This section of the user guide describes all the hardware modules and interconnections required to build the Portsdown transmitter.
Not all the hardware is required before the transmitter can start to be tested – an incremental approach is described here to allow constructors to gain confidence as they assemble the parts.
Each component that you need is described in more detail here. Suggested suppliers for each item are listed underneath the description in the quote box. A more concise shopping list is provided here: https://wiki.batc.tv/DATV_Equipment_Shopping_List.
Portsdown DATV transmitter built by Colin G4KLB in a Farnell case
- 1 Raspberry Pi 3
- 2 Power Supplies
- 3 Micro-SD Card
- 4 LCD TouchScreen
- 5 Touchscreen mount
- 6 GPIO Extender Card
- 7 IQ Filter and Modulator Board
- 8 Synthesized Local Oscillator Source
- 9 Local Oscillator Filters
- 10 Raspberry Pi Camera
- 11 Optional RPi camera Extension Lead Kit
- 12 Composite Video Capture Device
- 13 Audio Capture Device
- 14 4-Band RF Output Switch
- 15 4-Band Decode Switch
- 16 8-Band RF Output Switch
- 17 8-Band Decoder
- 18 Filters and PA drivers
- 19 Hardware shutdown
- 20 Output Attenuator
Raspberry Pi 3
The first item required is a standard Raspberry Pi 3. These are available from many suppliers but we suggest you just buy a vanilla Rpi3 without additional boards or SD cards as you will be using specific Portsdown hardware and software.
Raspberry Pi 3: CPC Farnell http://cpc.farnell.com/mksp2-raspberry-pi
You will also need a 5v power supply for the Raspberry Pi, and a low resistance Micro-USB supply lead (some cheap ones are not so good).
You should power the RPi through the Micro-USB power connector to take advantage of the input protection provided. Use a very short USB cable to your power supply as most of these cables are of very poor quality and cause significant voltage drop. If you see a "Lightning Flash" symbol at the top right of your LCD display it means that the voltage at the RPi is too low. You can measure the voltage between the GPIO connector pins 4 and 6.
A lot of strange problems, such as the Portsdown running very slowly, can be caused by poor PSU volts - the Rpi3 is actually specced for 5.1 volts, make sure your PSU delivers enough volts and current. See this post on the BATC forum for a real user experience!
You will also need a 12 volt DC supply to power the other Portsdown components.
You then need an SD card with the Portsdown software on it.
The easiest way is to buy a pre-programmed SD Card from the BATC Shop. (Item 1 in the Portsdown list).
OR if you want to have a go at loading the software yourself you will need to or purchase a good quality card from another source and follow the instructions described here. (The 16GB SanDisk Ultra is recommended - DO NOT use cheap copies)
SanDisk Ultra 16 GB Micro-SD Card: Amazon https://www.amazon.co.uk/s/?ie=UTF8&keywords=micro+sd+16gb+ultra
Once you have a card you will need to follow the initial set up guide
This is the minimum hardware configuration you need for testing the Portsdown transmitter! You can use an external PC to control the Portsdown in Console mode and run the QPSKRF or Ugly test mode using moving test patterns and images. (But as "ugly mode" only runs in reduced bandwidth or RB-TV mode, you will need a MiniTiouner to receive it.)
To use your Portsdown as a "proper" DATV transmitter you need the following components:
The touchscreen is required to control the transmitter without the use of another computer. The supported touchscreen is a 3.5 inch diagonal Waveshare Raspberry Pi LCD (Model A, not Model B). This is available from a number of suppliers for under £20. It uses a resistive touch panel and has a resolution of 480*320.
Do not buy a touchscreen that connects through the RPi HDMI connector - it is NOT supported.
If you do want a larger touchscreen, the 7 inch Element 14 touchscreen https://www.element14.com/community/docs/DOC-78156/l/raspberry-pi-7-touchscreen-display has been used successfully by a number of constructors, but continued support is not guaranteed.
Get the Waveshare display from Amazon: https://www.amazon.co.uk/Waveshare-Raspberry-Resistive-Interface-Rapsberry-pi/dp/B00OZLG2YS or several suppliers on ebay.
To mount the screen away from the RPi, you could use this extension cable: https://shop.cyntech.co.uk/products/26-way-gpio-ribbon-cable-extender-gert-cable?variant=613760449 Just make sure that you triple-check the connections and pin orientation before connecting the screen. They do not tolerate incorrect connection as several of us habe found out!
