20 MHz-3 GHz Programmable Chirp Spread Spectrum Generator for a Wideband Radio Jamming Application

Abstract

In this paper, an agile programmable chirp spread spectrum generator for wideband frequency-jamming applications from 20 MHz to 3 GHz is proposed. A frequency-mixing architecture using two voltage-controlled oscillators is used to achieve a wideband operating frequency range, and the direct digital synthesizer (DDS)-based chirping method with a two-point modulation technique is employed to provide a programmable and consistent chirp bandwidth. The proposed signal generator provides the various programmable FM signals from 20 MHz to 3 GHz with a modulation bandwidth from 0 to 400 MHz. The prototype successfully demonstrates arbitrary sequential jamming operation with a fast band-to-band hopping time of < 10 ${\mu}sec$.

1. Introduction

Radio frequency jamming is used to disrupt wireless communications by transmitting high-power interfering radio signals and decreasing the signal-to-noise ratio (SNR) of the nearby radio receivers [1]. Recently, jamming applications have expanded from military purposes into public uses and security applications. Because commercial wireless communications widely use frequencies ranging from 20 MHz to 3 GHz, the jamming frequency must cover this frequency range.

Fig. 1(a) shows a conventional frequency jamming signal generator. A white random noise source generates a wideband random noise signal, and a wideband power amplifier (PA) amplifies the noise signal, delivering the noise power to the antenna at the target frequency. To cover all the jamming bandwidth, a wideband PA is required [2]. Because the output power spreads over the entire frequency band, it is difficult to concentrate jamming power on the target frequency. Consequently, the power efficiency (Pout/PDC) becomes very low. The power efficiency can be improved by dividing the frequency band as shown in Fig. 1(b), which can concentrate the jamming power on each separate frequency band, where the chirp spread spectrum is generated using a voltage-controlled oscillator (VCO) and a linear frequency modulator (LFM). The chirp spread spectrum was originally designed for ultra-wideband communications for precision ranging or low-rate wireless networks [3, 4]. However, it can be applied to jamming applications. The narrowband jamming signal source is composed of a narrowband VCO, a narrowband PA, a switch (SW), and an LFM. The chirp spread spectrum is obtained by modulating each narrowband VCO [5]. The output frequency band is selected by the switch. This type of jamming signal generator can focus the required jamming power on the target frequency using a PA with relatively low P1dB; consequently, the power efficiency can be improved. The disadvantage is that this jamming signal generator is very complex to implement because of a large number of components, such as the VCOs, PAs, and SWs.

Fig. 1.Frequency jamming architectures using: (a) a white noise source and a wideband PA; (b) the narrowband VCOs and PAs, and (c) a wideband VCO and PA.

In this paper, a new wideband frequency-jamming signal generator using a wideband VCO, a wideband PA, and an LFM is proposed as shown in Fig. 1(c), where the LFM signal determines the operating frequency and modulation bandwidth, simultaneously. Because the proposed jamming generator can focus its power on the target band, it is also power-efficient. Additionally, a compact design is possible because of the small number of components.

 

2. Wideband Frequency Modulator Design

Because the required operating frequency range is nearly 200% of the center frequency, the design challenge of this work is to achieve a programmable chirp spread spectrum in this wide operating frequency range. Fig. 2 shows the proposed wideband frequency-mixing architecture. The wideband frequency range is realized using two identical high-frequency VCOs (VCOCF and VCOFM) and one frequency mixer. Fig. 3 shows the frequency characteristics of the employed VCO. The center frequency VCO (VCOCF) operates from 4220 to 7220 MHz and determines the output frequency of the jamming signal generator; the frequency modulation VCO (VCOFM) operates at the fixed base frequency (fbase) of 4200 MHz, and its frequency is linearly modulated by the LFM to form a chip spread spectrum. VCOCF and VCOFM are multiplied in a passive-type frequency mixer and filtered in a low-pass filter to reject the high-sideband signal. Only a low-sideband signal less than 3 GHz is selected for the output.

Fig. 2.The frequency-mixing architecture for the wideband chirp spread spectrum generator and the output spectrum.

Fig. 3.Frequency-jamming architectures with white noise

To generate a programmable chirp spread spectrum, a two-point direct VCO modulation technique is used as shown in Fig. 4(a) [6, 7]. The proposed signal generator should reconfigure the center frequency, the bandwidth (ΔfBW) of the chirp spread spectrum, the rate of frequency modulation (fRATE), and the chirp period (TPER). The center frequency of the jamming signal is controlled by a phase-locked loop (PLL) using a phase-frequency detector (PFD) and a charge-pump (CP). Because the loop bandwidth is set to be less than a tenth of fRATE, the center frequency and the frequency modulation can be performed independently.

