1. The concept of direct digital synthesis (DDS)
In 1971, the American scholar J. Tierncy, C. M. Rader and B. Gold proposed a new frequency synthesis principle that uses all-digital technology to directly synthesize the desired waveform from the concept of phase. Limited to the technology and device level at the time, its performance indicators are still not comparable to the existing technology, so it has not been paid attention to. In the past 20 years, with the improvement of technology and device level, a new frequency synthesis technology-direct digital frequency synthesis (DDS) has been developed rapidly, which is different from other frequency synthesis methods with superior performance and characteristics. The best in modern frequency synthesis technology. DDS is the abbreviation of Direct Digital Synthesizer. Compared with traditional frequency synthesizers, DDS has the advantages of low cost, low power consumption, high resolution, and fast conversion time. It is widely used in the fields of telecommunications and electronic instruments and is a key technology to realize the full digitization of equipment. DDS is an important design method today. The development of high-speed integrated circuits has further improved the performance of DDS. Combining it with traditional technology, the composition of various hybrid solutions will improve the performance of frequency sources to a new level. DDS is not only used in fields that traditionally need to use signal sources, but also will open up many new application fields.
2. Basic Principles
DDS technology is a synthesis technology that converts a series of digital signals into analog signals through DAC. One of the most widely used DDS methods is to use a high-speed memory as a look-up table, and then use a high-speed DAC to generate a sine wave that has been stored in digital form.
2.1" Phase accumulator part
The phase accumulator is composed of an N-bit adder and an N-bit accumulating register in cascade. Every time a clock pulse comes, the adder adds the frequency control data and the accumulated phase data output by the accumulation register, and sends the result of the addition to the data input end of the accumulation register. The accumulating register feeds back the new phase data generated by the adder after the previous clock is applied to the input of the adder, so that the adder continues to add the frequency control data under the action of the next clock. In this way, the phase accumulator performs linear phase accumulation under the action of the reference clock. When the phase accumulator accumulates the full amount, an overflow will occur to complete a periodic action. This cycle is a frequency cycle of the DDS synthesis signal. The overflow frequency of the amplifier is the signal frequency output by the DDS.
2.2 "Phase-amplitude conversion part
Use the data output by the phase accumulator as the sampling address, and perform phase-amplitude conversion on the sine wave waveform memory, and the output waveform amplitude can be determined at a given time.
2.3 Digital-to-analog conversion part
The DAC converts the waveform amplitude in digital form into an analog form signal of the required synthesized frequency. The low-pass filter is used to attenuate and filter out unwanted sampling components in order to output a pure sine wave signal.
For a phase accumulator with a counting capacity of 2N and a sine wave waveform memory with M phase samples, if the frequency control word is K, the frequency of the output signal of the DDS system is fo=fc×K/2N, and the frequency resolution is Δf =fomin=fc/2N.
3. DDS performance characteristics
DDS far exceeds the level that can be achieved by traditional frequency synthesis technology in terms of relative bandwidth, frequency conversion time, phase continuity, quadrature output, high resolution, and integration, and provides a system superior to analog The performance of the signal source.
3.1 "Extremely fast frequency switching speed
DDS is an open-loop system without any feedback link, and the frequency conversion time is mainly determined by the delay added by the LPF. If fc=10MHz, the conversion time is 100ns. If the clock frequency increases, the conversion time will be shortened, but it cannot be less than the delay time of the digital gate circuit. At present, the tuning time of DDS is generally on the order of ns, which is several orders of magnitude shorter than using other frequency synthesis methods.
3.2 Very high frequency resolution
From Δf=fomin=fc/2N, it can be seen that as long as the number of bits N of the phase accumulator is increased, an arbitrarily small frequency tuning step can be obtained. The resolution of most DDS is on the order of Hz, mHz or even μHz.
3.3 "Low phase noise and low drift
The frequency stability of the synthesized signal in the DDS system is directly determined by the frequency stability of the reference source, and the phase noise of the synthesized signal is the same as the phase noise of the reference source. In most DDS system applications, the reference frequency is generally generated by a fixed crystal oscillator, so its phase noise and drift characteristics are extremely excellent.
3.4 "Continuous phase change
Also because DDS is an open-loop system, when a frequency conversion command is added to the data input terminal of DDS, it will quickly synthesize the required frequency signal without any current pulse superimposed on the output signal, and the output change is a smooth This feature is unique to DDS during the transition process, and the phase is continuously changing.
3.5 Output a flat signal in a very wide frequency band
The minimum output frequency of DDS is the minimum resolution of the clock frequency used or the resolution of the phase accumulator. The Nyquist sampling theorem guarantees that the DAC can reproduce the signal at all frequencies up to half of the clock frequency, that is, the upper limit of the DDS frequency fomax is determined by the maximum clock frequency fc of the synthesizer (fomax = fc/2).
3.6 "Easy to integrate and adjust
Almost all components in DDS are digital signal processing devices, except for DACs and filters, no adjustments are required, which reduces costs and simplifies production equipment.
4. Application of DDS
When DDS came out, the speed limitation of DDS components and the noise caused by digitization, these two main shortcomings hindered the development and practical application of DDS. In recent years, with the development of ultra-high-speed digital circuits and in-depth research on DDS, the highest operating frequency and noise performance of DDS have approached and reached the equivalent level of phase-locked frequency synthesizers. With the development of this frequency synthesis technology, it has been widely used in fields such as communications, navigation, radar, remote control telemetry, electronic countermeasures, and modern instrumentation industry.
