The frequency of a driver source is multiplied in a nonlinear system to produce higher-order harmonic frequencies in a terahertz frequency multiplier. Frequency multipliers usually use planar Scotty varactor diodes, which take advantage of Gas substrate less technology to reduce substrate loss. BWOs or solid-state sources such as Gunn and IMPATT oscillators with relatively high output power in the range of 50 GHz to 150 GHz can be used as drive sources. Microwave frequency synthesizers can produce high output power above 100 GHz when combined with high-gain power amplifiers fabricated using monolithic microwave integrated circuit (MMIC) technology. 14 Series chains of frequency doubles and frequency triples produce the most effective terahertz frequency multipliers. 15 Frequency multipliers can produce signals up to 2 THz. 15,16,17 It is possible to produce terahertz signals with frequencies greater than 2.5 THz using a hybrid device consisting of a BWO and a chain of frequency multipliers. 18
An active frequency multiplier chain (Millitech AMC-10-R0000 with multiplication factor = 6) and a microwave frequency synthesiser (Agilent E8257D with frequency = 12.5-18.33 GHz) are used to create the first test source (output power = 4 dBm = 2.5 mW, frequency range = 75–110 GHz, and line width 0.6 Hz). The output frequency of this AFMC source is reliable and stable since the frequency synthesizer is synchronized to the Rb atomic clock. When the output frequency of the AFMC source was set to 100 GHz, we first measured the spectral line width of the fb beat signal. Figure 15.5 displays the RF spectrum analyzer’s linear-scale spectrum of this signal (resolution bandwidth (RBW) = 1 Hz and sweep time = 690 ms). The beat signal’s line width was 1.8 Hz as a result. This means that each mode of the PC-THz comb has a narrow enough line width to conduct high-precision frequency measurements. The fb beat signal, on the other hand, had a signal-to-noise ratio (SNR) of 40 dB. The detection limit of the THz power is calculated to be 36 dBm, or 250 nW, based on this SNR and the output power of the test source (+ 4 dBm = 2.5 mW). A detailed comparison of SNR between PCA-derived fb beat signals and electro-optic sampling can be found elsewhere.
We propose a scalable and cascadable “active frequency multiplier” architecture to generate THz from a mm-Wave tone based on the multiphase IL technique. This is where you’ll find the machine architecture.
To create a “active frequency multiplier,” the multiphase IL is combined with the subharmonic ILO (Figure 19.7). As the system’s first input, a mm-Wave or radio frequency (RF) signal source, such as a phase lock loop (PLL) or a free-running voltage-controlled oscillator (VCO), will produce the fundamental tone at f0. The N1th harmonics of the inputs are then extracted and injected into an oscillator running at N1f0. The ILO ensures that the input tone at f0 is frequency/phase synchronized. More significantly, it compensates for the conversion loss in harmonic generation by restoring the signal amplitude at N1f0 after frequency multiplication. The N2nd harmonic signals from the first ILO are then extracted and injected into the next ILO, which oscillates at N1N2f0 frequency. An “active frequency multiplier” chain is formed by cascading these subharmonic ILOs. Finally, the desired THz output signal can be extracted from the Mth harmonic of the last-stage ILO. It’s worth noting that if the multiphase IL technique is used for bandwidth extension and signal balancing, the device can be built quickly if all of the ILOs are multiphase oscillators.
Furthermore, the proposed “active frequency multiplier” chain structure can be realized in a multiring architecture if the subharmonic index and the multiphase ILO stage number are co-prime. Figure 19.8 depicts a special case of the Nth subharmonic IL and (N+1)-phase ILO. Assume that the outermost ring is a (N+1)-stage ILO with a counterclockwise phase progression at the input frequency of f0 and a phase difference of 2/(N+1) between each adjacent stage. Thus, at each point, the phases are 0, 2/(N+1), 4/(N+1), and so on. If the (N+1) multiphase signals’ Nth harmonics are used to injection-lock the second outermost ring ILO, the phases of these Nth harmonic signals are 0, 2, and 4, respectively. The second outermost ring should oscillate with its phase progression in the clockwise direction, that is, in the reversed direction compared to the outermost ring, using these harmonic signals for locking. If the Nth harmonics of the (N+1) signals from the second outermost ring ILO are used to lock the third ring ILO, the third ring ILO can oscillate in a counter-clockwise mode. While all the rings are mutually locked, the signal frequency is progressively multiplied up from the outer rings to the inner rings. Finally, since the innermost ring ILO oscillates at the highest frequency, its harmonic signals can be extracted and used to generate the desired THz output signal. This proposed ring structure architecture scheme achieves the desired phase progression while greatly simplifying high-frequency signal routing, which is extremely important in THz designs.
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