Saturday, May 7, 2011
【Weak current College】】 frequency meter basics.
<br> Electronic counters is a basis for measuring instruments and to date, more than 30 years of history. Early, designers goal mainly extended measurement range, plus improved measurement accuracy, stability, and these are also people measuring electronic calculator technical level, the .level of electronic counter price decision relied. At present these basic techniques perfected, mature. Application of modern technology allows you to easily incorporate electronic counter measure frequency limit is extended to the microwave band. <br> With the development of science and technology, users .of the electronic counter also put new demands. For low-grade products requires the use of convenient operation, measuring range (sufficient) wide, high reliability, low prices. For high-end products, the demand for a high resolution, high precision, .high stability, high measurement rate; in addition to the usual generic counters have the capabilities, as well as data processing, statistical analysis function, time domain analysis function, etc., or other functions, such as voltage measurement. These requirements are part of .already implemented or realized, but to really perfect realization of these objectives, for manufacturers, there are a lot of work to do, instead of on the face of it appears to be the development of the header. <br> <br> In the .testing distribution, microwave devices or products, you often need to measure frequency, typically these are more complex signal, containing complex frequency, modulation or containing unknown frequency components, frequency fixed or change, pure or overlay with interference, etc. In order to correctly .measure different types of signals, you must understand the test signal characteristics and frequency measuring instrument performance. Microwave counters generally use type of frequency spectrum analyzer or mixing circuit, also contains multiple time base, synthesizer, intermediate frequency amplifier etc. Although all microwave counters are .used to complete tasks, but count the manufacturer has a complex set of counters of design, making different types of counters performance and price will vary, so you need according to its additional features or prices to carefully chosen. <br> <br> On .the sensitivity and accuracy requirements <br> <br> In order to measure the microwave frequency, frequency meter must measure frequency points have sufficient sensitivity, because some of the instruments of the real-world performance than manual gives better indicators, such as when measuring .critical signal may have more flexibility. For example, microwave counters brochure gives 20GHz, sensitivity to-25dBm, can be successfully used to measure the frequency of the signal on-30dBm. Of course, if the rated maximum counter frequency 18GHz, because the counter .circuit cannot work more than the 18GHz, you can't even use it to measure in the signal on 0dBm 20GHz. Therefore, if you want to do an accurate measurement, it is important to ensure that the measured signal frequency and amplitude in measuring instruments .within the scope of the index. <br> <br> Brochures on test performance indicators for measuring instrument "accuracy" and the "resolution". Accuracy indicators show instrument readings nearly the extent of the actual signal frequency and resolution indicators show how small changes .in the frequency may be displayed on the instrument. If you need to have a resolution of 15GHz 1Hz, the equipment must be at least 11 digits displayed. High resolution can quickly detect smaller drift values and unstable value, but when reading does not fully represent .the accuracy of the instrument. <br> <br> The accuracy of the measuring instrument of choice <br> <br> Instrument of frequency measurement accuracy depends on the time base. Most of the instruments use 10MHz reference oscillator has a 10-7 .or 10-8 frequency accuracy and stability. High resolution-high precision easier to achieve because of the increased display digits than manufacturing more stable oscillation reference source to more easily. <br> <br> In order to improve the instrument of measurement accuracy and .stability, you can purchase a small thermostat reference oscillator as the time base. Good thermostat temperature stable to zero degrees, so you can ensure that the external temperature changes when the oscillator frequency changes quite small. Of course, the instrument's inherent accuracy depends on the .precision of manufacturing as well as calibration laboratory time base accuracy oscillator calibration; depends on the thermal stability crystal oscillator, and aging. <br> <br> Through the use of cesium frequency standard or GPS signals as a reference frequency source into the entire system .of all instruments that can maximize frequency measurement accuracy, so that the measuring instruments would not require precise timing and can reach 10-12-10-14 of frequency measurement accuracy, that is, you can achieve better than instrument maximum resolution higher frequency measurement accuracy .. <br> <br> May affect the counter options and applications there are several other worthy of consideration features such as: sampling time, measurement speed and tracking speed, these characteristics might affect the measurement results of accurate and timely processing of results. < .br> <br> The use of microwave counters <br> <br> If you want to measure the noise in the signal, harmonic or parasitic components, try not to use microwave counters. In the select measurement instruments you need to know before you .all to be logging signals, a report of non-positive test signal is clean (no noise), stationary, single frequency components, should develop test scenarios before Spectrum Analyzer first observation to be measured signal interference signal and noise level, and then look at .the counter of performance can allow these disturbances and still be able to successfully complete the frequency of measurement. For example: currently the interference signal than the measured signal at least 6dB, counter measured is the interference signal, which led to the error of the measurement .results. In General, on the interference signal and noise can be used to inhibit the attachment of the counter. If the measured frequency is less than the percentage, you can consider counter input setup a filter to suppress unwanted signal (Figure 1). If .you need to measure several signals frequency values vary greatly, you can use the tunable band-pass filter or high pass, low pass filter in order to measure the frequency of each signal. This avoids has always occupied a Spectrum Analyzer, because of the price .Spectrum Analyzer may be those accessories price of 10 to 20 times. <br> <br>> <br> <br> If you know the approximate test signal frequency (A), you can use filter to reduce known interference signal (B) ., and in the counter range other than the signal (C) or low level signal (D) does not treat the measuring signal interference of frequency measurement. <br> In some special testing, you may need other accessories, such as an RF amplifier .to zoom in on low level signal, or through an external mixer to measure out counter measuring range of frequency, of course, some counter to directly measure the frequency above 100GHz. In vehicle collision avoidance radar and low-power communication relay stations requires the performance .of frequency counter to measure. Also some counters measure signal level, period, pulse width and pulse frequency, select the counters you can use the test scenario testing apparatus. <br> <br> Closing remarks <br> <br> As a result .of microelectronics and computer technology, microwave frequency meter keeps progress, sensitivity, frequency range, function continuously increase. Some counters measure pulse parameters, and provides similar to frequency Analyzer screen display; on these features have different functionality for different sizes of many instruments, we .should regard the test require proper selection, to achieve the most economical and best results. <br> <br>.