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Scanlock ECM
Automatic bug finder

The Scanlock ECM was a Countermeasures Receiver (bugfinder), developed in 1990 by Audiotel in Corby (UK) as the successor to the Scanlock 2000. It uses the same novel harmonic scanning technique as its predecessors, but also offers a number of new facilities and operational modes.

Like earlier Scanlock receivers, the ECM can quickly scan the entire frequency spectrum between 10 MHz and 4 GHz for radio bugs with Amplitude Modulation (AM), Frequency Modulation (FM) or Subcarrier Modulation (SC).

In addition, the Scanlock ECM can detect Mains Carrier (MC) transmitters (power line bugs) and Current Carrier bugs. Unlike its predecessors, the ECM can also be used as a full-coverage radio scanner, with accurate frequency readout. In addition, an LF mode is present to detect eavesdropping devices operating below 10 MHz.
Scanlock ECM

The Scanlock ECM contains an harmonic receiver that is nearly identical to earlier designs in the Scanlock Mark VB and the Scanlock 2000. All parts of the Scanlock ECM are under digital control and the analogue local oscillator has been replaced by a digital PLL, allowing accurate frequency readout. Operation of the Scanlock ECM is not straightforward and requires good reading of the manual and understanding of the operating principle of the various modes. As an aid to the non-technical user, a number of built-in presets can be selected from a menu via the LCD display.

The Scanlock ECM superceeded all previous Scanlock models, but does not have a memory function that allows signals to be compared to those from a safe zone, like the Compuscan add-on for the Scanlock Mark VB did. Apart from that, the Scanlock ECM was a very versatile and compact bug tracer that allowed quick location of radio bugs, mains bugs and other cable bugs.

In 1992, the ECM was followed by the Scanlock ECM Plus that eventually evolved into the Scanlock Select Plus in 1996. It introduced signal analysis and comparison to previously stored scan data, and was the last logical step towards the fully PC-driven Scanlock M2 in 2000 [2].

Flightcase Scanlock ECM with shoulder strap Scanlock ECM Scanlock ECM in operation Scanlock ECM with HF antenna Scanlock ECM used in vertical position Control panel of Scanlock ECM in vertical position Manual tuning

All controls of the Scanlock ECM are nicely arranged at the front panel as shown in the diagram below. Nevertheless, operation of the receiver is not straightforward and requires in-depth knowledge of the principles behind it, not least because many different devices have been integrated into one compact portable design. To help the unexperienced user, a number of preset programs are selectable from a menu (using the yellow button marked 'MODE').

At the left are two antenna inputs, one for all RF frequencies between 10 MHz and 4 GHz (N-connector) and one for all CF frequencies below 10 MHz (BNC connector). For the RF and CF bands, two independent receivers are used. To the left of the display are 8 push-buttons that are used for selecting the type of modulation (AM, FM, SC, CP) and the type of scanning (SSS, SCAN and LOCK). The receiver can search for the Strongest Signal (SSS) or scan the entire band slowly (SCAN). LOCK mode is identical to the Sound/Wave mode (S/W) of earlier Scanlock receivers.

To the right of the display are buttons for manual frequency control (MANUAL) and for accessing the menu's (MODE and RESET). The LOCATE button is used to activate the bug-locating mode, in which the speaker will produce a Geiger-type ticker that gets faster when you approach the bug.

Control panel of Scanlock ECM in vertical position Scanlock ECM front panel Modulation and mode selectors Display and MODE selectors Controls (tuning, squelch and volume) Manual tuning HF antenna in use

Modes of operation
The Scanlock ECM can be used in a variety of modes and with various types of modulation, depending on the type of bug, the method of searching and the time available to do the sweep. The following modes are available:

  • Strong Signal Search (SSS)
    In this mode, the Scanlock ECM searches fully automatically for bugs in the room and locks itself onto the strongest signal that it finds, with a lock sensitivity of 1mV up to 4 GHz. In earas with strong broadcast transmitters, it may be necessary to move the receiver around the room whilst searching.

