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Carphone phreaking unit for ATF-1

The unit described here, is an electronic device for clandestine use – or phone phreaking – of the old Dutch ATF analogue carphone network, developed in the 1980s by the Counter Observation Team (COT) — a group of skilled hobbyist scanner listeners in the Amsterdam area (Netherlands).

It was suitable for use on the Dutch 1 analogue carphone system ATF-1, which was in use from 1980 to 1995, and operated in the 150 MHz VHF band. Unlike on modern systems – such as GSM – speech on ATF-1 was not encrypted. Anyone with a VHF scanner could monitor the mobile communications of service engineers, business men, captains of industry and politicians alike.

Furthermore, there was no authentication what­soever, and the digital modem signals – used for handshaking and handover – used an existing CPC protocol, of which the details were available.
Prototype of an ATF-1 carphone phreaking unit

The device is shown in the image above. It is housed in an aluminium enclosure with a 16-button keypad 2 on top, and a green and red indicator LED. It is powered by an internal 9V block battery, or by an external source, such as the 12V battery of a car. It is fully autonomous (not connected directly to transmitter or receiver) and mimics the modem signals from the carphone, by using two distinct in-band audio tones: 1950 Hz and 2070 Hz — for a digital '1' and '0' respectively.

Rather than stealing carphones and modifying them for clandestine use – as also happended in the Netherlands – the COT solution was entirely based on commercial-off-the-shelf equipment, and the home-made phreaking unit shown here.

The image on the right shows a complete setup as it was used by COT, neatly stowed is a plastic briefcase. At the right is an ICOM IC2E handheld transceiver that was meant for the 2m amateur radio band (144-146 MHz). It had been modified for the American 2m band (144-148 MHz), and was then used with a +600 kHz repeater shift.
Equipment for clandestine use of the Dutch ATF-1 carphone network [3].

This allowed the IC2E to use the first 10 input channels (148.410 - 148.590 MHz) of the ATF-1 base stations (148.410 - 149.130 MHz). For the return signal from the base station (153.010 - 153.730 MHz) a simple communications receiver or computer scanner was used. All the user had to do to initiate a call, is enter the desired subscriber number on the keypad, and hold the micro­phone in front of the miniature speaker of the control box, whilst keeping the PTT depressed.

The unit is in many ways similar to the so-called blue box, that was used from the 1960s to the 1990s, to make free phone calls on regular analogue wired telephone networks (POTS) in America and Europe [2]. This technique — commonly known as phone phreaking — was and is illegal [1].

  1. Compatible and interoperable with the German B-Netz.
  2. A later version had just 12 buttons on the keypad.

Prototype of an ATF-1 carphone phreaking unit Keyboard and indicators Equipment for clandestine use of the Dutch ATF carphone network PCB (with Motorola microcontroller removed) PCB - solder side
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Prototype of an ATF-1 carphone phreaking unit
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Keyboard and indicators
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Equipment for clandestine use of the Dutch ATF carphone network
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PCB (with Motorola microcontroller removed)
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PCB - solder side

Block diagram
The diagram below shows how phreaking of the ATF-1 mobile network was done. A complete setup consists of three independent units: a general purpose VHF FM communications receiver, a (modified) transmitter for the 2m amateur band (typically an ICOM IC2E), and the phreaking unit.

Note that the phreaking box is not connected to the transmitter and/or the receiver. It transmits the (emulated) modem signals through acoustic coupling of its speaker with the microphone of the transmitter. It was arguably the first hands-free configuration of a 1G car phone system.

The device is housed in an extruded eloxed aluminium enclosure with black plastic end panels. It measures 108 × 103 × 35 mm and weights 310 grams, including a 9V battery. The interior can be accessed by removing one of the plastic end panels and sliding out the PCB and the keypad.

The PCB measures 100 × 100 mm, and contains 26 parts. The circuit is built around a Motorola MC68705P3 microcontroller. It has 112 bytes of RAM and a built-in 1804 byte EPROM, that holds the firmware [A]. It can be erased by placing its window under an UV-light source for 10 min.

The image on the right shows the PCB from a prototype of the phreaking unit. The empty 28-pin socket at the far side is for the Motorola microcontroller, which has been removed here. It was a rather expensive part at the time, and has probably been reused in a different project.
PCB (with Motorola microcontroller removed)

At the right is a battery clip, to which the 9V NiCd cell was connected (removed here). At the centre is a 3-pin LM7805 voltage regular which provides the +5V for the microcontroller and the 8-pin audio amplifier in front if it. Although the text has been removed from this part — this was often done to make it more difficult to copy the design — it is clear that it is an LM386 or similar.

Circuit diagram
Below is the circuit diagram of the ATF-1 phreaking unit. At the top left is the power circuit, which allows the device to be powered by the internal rechargeable 9V NiCd battery, or by an external (12V) source, in which case the internal 9V battery is recharged. When the device is ON, an LM7805 delivers a stable +5V for the Motorola microcontroller and the LM386 audio amplifier.

The circuit is built around a Motorola MC68705 microcontroller, which has 122 bytes of memory (RAM) and 1804 bytes of erasable memory for storage of the firmware (EPROM). A quartz crystal provides a clock frequency of 2.1 MHz. The controller has two bi-directional 8-bit data busses (PA and PB) and one bi-directional 4-bit bus (PC). The PA-bus is used for scanning the keypad, by defining 4 bits as outputs and 4 as inputs. Of the PC-bus, one line is used for production of the audio signal (C0) and two for the red and green LED indicators (C1, C2). The PB-bus is unused.

At the bottom right is the audio amplifier, which is built around an LM386 integrated circuit (IC). The desired tones of 1950 and 2070 Hz are generated in software and are delivered at pin C0. A simple integrating network (1K + 10n) converts the square wave signal into a reasonably smooth sine wave, which is then supplied to the LM386 amplifier, and eventually to the speaker (SPK).

  1. Motorola MC68705P3 microcontroller, datasheet
    Date unknown. Retrieved February 2020.
  1. Anonymous donor, Prototype of ATF-1 carphone phreaking unit
    Received December 2019 - THANKS !.

  2. Wikipedia, Phreaking
    Retrieved May 2019.

  3. Wikipedia, Blue box
    Retrieved May 2019.

  4. Anonymous former KPN expert, Personal correspondence
    February 2020.
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Crypto Museum. Created: Tuesday 04 February 2020. Last changed: Tuesday, 03 March 2020 - 16:40 CET.
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