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USB-I2C bus control module available. |
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160m to 6m High perfomance down conversion Front End
Martein Bakker, PA3AKE has designed a HF Front End down converter based in part on the H-Mode Mixer or switched mixer developed by Colin Horrabin, G3SBI, that offers extremely third-order intermodulation intercept figures and high dynamic range.
Features are an MDS of -133dBm, SSB bandwidth, IIP3 of +51,2dBm with an IMD3DR (Third order intermodulation dynamic range) of 122.8dB on 40M!
All this is achieved with no special/expensive components.
Certainly, Martein’s design has broken the magic 120dB IMD3DR boundary on real life not on simulation software programs.
Martein has published a comprehensive documentation regarding his work at
http://www.xs4all.nl/~martein/pa3ake/hmode/
I resolved build it as a part of my High Performance Hybrid (Analogue/DSP) Receiver project. For so I acquired the full set of PCBs, QT quartz crystals and metal boxes through Martein’s group-buy at http://www.xs4all.nl/~martein/pa3ake/hmode/groupbuy.html |
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BPF Filter bank, H-mode mixer/Roofing Xtal filters and USB- I2C bus control module |
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Martein’s Front End project comprises two main units:
- Band Pass Filters bank unit.
- H- mode mixer, SSB and CW Roofing Xtal filters unit.
The Band Pass Filter bank includes filter boards for 160m-80m-40m-30m-20m-17m- 15m-12m-10m and 6m plus two extra slots for Spare bands, I2C Bus control board, Step attenuator board and 9MHz IF Notch board.
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BPF bank with removed cover
Size is 25cm x 16cm x 6cm (9.84x6.29x2.36inch).
Total weight with boards is 1.6 Kg.
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Boxed Motherboard bottom view
To achieve good stop band performance the bottom compartment has been divided in two sections.
This also improves stop band in the VHF region.
M2 brass nuts to fix the boards are soldered on the ground plane in order to improve they ground connection.
The motherboard ground plane is soldered to the enclosure walls resulting a robust assembly.
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Boxed Motherboard top view |
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Filter boards
They are three kinds of filter boards according to the toroid core size used:
- T94 core for 160m, 80m,40m,30m and 20m filter boards.
- T80 core for 17m,15m,12m and 10m filter boards.
- T50 core for 6m filter board.
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T94 core board type for 160m band. Three sections BPF configuration. |
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T80 core board type for 15m band. Four sections BPF configuration. |
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T50 core board type for 6m band. Five sections BPF configuration. |
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Relays on filter boards are small SMD telecom grade with DC "soak" contacts.
Martein recommends using Suflex 2.5% Polystyrene capacitors on all filters, due availability issues I used instead 1%/500V mica capacitors.
To reduce IMD, the flux inside the cores has to be keep as low as possible, for so the filters has been designed with inductance values as high as practical.
For the same reason toroidal cores grade were selected taken in account its μ value.
There are no variable capacitors. Fine tuning is performed moving the distribution of turns around the toroid.
A carefully PCB design improves the filter response. The backside of the PCB has vertical cuts on ground plane to prevent ground currents flowing from underneath one toroid to the next. With this cuts -100dB stop band is reached. |
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Filter board rear view detail.
Coils are secured in place by cable ties. |
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Alignment of filters
This is the tough part of the project.
Alike others filters with the same topology, toroid cores and fix capacitors values, the precise alignment of the BPF's to get minimal insertion loss and ripple in the pass band is performed moving the distribution of turns around the toroid, this ask for patient and time. |
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160m, 80m, 40m filter plots |
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9MHz Notch filter board
Attenuation at 9MHz > 85 dB
In a front end the IF frequency must be blocked as much as possible before the mixer.
The notch is placed behind the BPF's in the signal chain.
This is done to minimize the spectrum exposed to the notch filter. |
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0 to 30 dB Step Attenuator board
SMD telecom grade with DC "soak" relays switching in respectively 2dB, 4dB, 8dB and 16dB attenuation.
The IIP3 of the front end will be around +80dBm when the attenuator is set at 30dB!
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I2C control board
The twelve BPF filters and the Step Attenuator are relay-switched; the control is handled through I2C Bus protocol.
The board has a PCF8575 16 bit I2C Bus I/O-extender, two ULN2803A 8-bit relay drivers and 12V and 5V LDO regulators.
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USB-I2C Bus control Module for Front End project
(USB powered)
Martein’s Front End project does not gives software support to control the I2C Bus.
To solve this I designed a solution based on a PIC microcontroller 18F2455. It includes I2C Bus and USB peripherals that let's handle the Front End.
The software running on the PC takes control of all functions of the front end. In addition, it includes capability to change manually data and addresses on the I2C bus for experimentation purposes.
The Status bar indicates if the PIC module is connected to the USB port. It runs under Windows 2000, XP, Vista32 and windows 7 Operating Systems.
(Windows 2K users need to update Framework from Windows Update site)
The PIC I2C control module is powered from the USB port on PC.
It provides Reset push-button, three IDC 10 vias connectors for I2C lines matching with the standard pin out of the project; auxiliary connector for program the PIC on circuit (ICSP capability) and Expansion Bus, IDC 16 vias connector, for upcoming updates of the project.
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Assembled USB-I2C Bus control Module |
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Graphic User Interface on PC
| Download software |
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USB-I2C Bus Control Module program software for Front End project
Controls
| BPF bank:
- Bands
- Attenuator
- Manual control
- I2C addresses
- Status bar
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H-Mixer:
- Filters
- Squarer DAC
- Bias-point DAC
USB-I2C bus control module manual |
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The USB-I2C bus kit is available at Store page.
Kit contains:
- All parts needed to build the kit. Includes pre-progamed PIC microcontroller.
- I2C bus IDC-ribbon interconnection cable.
- USB cable.
- Assembly manual.
- CD ROM including Software program, Firmware and USB driver for Windows Operate System.
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What Next?
Martein is “cooking” new astonishing developments following the Front End project but no date forecast, in the meanwhile in order to test “in operation” the Front End project I need build:
A Local Oscillator covering the available bands with very low Phase Noise figure, a DDS appears to be the best candidate.
A post-mixer amplifier with good shape factor crystal filters and LNA just to compensate the filter losses between the front end and the IF, a Hybrid amplifier could be a good choosing taking in account the dynamic range of the Front End.
A High Dynamic range 120dB, 9 MHz IF/AGC, here the CDG2000 IF board is the immediate solution.
A 9MHz product detector, I want test the Softrock for this job. I could get binaural reception (Pseudo-stereo).
I have decided to start with the CDG2000 IF board for so I ordered the PCB it looks promising. Let see what happens.
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