The Red Pitaya board itself is a compact, open‑source FPGA‑based test and measurement platform. It’s often described as a Swiss‑army knife for electronics because it can act as an oscilloscope, signal generator, spectrum analyzer, VNA, logic analyzer, and more. It is fully open-source and widely used in industry, education, RF labs and hobbyist electronics.
The original STEMlab 125-14 board was introduced in 2013 when Red Pitaya was established, it has since evolved with newer models being released including the STEMlab 122-16 that was specificaly targeted at SDR developers, RF researchers and the amateur radio community.
I purchased my STEMlab 125-14 board from RS Components mid 2014 and the specifications of my board are as follows:
Processing & Architecture
• Xilinx Zynq‑7010 (ARM Cortex‑A9 dual‑core + FPGA)
• RAM: 512 MB (4 Gb)
• Storage: microSD (up to 32 GB)
RF Inputs (Oscilloscope ADC Section)
• Channels: 2
• ADC resolution: 14‑bit
• Sample rate: 125 MS/s
• Analog bandwidth: 50 MHz (‑3 dB)
• Input coupling: DC
• Input impedance: 1 MΩ ∥ 10 pF
• Full‑scale voltage: 2 Vpp (high‑gain jumper) - 46 Vpp (low‑gain jumper)
RF Outputs (Signal Generator DAC Section)
• Channels: 2
• DAC resolution: 14‑bit
• Sample rate: 125 MS/s
• Output amplitude: up to ~1.4 Vpp (depending on load)
• Output bandwidth: DC ~ 50 MHz
• Load impedance: 50Ω
I/O & Connectivity
• Digital I/O: 16× digital pins
• Communication interfaces: I2C, UART, SPI
• LAN: Gigabit Ethernet
It can be seen from the specifications above that the board is not ideal for RF purposes when you consider that the RF inputs are high impedance at 1MΩ and far from the 50Ω RF standard required for true RF use.
The newer STEMlab 122-16 is actually a better option if you want the best in performance and true 50Ω RF inputs but it does come at a cost. At the time of writing this article (Feb 2026) the STEMlab 122-16 is priced at 750,00€ and the STEMlab 125-14 is priced at 415.00€.
I produced the following chart so you can see what you get for the extra 335,00€
Dynamic Range (the holy grail)
Using the chart above, it is clear to see that more bits = more dynamic range, and there is a +24dB advantage between 12 bits and 16 bits.
In simplistic terms the dynamic range of a receiver is the range of signals measured in decibels, this is the minimum discernible signal received, taking into account a receiver's sensitivity and noise floor and the maximum signal received without distortion or overloading. A wider dynamic range is critical for separating small signals from large signals in crowded radio environments.
A 16-bit ADC has far more headroom than a 12-bit which means there is less intermodulation, less desensitization and a cleaner spectrum waterfall. If you are a contester or live in a high RF environment, the difference from 16-bits is huge.
Usable Sensitivity in the Presence of Noise More bits don’t improve the noise floor by themselves — the front‑end noise figure dominates that. But more bits preserve weak signals when strong signals are present. A 16‑bit SDR can decode a weak signal that a 12‑bit ADC would bury in quantization noise once the AGC backs off.
Quantization noise floor
Quantization noise is spread across the Nyquist bandwidth. A 16‑bit ADC has a much lower quantization noise floor, which matters when:
• You use wide FFT bins
• You do undersampling
• You run large FFT sizes for spectrum analysis
• You do multi‑tone or multi‑band DSP
Hermes‑Lite 2 (12‑bit) is excellent for the price, but its quantization floor is noticeably higher.
Undersampling performance
This is where the Red Pitaya 16‑bit boards shine. Because the analogue input bandwidth is high and the quantization noise is low, you can:
• Undersample VHF/UHF
• Do wideband spectrum analysis
• Capture strong + weak signals simultaneously
A 12‑bit ADC can undersample, but the dynamic range collapses quickly.
