tiny SCR Preselector

 
Microchip
Microchip
Microchip

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(last update 01/12/2009)

tiny SCR Preselector.

  Revision B

1.8MHz to 30MHz Tunable Band Pass Filter.

Software Controlled Radio Preselector via USB or I2C Bus

A less complex version of the SCR Preselector.

Assembled and kit versions.

 

tiny SCR Preselector Revision B comprises:

 

  • Toroid core changed from T200-2 to T200-7. Improved linearity. Better response on high frequency bands.

  • Optimized impedance match transformers. Less attenuation on band-pass. Better return loss and stop band.

  • Enhanced tuning function. Now more smooth tuning steps.

  • Software program revised regarding Windows 2000 users. It works on Windows 2K, XP, Vista32 and Windows 7 OS.

  • USB and I2C control boards no change.

 

Tiny User Manual

User Manual

Tiny Assembly manual

Assembly manual

 

 

The tiny SCR Preselector is a less complex version of the SCR Preselector.
It is oriented to complement radio projects due to its modular design.

There are no active switching devices in the signal path therefore it does not contribute to IMD (Intermodulation Distortion) itself.

Features:

■ Five bands tuneable Band Pass Filter bank for HF Bands covering 1.8 to 30 MHz.

■ Digital tuning.

■ High linearity. Suitable for Software Define Radios.

■ Removes the strong signals radiated back to the antenna from quadrature
    mixers in SDR applications.

■ Joined to a VFO can perform tracking tuning function.

■ Its modular design lets an easy implementation on your projects.

■ I2C Bus or USB control modules options.

■ Tin plated enclosure available.

 

USB control option


■ PC controlled through graphical user interface (GUI).

■ Full speed USB v2.0 compliant.

■ Plug and Play feature.

■ Runs under Windows 2K, XP, Vista 32 and Windows 7 OS.
   (Windows 2K users need to update Framework from Windows Update site)

 

I2C bus control option

 

■ 16 bits standard I2C Bus serial bus interface.

■ Support 100Kbits/s and 400kbits/s FAST I2C Bus protocols.

■ Auxiliary Input/Output ports to implement external controls and/or signalling.

■ Up to eight control modules can share the same I2C Bus.

■ Compatible with most microcontrollers.

The tuneable Band-Pass Filter bank module.

 

 

 

A five bands tuneable Band-Pass Filter bank make up the core of the tiny SCR Preselector.
On SDR applications, where high linearity is needed its passive design keeps a high IP3 factor thanks to an unique large coil wounded on a T-200 toroidal core, ultra low ESR, C0G (NP0) RF chip capacitors, low RF resistance magnetic shielded relays for tuning and telecom grade relays for band switching, instead of nonlinear semiconductors that cause distortion in front of strong signals and exhibit low tolerance to transients.

The tiny SCR Preselector incorporates on antenna input a gas discharge tube as surge suppressor providing safety against static spikes.

The five telecom grade relays are connected to its corresponding section of the coil through the Band Selector control on the program software.

A network of low ESR RF capacitors of 1 pF, 2 pF, 4 pF, 8 pF, 16 pF, 32 pF, 64 pF and 128 pF are connected to the coil in serial fashion emulating the variable capacitor needed to tune the series resonant circuit formed. Each capacitor is connected to the network by means of its associated tuning relay. The band-pass filter formed in this way is a classic serial tuned LC circuit, offering narrow bandwidth and low attenuation on the band-pass segment of frequencies.
The program performs the task of adding or subtracting capacitor values in binary fashion in direct relationship with the tuning control on the program software. Increments or decrements are done in steps of 1pF.


Available bands are:
• A) 1.8 MHz to 3 MHz • B) 3 MHz to 5.5MHz • C) 5 MHz to 10 MHz • D) 10.5 MHz to 21 MHz E) • 16 MHz to 30 MHz.

A carefully PCB layout design and strong L/C decoupling prevent stages to couple in to each other improving the stop band response and filtering noise from control lines.


Connection to the control module is done with 16 ways ribbon cable and IDC connectors on both sides, carrying control signals and power supply.
BNC connectors are used for Antenna Input and Preselector Output.
Dimensions are: 153x75mm. (6.02x2.95 inch)

 
 

Why a large toroid core T-200-7 is used in the tiny SCR Preselector?

The reason is to keep a high IP3 or in other words achieve tolerance to large signals.
On SDR radios the high linearity of quadrature mixers demands the best linearity possible to the ancillary connected before the mixer stage to avoid to degrading its IP3. Wide cores with large sectional area let wind more turns on it (more inductance); these parameters are essential to keep a low magnetic flux inside the toroidal core to achieve so mush linearity as possible.

On the other hand, the large core offers a non-loaded Q (efficiency) value exceeding 400, providing selectivity and high stop band characteristic, limiting the bandwidth in front of the mixer which improve signal to noise ratio, image rejection and prevents adjacent signals from reaching the circuits on the receiver side where 2nd order IMD and other spurs is most often created because of generalized use of nonlinear semiconductors especially on switching circuitry.

 

Transmission/Return loss of each band response plots.

 

Band A

Band A plot

(click to enlarge)

 

Band B

Band B plot

(click to enlarge)

 

Band C

Band C plot

(click to enlarge)

 

 

Band D

Band D plot

(click to enlarge)

 
 

Band E

Band E plot

(click to enlarge)

 

Broadcast Band rejection filter connected to the tiny SCR Preselector to remove interferences from AM stations.