Mounting the LCD Touchscreen can be a challenge!
This 3D print design mounts it flat to the panel: https://www.thingiverse.com/thing:1941701
This 3D print design mounts it at an angle: https://www.thingiverse.com/thing:1920486.
Note that, when mounted, none of the touchscreen sensitive areas should be in contact with the surround.
GPIO Extender Card
In the completed unit, there are 13 signals that need to be routed in or out of the GPIO interface on the RPi. Plus we need to break out all 40 pins of the GPIO, and have another connector on top for the LCD display. Because of this the Portsdown team have designed a PCB for the job - the blank PCB for this is available from the BATC shop.
Buying an extender card early in the project reduces the chance of damage to the RPi and allows these connections to be made safely.
The BATC card is very simple to construct with no components other than plugs and sockets.
For full construction details and a list of suppliers for the plugs and sockets see the GPIO breakout page
You can buy the GPIO Extender Card blank PCB from the Portsdown section in the BATC shop - item 6
IQ Filter and Modulator Board
The filter modulator board has been designed specifically for the Portsdown project. It takes the digital output from the Raspberry Pi and uses it to modulate the LO input from the ADF4351 module - the challenge has been to design a board that will cope with a wide range of symbol rates from 125 KS/s to 5 MS/s.
For more details see the Filter Modulator wiki page.
Ready assembled and tested boards are available from the BATC shop or you can buy a blank PCB and build the board yourself. However, the board uses a large number of small pitch surface mount components and is not a beginners project!
For more details on the card, including build instructions see this wiki page.
Ready assembled and tested boards are available from the BATC Shop - item 2.
Blank PCBs for home construction are available here - BATC shop item 3.
Synthesized Local Oscillator Source
The filter modulator board requires an on frequency local oscillator signal and the Portsdown transmitter software has been designed to control an ADF4351 chip to give an output on 146, 437 and 23cms.
Small PCBs with the IC and a reference oscillator are available from China through eBay. Be careful to buy the ones with black PCB lacquer and the 5x2 way connector – the green ones with the in-line connector have a reputation for being re-badged ADF4350s which do not have the same frequency range. As eBay links keep changing, you need to find one that looks like these:
Warning: A recent batch of Chinese boards have a 5V regulator fitted instead of the required 3v3 regulator. Please check your board before you apply power. Details here: https://forum.batc.org.uk/viewtopic.php?f=103&t=5375
It is possible to buy ADF4351 modules with a 10 or 25 MHz reference oscillator - the Portsdown software can handle either but you will need to set which oscillator you have during the initial set up.
Note that the boards take a 5V supply through the barrel connector - the pin labelled 3v3 is a low current regulated OUTPUT!
ADF4351 Synthesized Oscillator: Source from eBay (Remember to go for one of the black boards with the 5x2 connector.) http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=adf4351&_oac=1
Local Oscillator Filters
Testing has shown that, due to the square wave output of the ADF4351, filtering is needed between the output of the ADF4351 module and the modulator board LO input when used on 146 and 437 MHz. This filter only needs to be a simple low pass filter to remove the third harmonic and no filtering is required on 23cms.
The effect of running without a high pass filter on 146 and 437 is an imbalance in the modulator resulting in reduced MER and RF performance - it does not affect the transmitted spectrum and is OK to put on air.
The project team have designed a small multi band PCB automatically switched to the correct band by the Portsdown software.
A blank PCB is available from the BATC shop - item 4
Full construction details are available here.
You now have the components for a fully working Portsdown transmitter! See this page for suggestions on how to connect it all up.
However, you will need a camera and audio input to transmit pictures and sound.
Raspberry Pi Camera
If you want to transmit live pictures you will need a camera. The standard Pi camera gives excellent results but will need mounting in a housing.
Raspberry Pi Camera: CPC Farnell http://cpc.farnell.com/raspberry-pi/rpi-8mp-camera-board/raspberry-pi-camera-board-8mp/dp/SC14028
Optional RPi camera Extension Lead Kit
The RPi camera gives excellent results, but only comes with a short ribbon cable to connect it to the RPi.
It is possible to use some higher quality HDMI leads (the ground connections all need to be made) to extend the camera lead - this does NOT mean the signal out of the camera is HDMI, it is actually "Camera Serial Interface Type 2 (CSI-2)". We are just using HDMI cable as a useful extension lead.