  • Manual mode (SCAN)
    When searching for bugs with an extremely low RF output signal, or in areas with strong broadcasting stations or other sources of interference, it might be useful to conduct a manual search. In manual mode, the Scanlock has a typical sensitivity of -70 dBm up to 2 GHz. In this mode, the frequency can be adjusted by turning the multi-turn knob (with the built-in counter). Furthermore the meter can be used as a frequency indicator.

  • Soundwave mode (LOCK)
    In this mode, the Scanlock emits a continuous (audible) tone, that changes to an intermittent tone when the receiver detects itself (i.e. if it is heared through the bug). This mode should be used as a last resort, as it is likely to alert the eavesdropping party of the fact that a bug-sweep is taking place.

  • Locate mode (LOCATE)
    When the LOCATE button is depressed, the Scanlock provides an audible tone with a pitch that is proportional to the strength of the acquired signal. Starting off with a low frequency ticking sound, the pitch gets higher when approaching the bug. In this mode, the use of a pair of headphones is advised, as otherwise the eavesdropping party might become aware of the fact that a bug-sweep is taking place.

  • Cable mode (CF)
    When pressing the CF button, the Scanlock ECM is capable of detecting power line bugs (mains carrier bugs, or MC) or bugs that are connected to other types of cable. For this, the external LF interface is used. It is suitable for both the mains and for other types of cables, and should be connected to the lower antenna input (10 MHz -). If there are any power line intercoms or PLC modems in the building, you will probably hear them immediately. Note that an MC check has to be carried out on all wall sockets, as some of them may be powered from a different phase.

  • Telepone line bugs (RF)
    Telephone bugs are usually common radio bugs that are connected to the telephone line. They are commonly powered by the telephone line itself and are only activated if the line is in use. A special interface is supplied to allow telephone line bugs to be effectively detected and traced. The interface is connected in place of the telephone set.
Modulation types
The Scanlock ECM is suitable for reception of the following types of modulation:

  • AM
    This is for bugs that use Amplitude Modulation (AM). Not many bugs are of this type. It is generally used with low-frequency transmitters (below 80 MHz).

  • FM
    This is for the most common type of bugs that use Frequency Modulation (FM). These bugs generally operate at frequencies from 80 MHz onwards. Most of the cheaper commercial and homemade bugs are of this type.

  • Subcarrier (SC)
    With some of the more sophisticated bugs, the audio is modulated onto a non-audible subcarrier. As a result, the bug appears to be sending a silent carrier, whereas in fact it carries all of the sound it picks up in a room. In this mode, the Scanlock will adjust itself automatically to the required subcarrier frequency.  More information

  • Cable Frequencies (CF)
    This setting allows the mains power network and/or any other cables and cable networks to be checked for bugs. With transmitters of this type, such as power line bugs, the audio is often modulated onto a carrier of 120 kHz or higher. The Scanlock ECM can detect such carriers and demodulate them automatically.
Searching for radio bugs
Finding rado bugs with the Scanlock ECM is remarkably simple. All you need to do when first 'sweeping' a room for potential transmitters, is to select Strong Signal Search (SSS) and Frequency Modulation (FM) to find the most common type of bugs. The Scanlock will now automatically lock onto the strongest signal in its vicinity, as illustrated below:

With the receiver in location A, the Scanlock will probably lock immediately onto a strong broadcast station that is nearby. Such broadcast stations are generally much stronger than a potential bug in the room. When the Scanlock is moved around the room however, it will continue to scan and lock onto the strongest signal. When the receiver is in location B it will intermittently switch between the broadcast station and the bug, but when it is moved closer to the bug (C) the RF signal from the bug will be stronger than the broadcast station and the bug will be heard.

Searching for cable bugs
The Scanlock ECM can also be used for finding bugs that use an arbitrary pair of cables as their transmission medium. A well-known example of such a 'parasite' is a Power Line Bug, also known as a Mains Carrier Bug. Other cable pairs however, can also be used for this purpose (see below).

Bugs of this type, generally superimpose a modulated carrier between 25 kHz and 260 kHz onto the cable. As the carrier frequency is too high for the human ear, they are not noticed.

