RedPitaya SDR-TRX using STEMlab 122.88-16 (Version3)
The STEMlab 122.88-16 is an enhancement of the programmable measuring device STEMlab 125-14 from REDPITAYA. It is optimized for use in the SDR environment with regard to the system clock of 122.88MHz as well as the the RF inputs and outputs with an impedance of 50 ohms. SDR stands for Software Defined Radio.
With suitable "firmware" and appropriate PC software, the STEMlab can be used to build a full-fledged SDR transceiver for all HF bands from 160m to 6m. Pavel Demin has done pioneering work in this area. With his firmware and the PC software OpenHPSDR-Thetis , the SDR concept with the STEMlab became available for widespread use for the first time.
The
structure of such a TRX has already been described in various papers. More or
less detailed, more or less complex, more or 
less complete.
I got a lot of inspiration while working through these descriptions. I have taken up some ideas from them and incorporated some of them into my concept. The publications of DK5QH and DK7XL on the subject of "SDR on Red Pitaya" are particularly noteworthy.
The goal of my project was to build a simple, modular device for everyday use that can be easily integrated into the existing infrastructure of LAN/WLAN, PC, antenna + coupler, PA, etc.
The following documentation is not a building manual! Rather, it is intended to encourage one's own ideas and solutions. I’ll be glad to answer any questions you may have.
The PCBs or the production data of the described PCBs (Gerber files) are also available if required, of course.
Earlier realizations can be found here.
The schematics and layouts were created with DipTrace V4.
Most photos can be enlarged by clicking in.
In addition to the STEMlab, a complete TRX requires quite a bit of peripheral hardware. The main components are:
· Audio amplifier for speaker connection (optional)
· I2C-Bus Register and Buffer for controlling the filters and attenuators
· div. Optocoupler for galvanic isolation of the digital inputs/outputs from the STEMlab
· TX driver and PA to achieve an output power of +37dBm on all bands
· Lowpass Filter for transmission
· RX-Attenuator 0-10-20-30dB
· RX / TX switch
The various components are divided into two additional boards, as shown in the overview.
On the NF&Peripheral board, the audio codec is mounted via spacers and then the STEMlab board. The PA&Filter board can be placed underneath the NF&Peripheral board. This results in a complete transceiver module in a very compact design.
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The NF & Periphery Board
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On an area of 85x100mm, the NF & Periphery Board combines all components that are required for the Audio side coupling of the STEMlab. These are usually the audio codec as well as an audio amplifier for connecting a loudspeaker.
The board also accommodates the components for the digital connection of the internal peripherals. I.e. the I2C bus logic and the opto-couplers for PTT and Keyer as well as the PTT relay.
The 5V power supply for the STEMlab module is also integrated on the board with some protection circuits.
The technical details can be found in the Schematics and the BOM .
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The audio codec is attached to the equipped NFP board and fixed with spacers. The assembly is carried out overhead. This has simplified the routing of the NFP board somewhat.
The 3.3V power supply for the I2C bus logic components is generated on the STEMlab.
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By means of 20mm spacer bolts, the STEMlab is mounted above the NFP
The connection between STEMlab and board is done with flat ribbon cables.
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The PA & Filter Board
The PA & Filter Board
accommodates all components of the RF branches for RX and TX operation on a
base area of 100x100mm.
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At the top left are the voltage regulators for PA and BIAS.
In the RX branch there is only a 61MHz low-pass as an anti-alias filter. Its job is to keep strong signals from the 2nd and 3rd Nyquist zones away from the RX.
In addition, an RX attenuator is available. This is controlled via the ALEX mode of the Thetis software. Adjustable are 0..10..20..30dB.
The TX branch starts with a 3dB attenuator as the defined load for the STEMlab TX output.
The measured output power of the STEMlab is between +3dBm and +7dBm, depending on the frequency.
The attenuator is followed by a 20dB power driver. It is conventionally constructed with an OPA2674 OP-AMP.
The power amplifier is built as a push-pull amplifier with 2 LDMOS FETs (AFT05MS004NT1). This circuit has proven itself in the Hermes-Lite concept, for example. The output power is max. 37dBm.
The PA is followed by the obligatory low-pass filter. The filters for 5MHz, 10MHz, 20MHz and 30MHz are realized as
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The operating voltage for the OP-AMP as well as the BIAS supply of the PA module are only enabled during TX operation.
With the trimmers, the quiescent current of the FETs is adjusted to 55mA each. The quiescent current is temperature-stabilized and can be detected at the side measurement output with a resolution of 2.5mV/mA.
The assembly is controlled from the NFP board via ribbon cables. The RF connection to the STEMlab is established via short RG316 coax cables.
The dimensioning of the components can be taken from the BOM. The Schematic is stored here.
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The completed TRX
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The modules are connected by means of a ribbon cable. Only the power supply is wired separately.
Once the power supply, antenna and LAN cable have been connected, the STEMlab-TRX is ready for use.
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The assemblies are installed together with the power supply in a suitable housing.
A Schubert device housing, type 218, is used.
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The box is ready for use. |
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Last Update: 1.03.2024 DB8BH |
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