Improvements in receiver quality and tester speed have placed more stringent requirements on signal generator performance. As the frequency spectrum becomes more congested, the communications industry must develop new modulation technologies to increase component testing speed and performance and production capabilities. Therefore, it is now more important than ever before to require cost-effective, high-quality signal source solutions.
Similar to the evolution of cars to mobile phones, the performance of signal generators continues to increase and prices are decreasing. Customers and consumers continue to demand more features and performance and want lower prices.
RFIC design and cell phone production testing require signal sources to reduce phase noise and speed up frequency switching. This requirement is often contradictory. Therefore, in general, performance optimization can only be aimed at one of the requirements or the other, and it can rarely meet both requirements. The Aeroflex S-Series signal generator uses a characteristic frequency synthesizer design [Figure 1] to optimize both performances simultaneously, with a typical 1 GHz phase noise as low as -135 dBc/Hz while the frequency switching time is less than 100 μs. Frequency offset 20-kHz [Figure 2].
The advancement of modern device technology has enabled the miniaturization and simplification of the frequency synthesizer design. Compared with the previous generation products, its size and cost have greatly decreased. While the signal generator is smaller, lighter, and lighter, its function can also be continuously expanded as the product develops. This article focuses on the design principle of this frequency synthesizer and its effect on the output signal.
Multi-loop frequency synthesizerThe multi-loop synthesizer design has a wider frequency range (up to 6 GHz) and extremely high frequency resolution. This special design [Figure 2] uses two phase-locked loops (PLLs). A high-frequency, low-noise RF signal that provides step-adjustment in the desired range at coarse steps. The Direct Digital Synthesizer (DDS) provides precision frequency-resolved low-frequency signals that can be interpolated at coarse steps. Two signal inputs The second PLL generates the final output signal.
Low-noise high-frequency signal sourceAlthough the surface acoustic wave (SAW) oscillator has been a low-noise signal source in the 1GHz band, the use of a double-increased power VHF crystal is still the best method. The problem is that the high-frequency crystal oscillator can reduce noise, but it will increase the difficulty of the interpolation step. The low-frequency crystal oscillator can reduce the step size, but it will cause the noise to increase exponentially. In order to solve this kind of contradiction, adopt 135MHz crystal oscillator and fractional multiplier, signal frequency range is 967 MHz to 1350 MHz, step length is 22.5 MHz, compared with 22.5MHz crystal, the noise floor can improve 16dB when using 135MHz crystal.
Crystal designers face conflicting requirements. Good phase noise requires high power, reducing the tuning range. Due to the crystal oscillator aging of the tester's life cycle, the voltage-controlled crystal oscillator (VCXO) requires a small power and a wide tuning range to lock to a specific frequency. The solution is to use a high-power oscillator to reduce noise and digitally process crystal aging. The 135MHz crystal oscillator is a 10MHz standard reference frequency phase comparison oscillator that produces an accurate frequency output. The interpolated frequency synthesizer dynamically programs the frequency offset in real time.
Interpolation frequency synthesizerThe interpolating frequency synthesizer high frequency signal tunes the range 11.25 MHz to interpolate 22.5MHz. The output loop can add or subtract frequencies, so the required range is only half of the coarse step. Other basic features of this interpolated frequency synthesizer include:
The resolution range of the precision 0.01Hz synthesizer can be doubled to 6 GHz.
Offset VCXO rated frequency deviation
Apply broadband FM.
The 720MHz Voltage Control Surface Acoustic Wave Oscillator (VCSO) is a 10MHz reference frequency phase-locked oscillator used as a DDS clock. When the frequency synthesizer generates non-modulated CW, by adding one-level 720MHz signal, it further reduces the spurious signal that the DDS output is already low and filters out the low-frequency slight deviation. In this case, the frequency range is 22.5 MHz to 33.75 MHz. When a frequency synthesizer is required to generate a wide-band biased FM signal, the nominal interpolating frequency range is changed from 33.75 MHz to 45 MHz, and the interpolated signal can be floated within ±10 MHz to achieve a wide frequency deviation. The DDS output is used to directly change the frequency range from 23.75 MHz (33.75 MHz - 10 MHz) to 55 MHz (45 MHz + 10 MHz).
Output Addition LoopThe output loop adds two low-noise signals. The low-noise voltage-controlled oscillator (VCO) is rated for a frequency range from 500 MHz to 667 MHz and doubles to 1000 MHz to 1333 MHz. This signal is mixed with a low noise high frequency signal. Then, the interpolated signals are phase-compared using difference frequencies. After the phase detector output is low-pass filtered, the VCO is fed back to form a complete PLL.
An octave frequency synthesizer generally uses a VCO library to cover the output range because the low noise octave tuning range is not directly implemented. To achieve the desired tuning range, the 1/3 octave VCO can be amplified 34 or 5 times to cover an octave.
This multiplier uses less noise than previously designed. The 1000MHz to 1333MHz signal can be doubled again to reach 4 times the VCO frequency. This signal can be used to generate a 4x output directly or mixed with a background VCO signal to generate 3x and 5x VCO frequencies. Adjustable bandpass filter selects the upper and lower sidebands. This method of using multipliers and mixers ensures that the noise floor over the entire doubling frequency range is lower than with other methods.
The realization of fast frequency switching and its designImplementing frequency conversion within 100 μs while maintaining low noise further presents many challenges. There are several analog voltages in the frequency synthesizer to pre-tuned VCO and frequency-tunable varactor band-pass filters. It is flexible enough to complete the voltage tuning in a few microseconds and keep the noise below a few nanovolts. No drift after switching is a conflicting requirement. Our approach is to rigorously screen and select low-noise DAC bandwidth variable passive filters and filter capacitors with low dielectric absorption coefficients.
The PLL in the fractional multiplier and output add loop uses a mixer-based phase detector. Although the noise floor is very low, their disadvantage is limited capture range, which is a necessary condition to ensure the PLL loop bandwidth. Searching for oscillators and other traditional lock-in methods is too slow for this application. When the preset phase of the coarse adjustment adjusts the VCO frequency to the correct range, the VCO can be accurately adjusted using the digital phase detection technology (patented patent) before the phase lock. The FPGA compares the frequency of the two signals of the phase detector and adjusts the VCO to the correct frequency by modifying the preset voltage.
Since the PLL has a minimum bandwidth of 200 kHz, when the VCO is very close to locked, the set frequency can be locked very quickly and the frequency can be set to 0.1 ppm for 100 μs. The 1-GHz carrier frequency error range is only 100 Hz.
Frequency modulationThe frequency synthesizer uses standard two-point modulation to generate wideband bandwidth frequency offset modulation. The FM system can use advanced low-cost digital processing techniques to set the entire FM gain, matching the gain and delay of two internal calibration channels. The modulated signal can be applied to both the output VCO and the interpolation synthesizer. Due to simultaneous changes, the output PLL has no error at the phase detector. AC and DC input coupling, as well as phase modulation can also be handled digitally.
ConclusionThe frequency synthesizer uses the analog and digital combination technology to achieve the design goals, and the Aeroflex S-Series signal generator has excellent performance, which fully reflects the outstanding features of the Aeroflex product line that have continuously promoted the signal generator technology for decades.
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