used during spectrum sweep, providing much better spectral resolution.
In the scalar network analyzer mode, the original receiver bandwidth filter (RBW-filter) is fixed to 120kHz, which results wide noise bandwidth, limiting the maximum dynamic range to 50dB. The new firmware implements a very narrow bandwidth receiver filter, providing dynamic range up to 80dB.
The new firmware will bypass the on-board AD8307 LOG/RMS-detector with STM32F103's 12-bit ADC (analog-to-digital converter). Applying some simple DSP (digital signal processing), it is possible to implement highly linear, high dynamic range digital LOG/RMS-detector up to 80dB of dynamic range. The on-board, fixed 120kHz RBW-filter will be sampled by the ADC, and the new firmware will provide a set of digital filter with adjustable bandwidths.
The new firmware maintains 100% compatibility with the existing serial command protocol, which means that the updated LTDZ board can be used with the readily available PC GUI applications, such as WinNWT5, WinNWT4 and NWT4000lin.
LTDZ has two ADF4351 wide-band 35MHz-4400MHz Fractional-N synthesizers. The first ADF4351 is used as a signal generator, which is also used as a tracking generator in the scalar network analyzer mode. The second ADF4351 is used as an local oscillator (LO) for the receiver mixer in spectrum analyzer and scalar network analyzer modes.
The mixer output is feeding the 120kHz 5th order LC low-pass filter (120kHz RBW-filter). The output of the RBW-filter is connected to the input of the AD8307 LOG/RMS-detector. The detected signal level from AD8307 is then sampled by the STM32F103's 12-bit ADC, and the signal level is sent to PC application over the on-board USB-to-serial interface.
It was found out that without any modifications the dynamic range of the LTDZ 35MHz-4400MHz spectrum analyzer and tracking generator is around 45dB - 50dB. The linearity of the on-board AD8307 LOG/RMS-detector becomes less accurate as the signal levels become lower than -40dB (relative to the full scale).
The receiver bandwidth is fixed to 120kHz by the 5th order receiver bandwidth LC-filter. As the receiver architecture is direct-conversion zero-IF, the actual receiver bandwidth is twice the 120kHz ie. 240kHz. This wide receiver bandwidth makes the device almost useless for making any practical measurements for RF filters, antenna tuning, spectrum analysis etc.for amateur radio purposes.
The relatively high noise floor of the LTDZ hardware design, the wide 120kHz RBW-filter, and the spurious noise components limit the maximum available dynamic range of the AD8307 LOG/RMS-detector to 40-50dB. Using digital signal processing, some of theses limitations can be compensated for better dynamic range and performance.
Please find LTDZ hardware modifications here.
Soldered ST-Link V2 programming header to the PCB.
Added following decoupling capacitors:
2 x 22uF ceramic capacitors to the outputs of the AMS1117 +3V3 regulators.
1 x 10uF ceramic capacitor to VDDA power supply.
The on-board 120kHz RBW-filter was redesigned to give better low-pass filter response between 0Hz - 120kHz.
Fortunately the filter topology did not change, so modifying the component values is pretty easy to do.
Design specifications for the new 130kHz RBW-filter
Component values for the new 130kHz RBW-filter
LT-Spice simulation for the new and original 120kHz RBW-filter
Added 2 x 12 Kohm biasing resistors to bias the ADC-input to 1.5V-1.6V.
The other 12 Kohm resistor is connected between the output of the 120 kHz RBW-filter and the +3.3V linear regulator. The other 12 Kohm resistor is connected between the output of the 120 kHz RBF-filter and ground.
Finally, wired the output of the RBW-filter to the ADC input PA.2.
STM32F103 MCU datasheet and documentation.
ADF4351 PLL/VCO product-page and documentation.
IAM81008 double-balanced 5GHz mixer datasheet.
AD8307 LOG/RMS-detector product-page and documentation.
AMS1117 linear voltage regulator datasheet.
PS3120A switched capacitor dc-dc-converter datasheet.
Project source code, tools and libraries:
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LTDZ project source code from https://github.com/kalvin2021/ltdz-dsp
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ARM GCC cross-compiler
gcc-arm-none-eabi. -
STM32CubeF1-1.8.3.
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STM32F10x_StdPeriph_Lib_V3.5.0.
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ST-Link V2 Programming Utility.
The build environment is targeted for Linux platforms.
However, since Windows 10 has now Linux subsystem available, setting up the build environment for Windows 10 should be quite easy as well.
This project has been built & tested with the following ARM GCC compiler versions:
gcc-arm-none-eabi-10-2020-q4-major-x86_64-linux.tar.bz2gcc-arm-none-eabi-6-2017-q2-update-linux.tar.bz2
It should be possible to build this project successfully with any other recent ARM GCC cross-compiler version.
The ARM GCC cross-compiler package can be downloaded from here:
User may install the downloaded ARM GCC cross-compiler tool-chain into any suitable directory.
This documentation assumes that the ARM GCC cross-compiler tool-chain will be
installed into the user's home directory under directory opt.
