What is the "Acampo Transmitter"?
Simply put, the "Acampo Transmitter" is a Digital ATV Transmitter PiHat based upon the "Portsdown Transmitter"
Our approach is very different in scope and philosophy. In order to make this easier to follow, the "Acampo Transmitter" will be referenced as ATX and the "Portsdown Transmitter" will be referenced as the PTX.
After having purchased and built a few of the BATC Filter Modulator Boards from the BATC Group in the UK, our build team decided to build a similar transmitter based upon the work done by the BATC. However, here in Northern California we have some frequency constraints placed upon us by the FCC, the US Airforce and the repeater co-ordinating body in Northern California. This restricts where we can operate in the frequency band plan.
The BATC Filter Modulator Board has many functions and capabilities. Out here though we have chosen to use just a subset of just what that board can do. We bought the boards and only populated the parts we needed to operate in the way we have chosen to do. We don't need the range of Symbol Rates available or do we need the many frequency bands that the Filter Modulator Board supports. Although the new board will operate on other usable bands we have selected the frequency of operation to be the 1.2GHz band and the selected symbol rate to 2 M/s per second. What we have is a spectrally clean I/Q Modulator with sufficient output to drive our amplifier of choice (Down East Microwave 1.2GHz PA).
After deciding this is the best and cheapest way to build a suitable transmitter to fit our need, we took a look at making some changes
Our design team made a list of desired features vs non needed features.
1. Low cost to replicate
2. Ease of use
3. Small foot print in the shack
4. Operate as a portable rig
5. Keep the number of support devices to a minimum
To conserve space and build a smaller transmitter, we decide that the the following features were not needed:
1. Eliminate LCD Display
2. Eliminate the GPIO Extender board
3. Eliminate the 71MHz band, the 146 MHz band and the 437MHz band
4. Eliminate all symbol rates but the 2 M/s rate
5. Eliminate the PLL Filter board (no 437MHz operation)
6. Eliminate the Band Switch board
7. PI Camera use is a builders choice option
We still purchase our Easy Cap USB Dongles through the BATC Online Store.
Our cost of the blank BATC Filter Modulator board delivered with shipping and exchange rate increase was just over $23 US.
It was during this review process that we came up with the idea of building a DATV PiHat. The concept is obvious to any Pi user, build a board that plugs into the GPIO. This saves time, space and money in the long run. Fred Coe, WB6ASU took on the task of designing
a board based on the subset of parts used in the BATC Filter Modultor board and had a few made.
This transmitter can operate in various ways and modes, our version of build is stricly using the GPIO pins that the I/Q signals are sent to. These signals are buffered and fed to an I/Q Modulator IC along with a signal from the external VCO Board. The result of this combined signal is a DVB-S formatted RF Signal that is amplified by a low gain on board amp.
The output of that amp is sent to an external amplifier and boosted up before being sent to the antenna system.
Members of our team have purchased the DATV Express board when it was being produced. That board was expensive and much was required to support it. You needed the following items to put a picture on the air:
1. A laptop or a Desktop running linux or Windows
2. A hard to find video capture card or USB based video
3. 2 USB2 cables
4. Associated wall bug power supplies
This is a lot to carry around for portable operation. The cost to build this system can be as high as $600. That does not include the amplifier and antenna
Now there is an option, the ATX. It is a small low profile box. Plug in a USB Capture Device wired to a camera and microphone. Plug into an amplifier and you are on the air.
This box contains a Raspberry Pi 3B, a small yet surprisingly powerful little computer that runs this system very well. No extenal computer is required to operate it, the transmitter comes up running at boot time in the transmit mode. We simply key the amplifier bias to access the repeater. It is simple and clean in operation