Especially for these types of transmitters, the Scanlock ECM has a built-in Cable Frequency (CF) receiver that covers all frequencies below 10 MHz. It should be used with the supplied LF interface that is shown in the image on the right. One end of the LF interface is connected to the 10MHz- input of the Scanlock ECM whilst the other end is connected to a mains wall socket.
Checking the mains for power line bugs

As the mains network generally consists of three wires (Live, Neutral and Earth) and the bug only needs two wires, a rotary switch on the LF interface should be used to determine which wire pair is to be investigated (L-E, L-N or N-E). Note that this feature is new to the Scanlock ECM. The earlier Scanlock 2000 only checked for bugs connected between Live and Neutral. The new method was later als used in the separate TCM-03 Cable Checker. Searching for power line bugs requires all wall sockets to be checked. We will use the following situation as an example:

Example of a power line bug hidden in a room

In this case, a power line bug is mounted inside a wall socket at the bottom right (A). As this wall socket is connected to the R-phase of the house's mains network, it can not be detected from the wall socket at location B, which is connected to the T-phase. Wall socket C however, will yield a positive detection as it is connected to the same phase as the bug.

Mains power cables are by no means the only threat for carrying eavesdropping signals. In theory, any pair of cables that is present in the target room can be used as a possible medium.

Examples of such cable pairs are the speaker cable of the stereo, intercom cables, alarm cables and the thermometer cable of the central heating. Cables of this type can also be checked with the LF interface by connecting a flying lead with two crocodile type clips to it, as shown in the image on the right. In this case, the rotary switch should be set to the far right (CABLE).
Checking other types of cables

The crocodile clips are now connected to the wires to be investigated, but note that the input voltage supplied to the CABLE socket should not be too high. The LF interface can only be used for one cable pair, so the mains plug has to be removed when checking other types of cables. To ensure this is done, a sliding panel over the input sockets prevents the use of the other socket.

Scanlock ECM with cable interface Scanlock ECM with cable interface Checking the mains for power line bugs Checking the mains for power line bugs Checking other types of cables Two crocodile type clips Selecting mains input Selecting cable input

Searching for telephone bugs
With the appropriate interface, the Scanlock ECM can also be used for tracing telephone bugs. As bugs of this type are generally only activated when the line is active (i.e. the phone is in use), tracing them requires special tactics. The supplied Scanlock Telephone Interface is used for this.

First, one has to establish whether the bug is inside the telephone set, or connected to the line. If the Scanlock ECM picks up a bug when the handset is lifted, a bug has been positively detected. Next, the telephone set should be replaced by the Scanlock Telephone Interface shown in the image on the right. It is connected to the telephone line, the Recorder output of the Scanlock (at the rear) and the RF input (front).

Picking up the handset can now be simulated by sliding the LINE switch to the right. This 'claims' the line and should cause the LED to light up.
Scanlock ECM with telephone interface

If at this stage, no radio signal is detected, the bug was probably placed inside the telephones set. The telephone set should not be checked for typical phone bugs, such as a replacement microphone. If however, the radio signal is still detected when the line is engaged, we can assume that the bug is connected to the telephone line. The line should be checked physically.

Telephone interface with cables Telephone interface Rear view of the telephone interface Wires connected to the telephone interface Scanlock ECM with telephone interface Scanlock ECM with telephone interface Connecting the telephone interface to the recorder output

The Scanlock ECM was generally supplied with a number of accessories and training devices, packed together in a large sturdy flightcase. From the inventory in our Scanlock ECM kit and the user manual, we have assembled the following list of items that were available:

  • Scanlock ECM, main unit
  • Flightcase
  • User Manual
  • Telescopic antenna with N-connector (HF)
  • Telescopic antenna with BNC socket (LF)
  • Right-angle N-connector
  • Various radio bugs (for training)
  • Power line bug (for training)
  • Telephone interface
  • Low Frequency interface
  • Remote Alarm
  • BNC antenna lead
  • Telescopic hand antenna with BNC
  • Headphones
  • Mains lead
  • Shoulder strap

Flightcase, open, without Scanlock ECM

The purpose-made flight case is designed inside such a way that all items are nicely stored in their own tight-fitting compartments. The receiver is stored in the large rectangular space at the centre. Around the receiver are the telephone interface, a power line bug the LF interface, the external alarm and an (optional) FM/Subcarrier bug. Additional devices and training bugs can be stored in the remaining spaces. Cables and headphones are stored beneath the Scanlock ECM.

