Revision B
(last update 22/09/2009)
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High-performance Software Controlled Radio Preselector
TUNABLE BAND-PASS FILTER BANK FOR HF BANDS
Covering 1.8MHz to 30MHz in five bands.
Digital Tuning. Stand alone or USB control from PC.
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The SCR Preselector is available through
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The SCR Preselector in QEX magazine.
The SCR Preselector is featured in the May/June 2008 issue of QEX.
Original article
(Reprinted with permission © ARRL)
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SCR Preselector Brochure download
(Revision B) |
Features:
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■ Tunable Band Pass Filter for HF bands.
■ Covering 1.8 to 30 MHz in five bands.
■ Digital tuning.
■ Very high linearity (IP3).
■ Compatible with Software Define Radios.
■ Removes the strong signals radiated back to the antenna from quadrature mixers in SDR radios.
■ Joined to a VFO can perform tracking tuning function.
■ Standalone or USB controlled through a graphical user interface (GUI) on PC.
■ It can be commanded from an external control system.
■ High speed USB v2.0 compliant.
■ Plug and Play feature.
■ The software program on PC runs under Windows 2000, XP, Vista 32 and Windows 7 OS.
(Windows 2K users need to update Framework from Windows Update site)
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The Preselector an old concept to modern challenges.
Preselector for wireless. About 1914
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The Preselector is an almost forgotten concept as old as the Radio.
It is a unique tool, used for decades in radio design to contribute to selectivity and protect the early stages.
With the advent of the new SDR technology and the huge increase of the activity on HF bands, this old concept gives solution to modern challenges. |
The SCR-Preselector — A First Line of Defence
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The SCR Preselector is the first stage connected to the antenna system providing narrow band selectivity. It does not includes active switching devices like transistors and diodes in the signal path, it does not contribute IMD, Intermodulation Distortion, itself and provides safety under the electronic warfare conditions, which exist nowadays on the crowded bands.
On SDR applications, where high linearity is needed its passive design keeps a high IP3 factor thanks to its passive circuitry design in the signal path that uses large toroid cores and reliable magnetic shielded mechanical low RF resistance relays for tuning and telecom grade relays for switching functions, , instead of nonlinear semiconductors that cause distortion and exhibit low tolerance against strong signals and statics.
Ultra low ESR, C0G (NP0) RF chip tuning capacitors ensuring that performance does not degrade over time.
In Direct sampling SDR applications, the SCR Preselector gives protection against false images since the bank filters have an enhanced stop band.
These filter has the ability to provide a very sharp transition from pass band to stop band attenuating signals outside band that could mix with the sampling frequency of its Analog to Digital Data Converter.
In addition, the SDR-Preselector incorporates gas discharge tubes to surge suppressor on each antenna input providing safety against static spikes. Furthermore, the unused antenna input is short-circuited to ground.
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SCR Preselector Diagram.
(Click on picture to enlarge.)
The SCR Preselector. Open lid
The BCB Filter
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The SCR Preselector incorporates a Broadcast Band (BCB) high pass filter.
This is a so-called “brick-wall filter,” which attenuates spurious signals from strong AM broadcast stations. Such signals are well known enemies of shortwave receivers.
The BCB filter added to the Band Pass Filters bank stretches its effects into the medium Broadcast Band with a very sharp slope attenuating signals as much as 120dB.
The filter has minimum attenuation of all signals at 1.8 MHz and higher frequencies, including the CW portion of the 160 m band.
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BCB filter. Transmission/Return Loss plot.
(Click on picture to enlarge.)
Band Pass Filters. The core of the SCR Preselector
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A five overlapped bands tunable Band Pass Filter bank shapes the core of the SCR Preselector.
Each filter is a classic serial tuned LC circuit, and offers narrow bandwidth with little attenuation.On the upper side of frequencies its stop band is focussed on the adjacent band.
A network of high quality, high voltage, Ultra low ESR,precision capacitors of 1 pF, 2 pF, 4 pF, 8 pF, 16 pF, 32 pF, 64 pF, 128 pF emulates the variable capacitor needed to tune the series resonant circuit, ensuring that performance does not degrade over time.
The control program performs this task by adding or subtracting capacitor values in binary fashion in direct relationship with the tuning knob.
Increments or decrements are done in steps of 1, 2, 5 or 10 units.Available bands are:
Band A) 1.7 MHz to 4 MHz
Band B) 2.5 MHz to 6 MHz
Band C) 5 MHz to 12 MHz
Band D) 9 MHz to 21 MHz
Band E) 16 MHz to 30 MHz. |
Serial LC filters circuit diagram
SCR Preselector Transmission/Return Loss plots.
(Click on picture to enlarge.)
Why large toroid cores for 200 Watts transmitters are used in the SCR Preselector?
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The reason is to keep a high IP3 or in other words achieve tolerance to
large signals.
That is important on 160m, 80 and 40m bands, especially in multioperator
contest or DXpedition situations where large antennas are located in
close proximity to the operating receiver.
In SDR applications the high linearity of quadrature mixers are preserved.
On the other hand, large cores reduce the magnetic flux inside the
cores and offer high Q (efficiency) values, more than 300, providing
narrowband selectivity which improves signal to noise ratio and
prevents unwanted signals from reaching the circuits on the receiver
side where 2nd order IMD is most often created because the generalized
use of nonlinear semiconductors for switching functions.
