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A Software Controlled Radio Preselector a Tunable Bandpass Filter Bank for the HF Bands (1.8 Mhz to 30 Mhz)

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A Software Controlled Radio Preselector a Tunable Bandpass Filter Bank for the HF Bands (1.8 Mhz to 30 Mhz) Juan José de Oñate, MØWWA and Xavier R. Junqué de Fortuny P O Box 249, Hampton, Middlesex, TW12 2UY United Kingdom; [email protected] A Software Controlled Radio Preselector A Tunable Bandpass Filter Bank for the HF Bands (1.8 MHz to 30 MHz). The SCR-Preselector prototype is packaged in a Hammond Instruments case. Features: relays for switching functions, instead of software engineer, we started the job. First of • PC controlled through a graphical user nonlinear semiconductors that cause distor- all, because Xavier lives in Barcelona, Spain, interface (GUI). tion and exhibit low tolerance against strong and I live in London, England, we estab- • USB v2.0 compliant, for data commu- signals and static. lished a broadband link through the Internet, nication with PC. Plug and Play feature. Direct sampling SDRs are protected with audio/video capabilities. One year later, • Tested under Windows XP and Windows against false images since the SCR- the SCR-Preselector was born. Vista32 Operating Systems (OS). Preselector incorporates a low pass filter • Can perform its functions as a stand- (LPF) with an enhanced stop band of more Overview alone unit. than 60 dB. This filter has the ability to The core of the SCR-Preselector is the • Compatible with software defined provide a very sharp transition from pass tunable band pass filter bank, which is com- radios (SDR). band to stop band at a cut-off frequency of plemented with the necessary switching, pro- • Removes the strong signals radiated 30 MHz, attenuating signals in the VHF tection, filters, amplifier and control circuitry back to the antenna from quadrature mixers. region that could mix with the sampling as shown the block diagram in Figure 1 and • Improves your average receiver or scan- frequency of its analog to digital data con- 1 the assembled prototype of Figure 2. The ner to lead with strong signals, on the most verter. incoming signals pass through the antenna noisy and crowded bands. The SCR-Preselector incorporates gas selector relay, and then go to the bypass relay. • Connected to an exciter as a tunable discharge tubes to provide safety against From there, the signals either bypass the pre- band pass filter, it reduces harmonics gener- static and voltage transients. Further, the selector or enter a step attenuator that offers ated in the exciter. unused antenna input is short-circuited to a selection of 0 dB, 6 dB, 12 dB or 18 dB. • Complete receiver front-end for your ground. Those attenuation values match the standard projects. calibration of S meters. • Joined to a VFO, the preselector can Brief History Next, the input signals enter a broadcast perform a tracking tuning function. In September 2006 I attended a confer- band (BCB) high pass filter. This is a so- ence at the Whitton Amateur Radio Group, called “brick-wall filter,” which attenuates The SCR-Preselector — A First Line at which Jeffrey Pawlan, WA6KBL, had spurious signals from strong AM broadcast of Defense Against Interference and the kindness to give us during his European stations as much as 120 dB. Such signals are Static tour.2 The talk was all about Software well known enemies of shortwave receivers. The SCR-Preselector, pictured in the lead Defined Radio, and the WinRAD computer I have stretched the effects of this filter into photo, is connected in series between the program, how it came about, how it works the medium wavelength band. It has a very antenna and the receiver. Tuning to desired and where will be going in the future. sharp slope, as Figure 3 shows. The filter signal keeps the preselector bandwidth Motivated by his speech I became truly has minimum attenuation of all signals at centered at the operating frequency, adding interested in SDR. I began playing around 1.8 MHz and higher frequencies, including selectivity and protection to your receiver. with quadrature sampling detectors, also the CW portion of the 160 m band. The SCR-Preselector rejects or reduces called Tayloe detectors. This is a nice piece At this point, the input signals reach out of band unwanted interfering signals, of radio technology. Soon I noticed that the tunable band pass filter bank, which is improves signal to noise ratio and protects despite its excellent features, it still needs a arranged in five overlapping bands that are against interference from strong signals like good front-end circuit to condition signals user selectable. Each filter is a classic serial AM/FM broadcast stations. It can also be before they reach the mixer. I focused my tuned LC circuit, and offers narrow band- helpful in preventing interference during interest on this problem, and with the help of width with little attenuation. A set of eight multi-multi contest operations. my old friend Xavier Junqué de Fortuny, a switchable capacitors of 1 pF, 2 pF, 4 pF, The high linearity of the preselector is 8 pF, 16 pF, 32 pF, 64 pF and 128 pF emu- preserved thanks to a passive design that uses lates the variable capacitor needed to tune the large toroidal cores and reliable mechanical 1Notes appear on page 18. series resonant circuit. The control program Reprinted with permission © ARRL QEX – May/June 2008 11 Figure 1 — SCR-Preselector block diagram. 