Filters page

Heros Tech BCB filter

 

with BCB filter

Band A with BCB filter plot

(click to enlarge)

 

 

USB and I2C Bus Control modules
USB module
I2C module top
I2C module bottom
 

 

They are two optional control modules available: USB and I2C Bus

The tiny SCR Preselector USB is located in a separate module board to avoid introducing noise generated by the microprocessor itself and the noisy USB bus.
A 18F2455 PIC microcontroller is used. It incorporates a full speed (v2.0) USB port peripheral to communicate with the PC.
A four MHz quartz crystal is connected to the oscillator pins generating via internal PLL circuitry, the 48MHz clock for the microcontroller core and the 96 MHz clock to the USB peripheral.
In circuit serial programming (ICSP) connector is provided to let programming the microcontroller firmware without remove it from the PCB.
Two power drivers ULN 2803 ICs interface the PIC microcontroller output ports with the relays on the Band Pass Filter module.
Connection to the Multi-band pass filter module is done with 16 ways ribbon cable and IDC connectors on both sides, carrying control signals and power supply.
To suppress ESD and other transient over voltage events on the USB signals lines, an array of Avalanche Diode Array are connected to them.
A carefully PCB design and strong decoupling avoid introduce noise on the filters board.
Standard 2.5mm diameter power jack and type B USB connector are used.
Power supply: 12V DC/400mA max.

The tiny SCR Preselector I2C Bus control module, located in a separate module board, lets to handle easily the Band pass Filter board from an existing control system on your radio project due the simple implementation of the I2C protocol on any microcontroller.
It is based on the well know PCF8575 remote 16-bit I/O expander for I2C Bus. Two power drivers ULN 2803A interface the PCF8575 output ports with the relays. I2C addresses are configured through J4, J5 and J6 pin headers and J2 and J3 headers connect pull-up resistors to the I2C line if is needed.
Three auxiliary ports are provided for optional controls and/or signalling. J7 are Input/Output ports, 5volts level and J8 are buffered output ports at 12volts level.
J11 provides an open-drain interrupt output which can be connected to the interrupt logic of the microcontroller.
Connection to the Multi-band pass filter module is done with 16 ways ribbon cable and IDC connectors on both sides, carrying control signals and power supply.
A carefully PCB design and strong decoupling avoid introduce noise on the filters board.

Features of the module are:

- 16 bits standard I2C serial bus interface.

- 100Kbits/s and 400kbits/s FAST I2C Bus.

- Three auxiliary Input/Output ports for optional controls and/or signalling.

- Open-drain interrupt output.

- Sixteen latched outputs with high current drive capability.

- Programmable address by 3 hardware address pins.

- Optional I2C Bus lines pull-up resistors.

- Up to eight modules can share the same I2C Bus.

- Compatible with most microcontrollers.

- Power Supply: 12V DC/400mA max.

- Dimensions: 62x47mm. (2.44x1.85 inch)

I2C Bus control manual

I2C control module manual

Enclosure for tiny

 
 

The tiny SCR Preselector should be preferably encloused in a RF tight box due the antenna effect of its large coil that easily gathers nearby signals or noise.
To shield the tiny SCR Preselector against electric and magnetic fields a properly manufactured tin plate enclosure is offer including punched holes for connectors.

Dimensions are: 162x102x50mm. (6.378x4.016x1.969 inch)
 

The software

They are two versions of the program, Standard and Experimenter.

Both keep the same functionality, only the graphic user interface changes.

The program software on the PC runs under Windows 2K, XP, Vista 32 and widows 7 OS.
(Windows 2K users need to update Framework from Windows Update site)

The user, through the GUI, commands the tiny SCR Preselector via USB port.

The program can work at the same time with any SDR software running on the PC without interfering, allowing to the user to control both simultaneously.

 

Standard GUI version

 

1- Band selection.
By default Band A. Click over any button to change band.

2- Tuning Decrement
One step frequency decrement. One step increment of capacity.

3- Status bar

4- Tuning dial
Notice due to the response of series tuned circuits, the attenuation of the pass band filter increases with less capacity. A good practice is to avoid smaller values.

5- Tuning knob
Performs tuning moving left or right. Also by pressing left or right arrows on the keyboard or turning mouse wheel. Adjust to maximum signal or background noise.
The tune reference is expressed in picofarads with values between 1pF and 255pF.
Notice due to the response of typical series tuned circuits, the attenuation of the pass band filter increases with less capacity. A good practice is to avoid values smaller than 50 picofarads.


6- Tuning Increment
One step frequency increment. One step decrement of capacity.

7- Memories
There are up to five memories per band (25 in total).
Clear-Store and Recall functions for each memory position.
Store function lets write a text label.

 

Experimenter's GUI version

 

Tini SCR Preselector. Revision B. Assembled and kit versions available

 

The assembled version contains:

- tiny SCR Preselector. Band Pass Filter bank module and control module.

- IDC-ribbon interconnection cable.

- USB cable. (USB control module option)

- User manual.

- CD ROM including all software and USB driver for Windows Operate Systems.

The kit version contains:

- All parts needed to build the kit. Includes pre-progamed PIC microcontroller (USB option)
  and pre-winded impedance match transformers.

- IDC-ribbon interconnection cable.

- USB cable. (USB control option)

- Comprehensive assembly manual.

- User manual.

- CD ROM including all software and USB driver for Windows Operate Systems.

 

tiny SCR Preselector kit version. USB or I2C control

£ 89.00

tiny SCR Preselector assembled version. USB or I2C control

£ 119.00

I2C or USB control module only

£31.00

tiny enclosure

£ 24.00

 

 

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