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SSM2000 Datasheet(PDF) 10 Page - Analog Devices |
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SSM2000 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page SSM2000 REV. 0 –10– Noise is most objectionable at high frequencies (3 kHz–8 kHz). Therefore, only the VCF detector output signal is used to deter- mine the adaptive noise threshold. Figures 25a–c, are a series of circuits which illustrate how the noise threshold is derived. It is important to remember that the signal that is applied to the noise threshold detector circuitry has already been rectified and averaged. Hence, the lowest potential over a set period of time corresponds to the noise floor. Node A corresponds to the out- put of the VCF Detector, and Node B is proportional to the adaptive noise threshold. Figure 25a illustrates the condition where the potential at Node A is above the maximum possible potential for Node B. The maximum noise threshold is set by the potential placed on Pin 14. If the potential at Node B rises to a diode drop above Pin 14, then Q1’s emitter-base diode turns on and clamps Node B. This is represented by the current flow I2. However, if Node B has not yet risen to the maximum noise threshold level, then both Q1 and Q2 are OFF and the 35 nA current source is charging C1 (A.T. CAP). The auto threshold capacitor should be a ceramic or equivalent low leakage capacitor, because the charging current could otherwise be of similar amplitude to the capacitor leakage current. VEE A 20k Ω 20kΩ MAXIMUM NOISE THRESHOLD LEVEL 35nA 14 15 Q1 Q2 C1 0.22µF MINIMUM NOISE THRESHOLD LEVEL B I2 I1 t Q1 CLAMPS NODE B TO A DIODE DROP ABOVE THE POTENTIAL AT PIN 14 I1 IS CHARGING C1 B VOLTS A MAX MIN Figure 25a. Condition Where the Actual Noise Threshold Is Above the Maximum Noise Threshold Level Setting (Pin 14) Figure 25b illustrates the condition where the potential at Node A is between the maximum and minimum potentials for Node B. When Node A falls below Node B, then the emitter-base diode of Q2 turns ON causing Node B to follow Node A. Cur- rent I2 illustrates how the discharge current from C1 and the 35 nA current source are directed through Q2. Q2 shuts OFF the moment that Node A rises above Node B. This forces the 35 nA current source to begin charging C1 at a constant rate set by the value of C1 at Pin 15. VEE A 20k Ω 20kΩ MAXIMUM NOISE THRESHOLD LEVEL 35nA 14 15 Q1 Q2 C1 0.22µF MINIMUM NOISE THRESHOLD LEVEL B t I2 FLOWS DISCHARGING C1 CAUSING NODE B TO VOLTAGE FOLLOW NODE A I1 IS CHARGING C1 B VOLTS A MAX MIN I2 I1 Figure 25b. Condition Where the Noise Level Is Between the Maximum and Minimum Threshold Settings Figure 25c illustrates the condition where the potential at Node A is below the minimum potential for Node B. In this case the internal minimum noise potential causes a diode to turn ON. This clamps the Node A potential to the minimum noise thresh- old level. I1 represents the current flow in this condition. In addition, the 35 nA flows through Q2’s emitter-base diode as shown by I2. VEE A 20k Ω 20kΩ 35nA 14 15 Q1 Q2 C1 0.22µF MINIMUM NOISE THRESHOLD LEVEL B C MAXIMUM NOISE THRESHOLD LEVEL I2 I1 t I 1 AND I 2 FLOW CLAMPING NODE B TO THE MINIMUM NOISE THRESHOLD LEVEL B VOLTS A MAX MIN C Figure 25c. Condition Where the Noise Level Is Below the Minimum Noise Threshold Level Setting Simply subtracting the noise threshold from the average VCF HF control signal plus noise threshold and the average VCA control signal plus noise threshold will yield the final VCF and VCA control signal. This operation is accomplished with two internal difference amplifiers. Figures 26a–b shows the response of the detector that controls the VCF bandwidth and VCA gain respectively. Both L IN and R IN pins receive a 10 kHz tone burst. The lower trace of Fig- ure 26a shows the control voltage to the VCF (Pin 11) and the lower trace of Figure 26b shows the control voltage to the VCA (Pin 12). Note the quick rise and slow fall times. This allows fast adaptation to changed input signal conditions, while avoid- ing pumping effects and other sonic artifacts. 100 90 10 0% 100mV 100ms 500mV Figure 26a. VCF Control Voltage for a Tone Burst 100 90 10 0% 500mV 100mV 100ms Figure 26b. VCA Control Voltage for a Tone Burst |
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