HF Tunable Preselector-SCR-CAT 2020.

1.8MHz to 50MHz. 6 Bands. Tunable Digital Band-Pass Filter bank.

Featuring Computer Aided Transceiver (CAT) operation for remote tune.

Suitable to work with any Software Define Radio (SDR) or analogue radio.


The HF Preselector-SCR-CAT 2020 Tunable Digital Radio Preselector is suitable to work with SDR or analogue radios. It is connected in line between the antenna and your receiver or transceiver providing filtering of R.F.spectrum in order to reduce broadband energy and conforming signals before reaching the receiver. Useful in front of strong Near Vertical Incidence Signals (NVIS) propagation situation. In addition it provides rejection of signals from MW AM and VHF FM broadcast bands, removes strong broadband emissions radiated back to the antenna from quadrature mixers and attenuates signals in the VHF region that could mix with the sampling frequency of the analogue to digital data converter in SDR radios.

CAT remote control. Tracking feature
CAT (Computer Aided Transceiver), serial data communication protocol over Virtual Com Port (VCP) enables the SCR preselector-CAT to provide the user with a way to interface with several radios. The SCR Preselector-CAT acts as a tunable passive listener translating valid CAT commands into specific instructions.

Technical features:

■ Compatible with any Analogue and SDR radios.

■ Control software runs under Windows 8.1, Windows 10, Windows 11

■ Signed USB driver.

■ Six selectable bands and preselector By-Pass.

■ Featuring Five Tunable Band Pass Filter bank covering 1.8 to 30 MHz plus 50MHz (6m) band pass filter.

■ Embedded -110dB max stop band MW Broadcast Band Rejection Filter. (AKA BCB filter, Brick Wall filter, MW band killer filter).

■ All filters are LC passive. No active switching or amplifier devices in the signal path, (LNA Off), therefore it does not contribute to any IMD.

■ Very high linearity (IP3). Suitable for any Software Define Radios and analogue receivers.

■ PTT feature enables connecting in-line with a high power transceiver allowing to by-pass the preselector in transmission.(with Dual By-Pass relay accessory).

■ Two 50Ω antenna inputs and one 50Ω antenna output lets insert a converter, filter, etc between the antenna and the Preselector.

■ 0dB, 6dB, 12dB, 18dB input attenuator using Mini-Circuits PAT series dedicated attenuation cells.

■ Tuning coils wound on large toroidal cores T-106 size, rated for 100Watts RF applications, avoiding saturation due strong signals.

■ Band Pass Filters have Notch filters centered on the adjacent amateur bands giving an exceptional stop-band attenuation.

■ Digital tuning. Nine bits variable digital capacitor. One pF resolution. (Variable capacitor emulator).

■ Two embedded 7th order, 60MHz, Elliptic Low Pas Filter for rejection of signals from FM broadcast band and sampling frequencies from SDR radios.

■ Switchable low noise amplifier (LNA). 25dB Gain, 38dB IP3, 2.7dB Noise Figure.

■ 12V DC/1Amp DC output connector for an external by-pass dual power relay. (PTT or Keying line activated).


■ Full speed USB v2.0 compliant.
■ Plug and Play feature.
■ Signed USB drivers.
■ USB type B connector.
■ ESD protection on USB lines.
■ Optocoupled USB sense signal.
■ Optional USB Galvanic Isolator available.

I2C bus

(Customised external controller is needed)

■ I2C bus external lines Galvanically isolated (ADuM1251).
■ 8 bits standard I2C serial bus interface.
■ Supports 100Kbits/s and 400kbits/s FAST I2C Bus protocol.
■ Mini Din connector for I2C control lines.
■ Compatible with most microcontrollers.

■ Power supply: 12Volts DC/ 0.5Amp

■ Dimension: 220mm x 165mm x 52mm (8.661 x 6.496 x 2.047in).

■ Weight: 1.5Kg.


HF Preselector-SCR-CAT 2020 Tunable Digital Radio Preselector
Schematic Diagram. (click to enlarge)



SCR_HF tunable-Preselector_response plot. SCR_HF tunable-Preselector_User Manual
Scalar Network response plot. Preselector tuned on 40m band
(click to enlarge)
SCR Preselector-CAT user manual rev03.

HF Preselector-SCR-CAT 2020. Tunable Digital Radio Preselector. Video tutorials by Nigel Coleman, G7CNF


Phil McBride, VA3QR, has written a comprehensive review regarding the HF Preselector-SCR-CAT 2020



HF Preselector-SCR-CAT 2020. Tuning Tracking feature with FlexRadios.

FlexRadio signature radios. Tuning Tracking feature using DDUtil utility application.

DDUtil utility application supports the HF Preselector-SCR-CAT 2020 allowing tracking and sharing features from FlexRadio family series and other Remote Control apps.
DDUtil V3 (Data Decoder Utility) by Steven P Nance, K5FR is designed to enhance the operating experience for Flex Radio users by providing advanced connectivity to Peripheral equipment and radio control programs (RCPs) otherwise not available.
DDUtil may be used in a stand-alone mode or with up to six Radio Control Programs (RCPs) running simultaneously.
DDUtil automatically senses when a piece of hardware or a RCP is requesting radio information and responds accordingly.

DDUtil v3 Set up graphic user interface.

(You can also see: http://k5fr.com/DDUtilV3wiki/index.php?title=Other )



FlexRadio legacy radios models Flex-1500, Flex-3000, and Flex-5000 TuningTracking feature using PowerSDR ke9ns v2.8.0

DDUtil supports Flex legacy radios with some limitations. An excellent alternative is " PowerSDR ke9ns v2.8.0 " an ongoing success being performed by the independent developer Darrin L Kohn, KE9NS. The application perfoms tuning tracking among other features.
Link: https://www.ke9ns.com/flexpage.html



PowerSDR v2.8.0 by Darrin L Kohn, KE9NS



HF Preselector-SCR-CAT 2020. Tunable Digital Radio Preselector. Operating modes.

