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AD9874ABST Arkusz danych(PDF) 7 Page - Analog Devices |
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AD9874ABST Arkusz danych(HTML) 7 Page - Analog Devices |
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7 / 40 page  REV. A AD9874 –7– DEFINITION OF SPECIFICATIONS/TEST METHODS Single-Sideband Noise Figure (SSB NF) Noise figure (NF) is defined as the degradation in SNR perfor- mance (in dB) of an IF input signal after it passes through a component or system. It can be expressed with the equation Noise Figure SNR SNR IN OUT =× 10 log( ) The term SSB is applicable for heterodyne systems containing a mixer. It indicates that the desired signal spectrum resides on only one side of the LO frequency (i.e., single sideband); thus a “noiseless” mixer has a noise figure of 3 dB. The AD9874’s SSB noise figure is determined by the equation SSB NF P BW dBm Hz SNR IN =− × () {}−− 10 174 log where PIN is the input power of an unmodulated carrier, BW is the noise measurement bandwidth, –174 dBm/Hz is the thermal noise floor at 293 K, and SNR is the measured signal-to-noise ratio in dB of the AD9874. Note that PIN is set to –85 dBm to minimize any degradation in measured SNR due to phase noise from the RF and LO signal generators. The IF frequency, CLK frequency, and decimation factors are selected to minimize any spurious components falling within the measurement bandwidth. Note also that a bandwidth of 10 kHz is used for the data sheet specification. Refer to Figures 22a and 22b for an indication of how NF varies with BW. Also, refer to the TPCs to see how NF is affected by different operating conditions. All references to noise figures within this data sheet imply single-sideband noise figure. Input Third Order Intercept (IIP3) IIP3 is a figure of merit used to determine a component’s or system’s susceptibility to intermodulation distortion (IMD) from its third order nonlinearities. Two unmodulated carriers at a specified frequency relationship (f1 and f2) are injected into a nonlinear system exhibiting third order nonlinearities producing IMD components at 2f1 – f2 and 2f2 – f1. IIP3 graphically repre- sents the extrapolated intersection of the carrier’s input power with the third order IMD component when plotted in dB. The difference in power (D in dBc) between the two carriers and the resulting third order IMD components can be determined from the equation D IIP P IN =× 23 (– ) Dynamic Range (DR) Dynamic range is the measure of a small target input signal (PTARGET) in the presence of a large unwanted interferer signal (PINTER). Typically, the large signal will cause some unwanted characteristic of the component or system to degrade, thus making it unable to detect the smaller target signal correctly. In the case of the AD9874, it is often a degradation in noise figure at increased VGA attenuation settings that limits its dynamic range (refer to TPCs 15a, 15b, and 15c). The test method for the AD9874 is as follows. The small target signal (an unmodulated carrier) is input at the center of the IF frequency, and its power level (PTARGET) is adjusted to achieve an SNRTARGET of 6 dB. The power of the signal is then increased by 3 dB prior to injecting the interferer signal. The offset frequency of the interferer signal is selected so that aliases produced by the decimation filter’s response as well as phase noise from the LO (due to reciprocal mixing) do not fall back within the measurement bandwidth. For this reason, an offset of 110 kHz was selected. The interferer signal (also an unmodulated carrier) is then injected into the input and its power level is increased to the point (PINTER) where the target signal SNR is reduced to 6 dB. The dynamic range is determined with the equation: DR P P SNR INTER TARGET TARGET =+ – Note that the AD9874’s AGC is enabled for this test. IF Input Clip Point The IF input clip point is defined as 2 dB below the input power level (PIN), resulting in the clipping of the AD9874’s ADC. Unlike other linear components that typically exhibit a soft compression (characterized by its 1 dB compression point), an ADC exhibits a hard compression once its input signal exceeds its rated maximum input signal range. In the case of the AD9874, which contains a - ADC, hard compression should be avoided because it causes severe SNR degradation. |
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