12 QEX – May/June 2008 Reprinted with permission © ARRL Figure 2 — This photo shows the main circuit board for the preselector. Figure 3 — The SCR-Preselector was tested with an N2PK Vector Network Analyzer. Point A: –120.2 dB at 1.07 MHz; Point B: –81.12 dB at 1.4 MHz; Point C: -3.5 dB at 1.8M Hz; Point D: –39.5 dB at 3.6 MHz; Point E: –48.1 dB at 7.05 MHz. Figure 5 — Here is a screen shot of the SCR- Preselector Graphic User Interface display on my computer. Figure 4 — A photo of the front panel circuit board module, assembled on the front panel of the Hammond Instrument case. performs this task by adding or subtracting width to 30 MHz. done when the USB cable is plugged in or capacitor values in binary fashion, in direct To make the SCR-Preselector compatible unplugged. This adds a “Plug and Play” relationship with the tuning knob operated by with computerized receivers we needed it to feature to the project. The program can work the user. Increments or decrements are done operate not only as a stand-alone unit, but at the same time with any SDR software in steps of 1, 2, 5 or 10 units. One of five tor- also to perform its functions under computer running on the PC, allowing to the user to oidal inductors is selected by the front-panel control. For all that, a PIC microcontroller control both simultaneously. switch to form the filters. The selectable 18F4455 was selected, which includes an bands are: integrated USB peripheral, 35 I/O ports and Circuit Description Main board • A) 1.8 MHz to 4 MHz in circuit programming (ICSP), among other The SCR-Preselector main board sche- • B) 3 MHz to 6 MHz features. It is assembled on a front panel matic is shown in Figure 6. K19 is the • C) 4 MHz to 10 MHz circuit board module apart from the main antenna selector relay. The SCR-Preselector • D) 9 MHz to 18 MHz circuit, along with a liquid crystal display is designed to operate with antennas (and • E) 16 MHz to 30 MHz. (LCD) and two incremental rotary encoders feed lines) that have a 50 impedance. An Filtered signals are amplified or bypassed for band switching, tuning and other func- Ω optional external device, such as a converter, in the following stage, a low noise amplifier tions. See Figure 4. Also, it offers the pos- amplifier or filter can be inserted between J3 (LNA). I chose a Gali74 MMIC device, with sibility to command the SCR-Preselector by and J2, with antenna 2 selected. an average gain of 25 dB, a noise figure (NF) other external means. Relays K20 and K21 drive signals to J6, of 2.7 dB and high dynamic range.3 Because The software program on the PC runs the preselector output in bypass mode, or to it is a high gain, wideband amplifier that oper- under Windows XP or Windows Vista32. The the attenuator section. Switching to either ates to more than 1 GHz, we need to “hold user, through the GUI, commands the SCR- K22, K23, or both configures the desired its horses.” The gain of this amplifier can be Preselector in the same way as it does in attenuation, or bypasses signals. Next, the regulated through the input attenuator. An stand-alone mode. See Figure 5. Switching circuit built around C9 to C15 and L6, L7, LPF connected at the output, limits the band- between USB or standalone modes is Reprinted with permission © ARRL QEX – May/June 2008 13 14 QEX – May/June 2008 Reprinted with permission © ARRL Figure 6 — The schematic diagram of the SCR-Preselector main circuit board. L8 shapes a sharp BCB HPF of ten ele- ments. T80-2 powdered iron toroidal cores are used to achieve large Q values. Other characteristics are the low pass band inser- tion loss — less than 1 db — and the flat pass band response. The characteristic impedance of the circuit, 50 Ω, is converted to a typical low level for the series band pass filter by means of T1, a wide bandwidth UNUN trans- former arranged to improve the filter high- frequency response.5 A set of eight capacitors, C1 to C8, and relays K11 to K18, form the “switching capacitor” section or “variable capacitor emulator” that is connected to relays K1 to K5.
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