$ mkdir -p ~/opt
$ cd ~/opt
$ tar xjf ~/Downloads/gcc-arm-none-eabi-_version_-linux.tar.bz2
Checking the ARM GCC compiler version:
$ ~/opt/gcc-arm-none-eabi-10-2020-q4-major/bin/arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Arm Embedded Toolchain 10-2020-q4-major) 10.2.1 20201103 (release)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
The STM32CubeF1-1.8.3 Github-repository can be found here:
https://github.com/STMicroelectronics/STM32CubeF1/tree/v1.8.3
Download the zip-package from this link:
https://github.com/STMicroelectronics/STM32CubeF1/archive/refs/tags/v1.8.3.zip
Note: This document and the project's Makefile uses the same default installation
location for the zip-package that is used by the official STM32Cube ie.
~/STM32Cube/Repository/.
Should the user install the zip-package into some other location, it is required
to update the BSP_ROOT in the project's Makefile.
Install the downloaded zip-package into home directory under directory STM32Cube/Repository/STM32CubeF1-1.8.3:
$ mkdir -p ~/STM32Cube/Repository
$ cd ~/STM32Cube/Repository
$ unzip ~/Downloads/STM32CubeF1-1.8.3.zip
The STM32F10x_StdPeriph_Lib_V3.5.0 can be found here:
https://www.st.com/en/embedded-software/stsw-stm32054.html
Note: This document and the project's Makefile uses the same default installation
location for the zip-package that is used by the official STM32Cube ie.
~/STM32Cube/Repository/.
Should the user install the zip-package into some other location, it is required
to update the BSP_ROOT in the project's Makefile.
Install the downloaded zip-package into home directory under directory STM32Cube/Repository/STM32F10x_StdPeriph_Lib_V3.5.0:
$ mkdir -p ~/STM32Cube/Repository
$ cd ~/STM32Cube/Repository
$ unzip ~/Downloads/en.stsw-stm32054_v3.5.0.zip
Updating the firmware will require use of ST-Link V2 USB-dongle and the utility.
It is important to be aware that some cheap ST-Link V2 clones sold online may have wrong pin numbering on the USB-dongle's enclosure.
It is possible to open the enclosure of the USB-dongle, and check the pin names from the PCB. Checking the actual pin numbering will reduce the amount of frustration experienced while trying to get the firmware update completed successfully.
Use only the following ST-Link V2 signals: GND, SWDIO and SWDCLK.
WARNING: DO NOT CONNECT THE +3V3 OR +5V SIGNALS BETWEEN LTDZ AND THE ST-LINK V2 DONGLE!
Otherwise you will most probably damage your LTDZ or ST-Link V2 dongle.
This project has been tested with ST-Link V2 version 1.7.0. Any other recent ST-Link V2 tool version should work ok.
For the Windows build environment, a native Windows ST-Link V2 tool needs to be installed instead of this Linux version of the utility.
The ST-Link V2 version 1.7.0 for Linux and Windows can be downloaded from the following link:
https://github.com/stlink-org/stlink/releases/tag/v1.7.0
It is also possible to build the tool from the source code.
Clone the LTDZ source code repository into user's home directory under ltdz-dsp:
$ cd ~
$ git clone --depth 1 https://github.com/kalvin2021/ltdz-dsp.git
Update the firmware source and build the LTDZ firmware for STM32F103:
$ cd ~/ltdz-dsp
$ git pull
$ export GCC_PATH=${HOME}/opt/gcc-arm-none-eabi-10-2020-q4-major/bin
$ make
Flash the firmware into the LTDZ board using ST-Link V2:
$ make flash
Use only the following ST-Link V2 signals: GND, SWDIO and SWDCLK.
WARNING: DO NOT CONNECT THE +3V3 OR +5V SIGNALS BETWEEN LTDZ AND THE ST-LINK V2 DONGLE!
Otherwise you will most probably damage your LTDZ or ST-Link V2 dongle.
The ltdz_adc_read.py utility can be used for capturing, plotting and saving
the ADC sample buffer data.
For example, the following will sweep the frequencies from 90MHz to 120MHz with a frequency step size of 1MHz (using 30 steps).
$ python3 util/ltdz_adc_read.py -p /dev/ttyUSB0 --start 90m --end 120m --step 1m
The utility will plot the ADC sample buffer data in time-domain, and compute the FFT of the samples for each sweep step (using Hanning window).
The utility will also plot the overall signal spectrum over the frequency sweep range.
Note: The ADC sample buffer is by default 4096 samples long. Transferring the complete ADC sample buffer after each frequency step over the USB serial line with 57600 bits/s will take some time.
The utility will be used during firmware development for examining the ADC sample buffer during the frequency sweep.
The captured ADC sample buffer data can be saved in Matlab/GNU Octave ASCII-format for later analysis.
Here is a screen-capture from measuring a FM-band band-reject filter between frequency range 65MHz - 130MHz (65 steps):