Practical real‑world difference
12‑bit (Hermes‑Lite 2)
• Excellent for HF
• Very cost‑optimized
• Needs filtering and careful gain staging
• Can overload in urban RF environments
• Great for ham use, not ideal for lab‑grade work
14‑bit (Red Pitaya 125‑14)
• Noticeably better blocking
• Good for HF and low VHF
• Still needs a proper RF front‑end for SDR use
• Very flexible because of the FPGA
16‑bit (Red Pitaya 122‑16)
• Lab‑grade dynamic range
• Handles brutal RF environments
• Much cleaner waterfalls
• Better for multi‑signal DSP
• Better for undersampling
• More forgiving front‑end requirements
The bottom line
- HF ham radio → 12‑bit is fine
- HF in a noisy city → 14‑bit is noticeably better
- Serious SDR work, spectrum analysis, multi‑signal DSP, or VHF/UHF undersampling → 16‑bit is a big upgrade
If you discard the fact that the STEMlab 125-14 has 1MΩ inputs and look at the remaining specifications, it can be seen that the cheaper 125-14 board is still a very capable option for building an SDR transceiver.
In order to utilize the 125-14 board for an SDR receiver or transceiver, the high impedance inputs must be modified to function as 50Ω RF inputs, and this can be done in several ways.
- Quick‑and‑dirty: Add a simple resistive pad to the Red Pitaya's inputs to lower their impedance so they appear as 50Ω to the antenna.
- Add
a wideband impedance matching transformer to the inputs, not ideal because the RP board still sees a
very high impedance that is just scaled by the transformer's turn ratio.
- Leave the 125‑14 hardware untouched and build a small RF front‑end that presents 50 Ω to the antenna and conditions the signal for the Red Pitaya’s high input impedance.
- Hack the hardware and bypass the existing Red Pitaya's input components, filters and op-amps. This option is considered the best, as it, in effect, connects the antenna straight to the LTC2145-14 analogue to digital converter IC's inputs.
Option 4 is the best method but the most invasive and hard core of them all, as it requires the removal of some tiny 0603 size SMD components that measure approximately 1.6mm by 0.8mm. You should therefore only tackle this option if you have good soldering skills & experience with SMD components and are willing to accept the risks involved carrying out the modifications.
For my STEMlab 125-14 board, I decided to go with option 4 and followed some information that I found online by a Russian radio amateur. For the first step, you remove all of the components shown within the yellow outlined area as per the image below. Note for clarity I have also outlined the individual components in blue!
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Red Pitaya 125-14 Front End Modification Image |
I used a fine tipped soldering iron and my hot air workstation to remove the components, taking care not to damage the PCB or header connector that sits adjacent to a number of SMD components.
Once all the components have been removed, the next step is to build a replacement input circuit to feed the ADC chip.
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Schematic diagram of the replacement Red Pitaya input circuit. |
I built the above circuit on a small piece of FR4 strip board mounting some of the components on the top of the board and some on the underside.
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The above image shows the replacement input board installed in the area previously occupied by the manufacture's input circuit. For clarity I have photo edited the image so it is clearer and easier to follow.
Once the input stage modification is done, that is the difficult part out of the way. The final modification is carried out on the underside of the Red Pitaya board and again to the ADC circuit. The mod involves adding a capacitor, removing a resistor and fitting another resistor to a new component location.
Here is an image I created that illustrates where the components are added and the resistor that must be removed. The box outlined in yellow is a zoomed-in section of the PCB modification area, this has been overlaid onto the full PCB image to provide clarity for the modification.
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| Modifications to the underside of the Red Pitaya Board. |
That concludes the preparation of the board for my SDR transceiver build and it also brings it more inline with the more expensive Red Pitaya 122-16 model.
It should be noted that there is an additional modification that can be carried out to improve the transmit performance and deliver a cleaner signal on the 50 MHz (6 Meter) band. It involves replacing two OP-AMPS in the transmit signal path but as I do not intend to operate my transceiver on this band, I will not provide any further details of that modification here.
Part 2 of my SDR Transceiver build will cover the design and build of a dedicated receive module complete with bandpass filters, RF-PreAmplifier and a Digital Attenuator.
Project files will be made available via the Groups.io platform by joining my G6LBQ community group, where you can discuss my projects, ask questions and help others.
Joining my group is free. Just click on the button below.
Until next time...
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