Fig. 4.(a) Two-point direct VCO modulation technique and (b) chirp control voltage of the VCO

The chirp voltage is realized using a direct digital synthesizer (DDS) and a digital potentiometer (DP), as shown in Fig. 5. The DDS uses a reference frequency (fCLK) of 100 MHz. The accumulated phase is converted to a triangular waveform to generate a linear chirp voltage. The digital potential meter controls the magnitude of the output chirp voltage with 8-bit precision. The output frequency (fOUT) of the chirp generator depends on fCLK, the length of the phase accumulator bits (n), and the binary tuning word (M). It is provided as follows:

Fig. 5.DDS-based linear frequency modulator.

ΔfBW is determined by the chirp voltage VFM and the VCO gain KVCO as ΔfBW = KVCO ⋅ VFM. The frequency chirp period TFM determines the frequency sweeping rate, and TPER sets the jamming time on the target frequency. Because the nonlinear gain characteristics of the VCO cause harmonic power generation, the magnitude of the chirp voltage is limited to a value with a harmonic rejection rate of 40 dBc.

Fig. 6 shows the schematic diagram of the proposed wideband chirp spread spectrum generator, which includes a fast PLL for VCOCF, a slow PLL for VCOFM, a linear frequency modulator, and a mixer with a programmable gain amplifier (PGA). A fast locking time is required for VCOCF because it determines the minimum hopping time of THOP. Because the VCOs are controlled by the PLLs, the hopping time is determined by the settling time of the PLL. The settling time is determined by the loop bandwidth of the PLLs, which is a function of the charge pump current ICP, the loop-filter capacitance of C1, and the frequency-dividing ratio of N [8]. The approximate settling time is

where KVCO is the gain of the VCO. In our design, the settling time of the fast PLL is set to <10 μsec. The VCOFM in the slow locking loop is controlled by both the PLL and LFM, simultaneously. To allow them to coexist without interfering with each other, a two-point modulation technique is employed, where the loop-filter bandwidth of the PLL is set to <1 kHz, while the frequency of the LFM is limited to >10 kHz [9]. After two voltages are added, its voltage levels are shifted from 0-5 V to 0-12 V because the required control voltages for VCOFM are in the range of 0-12 V. Fig. 7 shows a control diagram of three consecutive chirp frequencies and an illustration of the chirp spectrum.

Fig. 6.Schematic diagram of the proposed wideband jamming signal generator.

Fig. 7.Illustration of three consecutive chirp frequencies.

The SPST switch breaks the signal path during the band - to - band transition to remove the unwanted frequency occurrence, and the wideband programmable gain amplifier (PGA) provides a gain control of 32 dB.

 

3. Experiment Results

Fig. 8 shows a photograph of the chirp spread spectrum signal generator, which is integrated on a 4-layer FR-4 PCB with an area of 95×145 mm2. The prototype chirp spread spectrum signal generator is encapsulated in an aluminum case to remove mutual coupling between the components.

Fig. 8.Photographs of the signal generator

Fig. 9.Chirp bandwidth according to the control digit

The proposed chirp spread spectrum generator shows full operation over the frequencies of 20 MHz to 3 GHz. Fig. 8 shows the measured chirp bandwidth according to the control digit. The maximum chirp bandwidth is approximately 400 MHz in one frequency slot time, and the minimum chirp resolution is approximately 1.5 MHz. The various chirp spectrums for various jamming sequences are controlled according to the operating frequency (fC), modulation bandwidth (ΔfBW), modulation period (TPER), modulation rate (fRATE), and output power (POUT).

Fig. 10(a) shows the output spectra with the chirp bandwidth for 0, 10, and 400 MHz, respectively. Fig. 11 shows the chirp signals of the 10-MHz and 400-MHz chirp bandwidths, respectively. Fig. 12 shows the chirp spread spectrum of two programed jamming sequences. The first graph in Fig. 12(a) is obtained using 10 jamming sequences with 10 different frequency steps and constant output power, showing consistent modulation bandwidths and greater than 44 dB of harmonic suppression. The output power errors over the whole frequency range <3 dB. The second graph shows a chirp spread spectrum with five different frequency steps and the power variations.

Fig. 10.The output spectrums with fBW of (a) 0 MHz, (b) 10 MHz, and (c) 400 MHz

The frequency hopping time can be estimated by measuring the VCOCF control voltage. Fig. 13 shows the measured VCO control voltage. After the digital control data transfer over 70 μsec, the carrier frequency is changed within 10 μsec. If the frequency jamming sequence can be stored in memory, the control data transfer from the external computer is not required; therefore, the hopping time becomes <10 μsec. Table 1 summarizes the important features of the proposed programmable chirp spread spectrum generator

 

4. Conclusion

In this paper, a programmable chirp spread spectrum generator is presented. By adapting the wideband frequency mixing architecture as well as fast and slow PLLs, a wideband operating frequency range from 20 MHz to 3 GHz and an agile frequency hopping time were achieved. Additionally, a direct two-point modulation technique was used with a DDS-based frequency chirp controller for the programmable modulation bandwidth from 0 to 400 MHz with resolution frequency of 1.5 MHz. The prototype successfully demonstrates a sequential chirp spread spectrum with a band-to-band hopping time of <10 ×sec, which can be applied to a frequency jamming signal generator.