Stand alone mode.
Because the dynamic environment in which you operate demands that you exercise the most effective command possible, the HF Tunable Digital Preselector is designed with the most important functions immediately available for adjustment so control knobs and pushbuttons are reduced to minimum possible.
Two encoders with a dynamic LCD display are located on front panel for any operation.
The large flywheel-tuning knob with finger hole is the main tuning control. Pushing it quits actual screen and enters the main menu.
On its left hand, a second knob features menu navigation and selection.
A combined operation of rotating and pushing knobs does rolling and selection of all available functions.

  LCD menus. See User manual for details.  

Bands A-B-C-D-E


Band F


Band selection


Memory bank


Operation mode menu

Tracking mode


Graphic User Interface (GUI) console on PC. Manual remote tuning operation mode.

HF Preselector-SCR-CAT 2020 is commanded by the user by means of the GUI console on PC or in stand-alone mode.
The console runs under Windows 7, Windows 8/ 8.1, Windows 10 and Windows 11.
The GUI context menu provides additional options and utilities.



Graphic User Interface console on PC.




CAT remote control. Kenwood TS-2000 compatible.

CAT (Computer Aided Transceiver), serial data communication protocol over Virtual Com Port (VCP) enables the SCR Preselector 2020 to provide the user with a way to interface with several radios. The user can track the SCR Preselector 2020 from remote control software applications.
Refer to your specific radio software application manual regarding CAT operation.
The SCR Preselector 2020 acts as a passive listener translating valid CAT commands into specific instructions.The microcontroller on board performs valid Kenwood TS-2000 compatible CAT commands.


SCR Preselector-CAT on Virtual Com Port.


Internet/Local network remote operation mode.

The HF Preselector-SCR-CAT 2020 supports connections via the Internet and Local network.To make use of this feature execute the GUI application software on your computer and on a remote one.The protocol only uses TCP/IP; UDP which by definition is unreliable is not used.

Remote server (preselector side)
To start the communication open first the Remote Server.
The connection console is displayed showing the address in dotted format of the computer where the server is running and the port number.
Click on Start button. If the connection is successful you will start to receive commands from the client computer after the remote client is configured and sends commands. Current commands are traced on the console. The SCR Preselector 2020 will respond in accordance with valid CAT commands.



Remote Client (remote control side)

To connect to the server make sure the server is started and listening. Open the Remote Client.
The connection console is displayed showing the default address of the client computer in dotted format and the port number, default port 51000.
Write down the server IP address in the Host IP text box and start operating the SCR Preselector 2020 application.
Executed commands on the GUI are traced on the console. Also valid CAT commands can be send from the command text box.


I2C bus remote operation mode. (Customised external controller is needed)

HF Preselector-SCR-CAT 2020 features remote control mode from a dedicated external radio or digital system supporting I2C bus data communication system.
It operates fully independent of other remote control modes.
I2C lines are Galvanically isolated by means of ADuM1251 a hot swappable digital isolator specialised device from Analog Devices Inc.

For more details regarding I2C bus remote operation mode see the user manual.


  WARNING: If the preselector is connected in line with a transceiver, take precautions to prevent transmitting through it. A 100 Watts power rating High Frequency, High Power, Dual Port By-Pass relay accessory is available in order to detour the preselector in transmission switchover.

NOTE: Some FlexRadio models include RF by-pass power relay embedded. See primary antenna ports configuration section on specific FlexRadio Owner Manual Antenna configuration options.

Optional Accessories


General purpose LF-HF-VHF-UHF, Dual Port, by-Pass relay

100 Watts RF power rating

DC - 1300MHz

It is designed to by-pass the Input and Output RF ports of any device in line with your transceiver or transmitter
A typical application includes inserting between antenna and a transceiver or transmitter a preselector, preamplifier or filter device.

On transmission switchover, via PTT or Keying line, the Input and Output RF ports of the device are by-passed avoiding being overloaded


USB Galvanic Digital Isolator

For SDR radios is designed to break the galvanic connection between USB host PC and the attached SDR radio peripheral avoiding that whatever power and ground noise from the PC side do not influence the sensible radio on the other side.

The resultant galvanic isolation improves common-mode voltage, prevents ground loops, a common cause of interference and data distortion issues, improves reception, as the noise floor in the receiver is lowered, and removes spikes and birdies on the PC panoramic screen.

In addition provides isolation of 1000V AC between the host PC and the radio peripheral that guard against surges that may come from the antenna connected to the receiver.




10KHz to 60MHz Antenna Galvanic Isolator.

Removes local pick-up noise by the antenna feedline.

Specifically recommended to work with SDR radios.
Suitable for analogue radios.
Indoor mount.




PTT foot switch

Robust momentary action foot operated switch with non slip base.

• 1.8m cable length.
• Standard 3.5mm jack plug. Other connectors available, Please ask.
• Dimensions 74 x 75 x 32mm
• Rated 3A 250V AC






HF Preselector-SCR-CAT 2020. Tunable Digital Preselector.


The set includes:
Plug and Play HF Preselector-SCR-CAT 2020 Tunable Digital Radio Preselector, User manual in CDROM format, Two BNC-BNC coaxial cable tails, USB cable, Power supply cable with EMI filter and Auxiliary connectors. SKU: HS-12003CAT

SCR Preselector-CAT