Kits are available to connect a female HDMI connector to the RPi camera connector, and another female to the camera, allowing the use of domestic 1, 2 and 3m HDMI leads.
RPi Camera HDMI Lead Extension Kit: https://www.tindie.com/products/freto/pi-camera-hdmi-cable-extension/
There are 2 modifications that can be made to enable lower quality HDMI leads (those without all the ground connections made, or even the CEC wire on pin 13 missing) to be used. Full details of these modifications are available here.
Composite Video Capture Device
The transmitter is also capable of working with a USB-connected “EasyCap” dongle to capture PAL composite video from camcorders and analogue mixers etc.
There are at least 4 similar devices being marketed under this name on eBay, each with different chipsets, but only the Fushicai "USBTV007" brand is guaranteed to work.
To help make sure you buy a unit that will work with Portsdown we are now stocking the Fushicai USBTV007 Audio-Video Grabber which is guaranteed to work in the BATC Shop - item 8. More information on the device can be found here.
Audio Capture Device
eBay provides a good source for very cheap audio capture USB dongles, which can provide an audio input to go with the RPi camera.
Audio Capture Device: Source from eBay - for example: http://www.ebay.co.uk/sch/Laptop-Desktop-Accessories/31530/i.html?_from=R40&_nkw=usb%20audio%20adapter&_dcat=75518&Channels=2%252E0&rt=nc&_trksid=p2045573.m1684
The software version 201707220 introduced audio source switching between audio and video capture devices. A list of devices that are supported by this audio switching is here.
That completes the basic Portsdown transmitter shopping list
However, the team has design some additional components to enable you to build a complete DATV transmit system
4-Band RF Output Switch
The Portsdown team have developed a 4-Band RF output switch capable of switching the low level output of the Portsdown filter modulator board to separate amplifiers for the 70, 144, 437 and 1255 MHz bands. Note: due to potential high insertion loss, it is not recommended to use the switch above 23cms.
Full details including parts list and schematic are available on this wiki page.
4-Band Decode Switch
The Portsdown provides band switching and PTT outputs on the GPIO pins - these outputs need to be decoded to provide an output for each band.
The Portsdown team have designed a simple board for switching between 4 bands and buffering the PTT signal which can be either constructed on 0.1” perforated board (veroboard) or on a PCB - this can be home etched.
Full details on the board including circuit etc are available here. Blank PCBs are available in the BATC shop - item 5.
8-Band RF Output Switch
For those of you who also want to drive Microwave transverters from your Portsdown, the team are developing an 8-Band RF output switch that includes an RF amplifier. It is designed to be used with the Output Attenuator (see below). It will be capable of amplifying the variable level from the attenuator and switching it to separate amplifiers for the 70, 144, 437 and 1255 MHz bands, and to 4 transverters for the higher bands. PCBs are on order and will be available from the BATC Shop.
Full details including parts list and schematic are available on this wiki page.
This 8-band decoder is designed to work with the 8-band RF output switch and the "Portsdown 2018" software. It will provide switching signals (either always on, or PTT activated) to 4 Power Amplifiers and 4 transverters, and it also enables you to set which VCO filter should be switched in for the 4 transverter outputs. PCBs are on order and will be available from the BATC Shop.
Full details on the board including circuit etc are available here.
Filters and PA drivers
It is absolutely essential that the Portsdown transmitter output is followed by suitable bandpass filters.
These are not included as part of the project but a number of designs and suppliers are listed here.
In order to prevent SD card corruption by just removing the power to shut the system down, it is recommended that you include a shutdown switch.
When the facility is enabled (not by default), Pin 15 on the GPIO port signals the RPi to shutdown cleanly.
It is held low by a pulldown resistor in the RPi; when taken to 3.3v through a pushbutton it will command the RPi to shutdown. The signal on pin 13 can be used to illuminate an LED through a current limiting resistor. This LED illuminates when the RPi is active, and extinguishes when the software has shutdown and it is ready for power-off.
The RPi can be restarted from the "ShutDown" state without recycling the power by momentarily shorting the 2 "RUN" pads that can be found behind the USB Connectors.
You can use an electronically switched attenuator to adjust the transmit drive level for each band. Attenuators based on the PE4302, HMC1119 or PE43703 (preferred) are supported.
These attenuators are available on eBay. Full details and wiring instructions are detailed here: Output Attenuator.