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The joy of operating
Because the dynamic environment in which you operate demands that you exercise the most effective command
possible, the SCR Preselector is designed with the most
important functions immediately available for adjustment so control knobs and pushbuttons are reduced to minimum possible.
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Two main Multi-Function buttons for effortless operation
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A combined operation of rotating and pushing knobs lets shift and select all available functions.On its left hand, a second knob features Band and Function selection. Pushing it lives the Tuning Function and enters the Function Menu. The large flywheel-tuning knob with finger hole is the main tuning control. Two knobs/push-buttons together a dynamic LCD display is located on the front panel for total operation. |
Functions
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■ Tuning
The central frequency of preselector is tuned by means of the large flywheel knob labeled “Tune”.
The value, expressed in pF, is show dynamically on the LCD display.
Arrows on display indicates if the frequency shifts up or down. Pushing the tuning knob enters the Functions Menu.
Tuning of the SCR Preselector is simple, just turn the Tune knob and peak the background noise or signal strength.
■ Tuning Steps
Tuning can be incremented or decremented in steps of 1/2/5 and 10 units selecting “Step” in Functions menu.
■ Five Bands selection plus By-Pass
Pushing the smaller knob on the front panel labelled “Band” enters the Band menu. Rotating left or right and pushing again the wanted band is selected. This knob also executes “Select” when the Functions menu is explored.
■ Five Read/Write Memories Bank
Ten memory positions are available through the Functions menu, five positions to storage and five more to recall.
The working frequency and all present configurations are storage on the selected memory position. |
Superb Dynamic LCD Display
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A high brightness LCD display of three lines, sixteen characters contains the block diagram of functions its status and dynamic tune value.
Included in the display are Band Select, Tune value, Tune Step, Memory and Antenna selection. |
Two Antenna connectors plus In Line External Device insertion.
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Simplify Diagam of antenna port inputs
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The "ANT1" and "ANT2" BNC antenna sockets on the rear panel allow connection of two different 50Ω antennas, which can be selected through the function menu “ANT”.
A separate BNC socket “ANT1 OUT” allows inserting in the preselector line, an external device such as Converter, Amplifier or special Filter, between it and “ANT2” socket.
Gas discharge tubes to surge suppressor are connected to ANT1 and ANT2.
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Four-Level Input RF Attenuator
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To adjust the preselector gain, or for comfortable listening to extremely strong local signals,
the Attenuator circuit provides 6/12/18 dB of gain reduction plus "0" by-passed.
Those attenuation values match the standard calibration of S-Meters.
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Low Noise amplifier
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The wide variations in noise and signal levels on the HF bands demands a customized approach to the preselector gain.
The SCR Preselector provides and optimized preamplifier for HF bands.
If preamplification is not needed, Switching to “A -“ in the Functions menu will bypass
the RF preamp stage, routing the RF energy directly to the next stage improving the 3rd-order Intercept Point accordingly.
The SCR Preselector incorporates a Low Noise Amplifier Gali74 a MMIC device, with an average gain of 20 dB, a noise figure of 2.7 dB and high dynamic range.
The gain of the amplifier can be regulated by means of the input attenuator.
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USB bus for communication with PC
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At any time, the control of the SCR Preselector can be switch to PC, connecting the USB cable to the socket labelled “USB” on the rear panel, Plug and Play feature.
All available functions are replicated on the Graphic User Interface (GUI).
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The Software Program.
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The software program on the PC runs under Windows 2000, XP, Vista32 and Windows 7.
Windows 2K users need to update Framework from Windows Update site.
The user, through the GUI, commands the SCR Preselector in the same way as it does in stand-alone mode.
The program can work at the same time with any SDR software running on the PC, allowing to the user to control both simultaneously.
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The SCR Preselector Graphic User Interface on PC
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1-Configuration memories.
Click over any previously memorized frequency (in bright red) to recover its frequency
in the tuner. Remember to select first the band in which the frequency was stored.
2-Storing configuration into memory. .
To store the working frequency and all present configuration, the memory feature is activated by clicking on the MEM button. Memories 1 to 5 will flash. Next, click on any of the memories to store. If the memory had any previous preset stored, it is overriden.
There are up to five memories for each band (25 in total).
In order to clear a memory, doble-click over its number.
3-By-Pass and Band selection.
By default By-Pass. Click over any band to start working.
4-Attenuator.
When needed, click and add 6dB, 12dB or 18dB of attenuation. Click over 0dB for no attenuation.
5-Preamplifier ON/OFF
Alternatively set ON (light on) and OFF (light off) preamplifier.
6-Antenna1/Antenna2 selection.
Click over antenna1 or antenna2 as desired.
7-Tuning steps.
Set tuning steps x1 (fine), x2, x5 and x10 (biggest step).
8-Tuning.
Performs tuning by pressing left or right arrow on the keyboard or turning mouse wheel.
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External control
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The SCR Preselector incorporates three 6B595 eight bit serial power driver shift registers to interface relays on the main board with the control system.
Power drivers can be commanded from any external compatible logic control, due their simple codification.
The control signals needed, Data, Clock and Strobe are addressed to the 10 vias IDC connector J8 on the main board.
Data signals are clocked to the three ICs and each one is latched independently by means of its strobe line.
External signals are routing to IDC connector J8. 5VDC to power external circuitry are
available on pins 6-8-10
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Control Diagram
The SCR Preselector is available through
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