4.1 "High-precision signal of real-time simulation
Store a large amount of non-sinusoidal waveform data such as sine wave, square wave, triangle wave, sawtooth wave, etc. in the waveform memory of DDS, and then control these data by manual control or computer programming, and you can change the waveform of the output signal at will. Using DDS's characteristics of fast frequency conversion, continuous phase conversion, and precise fine-tuning step, combining it with a simple circuit constitutes the best way and means to accurately simulate various signals. This is incomparable to other frequency synthesis methods. For example, it can simulate a variety of waveforms such as nerve pulses and reproduce the waveforms captured by a digital storage oscilloscope (DSO).
4.2 "Realize various complex ways of signal modulation
DDS is also an ideal modulator, because the three parameters of the synthesized signal: frequency, phase and amplitude can be precisely controlled by the digital signal, so DDS can accurately control the phase of the synthesized signal by presetting the initial value of the phase accumulator. So as to achieve the purpose of modulation.
In modern communication technology, there are more and more modulation methods, BPSK, QPSK, MSK all need precise phase control of the carrier. The phase accuracy of the synthesized signal of DDS is determined by the number of bits of the phase accumulator. A 32-bit phase accumulator can generate 4.3 billion discrete phase levels, and the phase accuracy can be controlled within the range of 8×10-3 degrees. Therefore, when switching the frequency, only pass the initial phase accumulator preset Value, you can precisely control the phase of the synthesized signal, and it is easy to implement various digital modulation methods.
4.3” Realize fine frequency adjustment, as an ideal frequency source
DDS can effectively achieve fine frequency adjustment, and it can replace multiple loops in many phase-locked loop (PLL) designs. Maintaining an appropriate frequency division ratio relationship in a PLL can organically combine the high frequency resolution and fast conversion time characteristics of DDS with the characteristics of high output frequency, low spurious noise and low clutter of the phase-locked loop, so as to achieve More ideal DDS+PLL hybrid frequency synthesis technology.
PLL is used to cover the required operating frequency band during coarse frequency adjustment, and the appropriate frequency division ratio can be selected to obtain higher phase noise, while DDS is used to cover those coarse adjustment increments and realize fine frequency adjustment within it. This kind of scheme satisfies the stringent technical requirements of various systems for frequency sources with its superior phase stability and extremely low microphonic effect. This is also the most widely used method for developing and applying DDS technology. The frequency synthesizer composed of this scheme has been realized at a very high frequency.
Of course, the application of DDS is not limited to these, it can also be used in nuclear magnetic resonance spectroscopy and its imaging, detection instruments, etc. With the rapid development of DDS integrated circuit device speed, it has become an important and flexible design method that can be used to meet system frequency requirements.
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How far(long) the transmitter cover?
The transmission range depends on many factors. The true distance is based on the antenna installing height , antenna gain, using environment like building and other obstructions , sensitivity of the receiver, antenna of the receiver . Installing antenna more high and using in the countryside , the distance will much more far.
EXAMPLE 5W FM Transmitter use in the city and hometown:
I have a USA customer use 5W fm transmitter with GP antenna in his hometown ,and he test it with a car, it cover 10km(6.21mile).
I test the 5W fm transmitter with GP antenna in my hometown ,it cover about 2km(1.24mile).
I test the 5W fm transmitter with GP antenna in Guangzhou city ,it cover about only 300meter(984ft).
Below are the approximate range of different power FM Transmitters. ( The range is diameter )
0.1W ~ 5W FM Transmitter ：100M ~1KM
5W ~15W FM Ttransmitter : 1KM ~ 3KM
15W ~ 80W FM Transmitter : 3KM ~10KM
80W ~500W FM Transmitter : 10KM ~30KM
500W ~1000W FM Transmitter : 30KM ~ 50KM
1KW ~ 2KW FM Transmitter : 50KM ~100KM
2KW ~5KW FM Transmitter : 100KM ~150KM
5KW ~10KW FM Transmitter : 150KM ~200KM
How to contact us for the transmitter?
Call me +8618078869184 OR
Email me [email protected]
1.How far you want to cover in diameter ?
2.How tall of you tower ?
3.Where are you from ?
And we will give you more professional advice.
About Us
FMUSER.ORG is a system integration company focusing on RF wireless transmission / studio video audio equipment / streaming and data processing .We are providing everything from advice and consultancy through rack integration to installation, commissioning and training.
We offer FM Transmitter, Analog TV Transmitter, Digital TV transmitter, VHF UHF Transmitter, Antennas, Coaxial Cable Connectors, STL, On Air Processing, Broadcast Products for the Studio, RF Signal Monitoring, RDS Encoders, Audio Processors and Remote Site Control Units, IPTV Products, Video / Audio Encoder / Decoder, designed to meet the needs of both large international broadcast networks and small private stations alike.
Our solution has FM Radio Station / Analog TV Station / Digital TV Station / Audio Video Studio Equipment / Studio Transmitter Link / Transmitter Telemetry System / Hotel TV System / IPTV Live Broadcasting / Streaming Live Broadcast / Video Conference / CATV Broadcasting system.
We are using advanced technology products for all the systems, because we know the high reliability and high performance are so important for the system and solution . At the same time we also have to make sure our products system with a very reasonable price.
We have customers of public and commercial broadcasters, telecom operators and regulation authorities , and we also offer solution and products to many hundreds of smaller, local and community broadcasters .
FMUSER.ORG has been exporting more than 15 years and have clients all over the world. With 13 years experience in this field ,we have a professional team to solve customer's all kinds of problems. We dedicated in supplying the extremely reasonable pricing of professional products & services. Contact email : [email protected]
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For Warranty .
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