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Minilock 6910
Programmable Precision Measurement Receiver

Minilock 6910 was a digital programmable precision measurement receiver, developed and built by Schlumberger in München (Germany) around 1990, as the successor to the Minilock 6900. The receiver covers all frequencies between 10 kHz and 1890 MHz and supports all common modes (modulation types), including AM, FM, PM, USB and LSB with a variety of bandwidths.

The receiver was commonly used by a number of agencies around the world, for interception and fingerprinting of suspicious radio signals and for frequency spectrum monitoring, for example in Germany, The Netherlands, and South Korea.

The basic 6910 system consists of two units: a smaller unit that contains the power supply and the control panel, and a larger 'black box' that contains the actual receiver parts. Optionally, the set could be completed with a 6901 Panoramic Display and a 4940 Antenna Multiplexer. The units are linked together via cables at the back.
  
Minilock 6910 control panel

Between the RF Unit and the Control unit, three coaxial cables, a power cable and a data cable are needed. For this reason, the two units can not be separated more than approx. 20 cm. The optional 6901 Panoramic Display, that could also be used with the Minilock 6900, is connected via three coaxial cables that can have any length. The optional 4940 Antenna Multiplexer can only be used if the (optional) 6906 Control Adapter is present at the rear of the 6910 RF Unit.

Minilock 6910 control panel 6910 Control Unit 6910 RF Unit 6901 Panoramic Display 4940 RF Multiplexer

Block diagram
The block diagram shows how the various parts of the Minilock 6910 are connected together. At the top left is the antenna input, that is passed via a switchable attenuator to a pre-selection filter and then to a low-noise low-distortion pre-amplifier. From there is is mixed with the first Local Oscillator signal (LO), passed through the 1st IF-stage and then mixed with the 2nd LO, resulting in a 10.7 MHz signal. At the right are the IF and the various (demodulated) outputs.


At the bottom right is the Audio Frequency amplifier (AF) and the speaker output. The local oscillators, the attentuator, the pre-selection filters and the various IF stages, are all controlled by the CPU, depending on the current frequency band. The CPU has various interfaces to the outside world and for connecting accessories, such as the 4940 Antenna Multiplexer or an XY plotter.

Components
6910 Control Unit
The Minilock 6910 consist of two major building blocks: the RF Unit and the Control Unit. The two units are connected by means of a series of cables at the rear. All controls are on the Control Unit that was usually mounted on a desk. If space was limited, the RF Unit was often mounted below the desk. The distance between the two unit is, of course, limited by the supplied cables.

The control panel is a separate unit that is usually mounted at the front size of the Control Unit, as shown in the image on the right. In that case, the Control Unit is placed on the desk. To the right of the control panel are two power buttons: a red one and a black one.

If space is limited, the control panel can also be detached from the Control Unit, allowing both (large) units to be mounted below the desk, whilst the control panel was within reach of the operator. The extenstion cable allows the control panel to be placed approx. one meter away.
  
6910 Control Unit

The red power button is used to turn the entire system ON or OFF. After switching the Minilock 6910 ON, maximum stability is obtained after 20 minutes. The black power button can be used to put the system in STANDBY mode. In this mode, the entire receiver is powered down, except for the internal 10 MHz reference oscillator. As a result, the receiver will have its optimum stability when waking up from STANDBY mode. The power buttons are part of the Control Unit and can not be removed with the control panel. They should stay within reach of the operator.

6910 Control Unit Minilock 6910 control panel Adjusting the frequency Control Unit interior Control Unit rear view Control Unit interior Control Unit interior Cables

6910 RF Unit
The RF Unit is the larger of the two units. It measures 444 x 236 x 490 cm, weights 19 kg and fits a standard 19" rack. It contains all HF, IF and AF components and is controlled by the 6902 Control Unit. The antenne is be connected to the N-connector at the rear. All frequency and band dependent parts and filters inside the RF Unit are controlled by an 6800 microprocessor.

The 6910 RF Unit is usually placed on top of the 6910 Control Unit, so that the control panel is easily accessible when the reciever is placed on a table. It is also possible however, to place the RF Unit under the Control Unit. Furthermore, the Control Panel can be detached (see above).

In order to obtain maximum precision and accuracy, the receiver uses its own built-in temperature stabilized 10 MHz reference oscillator. A few minutes after turning the receiver on, the stability of the reference oscillator is good enough for general use.
  
6910 RF Unit

Maximum stability however, is obtained after several hours. For this reason, the receiver can be put in STAND BY when unused, by means of the ST BY switch. This leaves the reference oscillator powered. The Control Unit is locked to the same 10 MHz reference signal via a coax cable at the back. If an even better precision and stability is required, the internal reference oscillator can be locked to an external 10 MHz source, e.g. from a Rubidium, DCF or GPS Standard.

6910 RF Unit RF Unit, rear view 6906 Control Adapter, mounted at the rear of the RF Unit Cables

6901 Panoramic Display
In order to get a real-time view of the frequency spectrum around the currently received frequency, the optional 6901 Panoramic Display can be used. The Display Unit, that could also be used with the earlier Minilock 6900, is housed in a 19" case that can be stacked with the other units. Alternatively, the Panoramic Display can be mounted elsewhere in a desk.

It connects to the RF unit by means of three coaxial cables.
  
6901 Panoramic Display

6901 Panoramic Display 6901 Panoramic Display, front view Panoramic Display, rear view

4940 RF Multiplexer
The Antenna Multiplexer allows size different antennas to be connected to the Minilock 6910. Only one of these antennas will be connected to the receiver at any time, controlled via a 25-way interface at the rear. The 4940 Multiplexer can only be used if the 6906 Control Adapter is installed at the rear of the 6910 RF Unit.

The Minilock 6910 can be programmed to automatically select the appropriate antenna for any given frequency band. In addition, the 4940 has extra switched outputs that can be used for a variety of functions, such as selecting an amplifier or attentuator.

The 4940 Multiplexer if fully controlled by the Minilock 6910, but can also be operated manually, using a series of push-buttons at the front panel.
  
4940 RF Multiplexer

4940 RF Multiplexer 6 inoputs and 1 output on the front panel of the 4940 RF Multiplexer (N-connectors) Manual controls on the 4940 RF Multiplexer Rear view of the 4940 RF Multiplexer Expansion sockets at the rear of the 4940 Connection to the Minilock 6910 6906 Control Adapter, mounted at the rear of the RF Unit

Technical specifications
  • Frequency range: 10 kHz - 1.89 GHz (in 10 Hz steps)
  • Antenna input impedance: 50 Ω
  • Preselection: Automatic, 19 ranges
  • Input attenuator: 0, 3, 10, 20, 30, 40, 50, 60 dB
  • Input pre-amplifier: (selectable) 15 ± 1 dB
  • Stability: 10-7 after 20 minutes at 25°C
  • Temperature drift: 2·10-9/°C
  • AF Bandwidth: 1-100kHz in 0.5kHz steps
Intermediate Frequency (IF)
  • 829.3 MHz (0.01-520 MHz)
  • 229.3 MHz (520-1010 MHz)
  • 250.7 MHz (1010-1370 MHz)
  • 950.7 MHz (1370-1890 MHz)
References
  1. Schlumberger Technologies GmbH, Minilock 6910 Bedieningsanleitung
    Version 1-2, July 1990 (German).

  2. Schlumberger Meßgeräte GmbH, Minilock 6910 service manual - Vol 1
    Volume 1 (German). November 1990. (10MB PDF file).

  3. Schlumberger Meßgeräte GmbH, Minilock 6910 service manual - Vol 2
    Separate page with full service manual Volume 1 and Volume 2.
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