Headphones Mains power line bug (MC) Telephone interface with cables Cable with two crocodile type clips for checking cables Checking the mains for power line bugs N-type antenna extension lead LF antenna (left) and HF antenna (right) Remote alarm unit

Training devices
In order to get acquinted with the Scanlock ECM and with finding bugs, it is necessary to train regularly with the equipment. For this purpose, Audiotel and other manufacturers supplied a range of training bugs. Some of these were real bugs that were converted for training use.

Subcarrier bug
For training purposes a small radio bug in a metal enclosure was often supplied with the Scanlock ECM. Offering two types of modulation (FM and SC) it can be used as an ordinary FM radio bug as well as a subcarrier bug.

Please note that the possession and use of this type of bug might be subject to local laws.

 More information
FM/SC training transmitter with wire antenna

Power line bug
For training purposes, Audiotel created the simple MCX Power Line Bug, that is shown in the image on the right. It consists of a small PCB that is powered from the mains. A sensitive microphone picks any sound, and the modulator injects the signal back onto the mains.

 More information
Interior of the SL-MCX power line bug

Mactron Mark II bug
For traning purposes, a variety of RF bugs were used. Although Audiotel offered a range of battery powered radio transmitters, some users preferred the powerful Mactron Mark II shown in the image on the right. It was crystal based and was powered by a standard 9V battery.

It is shown here with an external plug-in mike. Please note that the possession and use of this type of bug might be subject to local laws.

 More information
Mactron Mark II bug with external microphone

Phone mike bug
Another professional bug that was often used for training purposes, is the so-caled Phone Mike Bug, a small RF transmitter that looks exactly like the standard microphone of an old telephone handset.

It was powered by the telephone itself and was only on the air when the handset was picked up.

 More information
Placing the radio bug inside the handset

The Scanlock ECM is housed in a heavy aluminium case that can be accessed by removing the top panel (when the receiver is in horizontal position). The batteries and the speaker are mounted to the inside of the top panel and are connected to the main unit by means of flying wires.

The interior roughtly consists of three parts: the main board that contains all analogue and digital components, a Power Supply Unit (PSU) that is mounted at the right side (when seen from the front) and the controls board with the LCD display that is mounted behind the front panel.

The main board takes up most of the available space inside the receiver. In consists of a professionally designed double layer PCM with all first class components mounted on the top surface. The image on the right shows the harmonic receiver, located in the rear left corner.
Front-end (scanlock harmonic mixer)

The other parts of the main board contain the separate CF receiver, for frequencies below 10 MHz, the audio circuits, with tone and ticker generators for the LOCK and LOCATE modes, and the AMD 87C521 microcontroller. A separate I/O chip is used for driving the LED bar.

Scanlock ECM interior Main board Front-end (scanlock harmonic mixer) Audio circuitry CF receiver Microcontroler LED driver PSU

AM   Amplitude Modulation
CF   Cable Frequency (signals below 10 MHz)
FM   Frequency Modulation
HF   High frequency (signals above 10 MHz)
LF   Low Frequency (signals below 10 MHz)
MC   Mains Carrier / Mains Cable
RF   Radio Frequency (signals of 10 MHz or higher)
SC   Subcarrier modulation
SSS   Strong Signal Seeking

  1. Audiotel International Ltd. Scanlock ECM Countermeasures Receiver
    Scanlock ECM sales leaflet. © 1984, Audiotel.

  2. Audiotel International Ltd.
    Audiotel website, 18 June 2003. Retrieved via WayBack Machine.

  3. Audiotel International Ltd. Scanlock ECM Operators Manual
    Issue 3.4, April 1991.
Further information
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Crypto Museum. Created: Wednesday 28 March 2012. Last changed: Wednesday, 12 April 2017 - 09:44 CET.
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