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Winter 2005 Issue |
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Receiver Dynamic Range–Part 2 |
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In part one of this article we covered one-,
two-, and three-signal dynamic range and the practical measurement of
noise floor, as well as crystal notch filters. In part two, we cover the
practical details of DR2 and DR3 measurements, and conclude with a
discussion of how much DR we really need. For a conventional receiver, the level of the weak signal makes no difference as long as the dynamic range of the IF and audio sections of the receiver under test is adequate. It makes no difference at all if the AGC is enabled or not, if the spectrum analyzer at the loudspeaker output is being used to simultaneously monitor the desired weak signal and the noise floor. To check the dynamic range of the IF and audio sections, increase the level of the desired signal and monitor S/N. At some point, S/N will no longer increase in proportion to the input level. This is the level where IF or audio noise is no longer small compared with the noise floor at the antenna input, which has been reduced by AGC action, or it is the level where the signal does not increase anymore because of clipping. If the receiver under test has a dual-loop AGC, the level where an RF attenuator is inserted will be clearly visible in the plot of S/N versus input power. DR2 measurements have to be made with the weak signal well below this level. The way the two signal generators are combined is very uncritical for DR2. The power level needed for the strong signal is below 0 dBm, and the weak signal may be somewhere around –130 dBm. A simple resistive network will be fine; connect the strong generator through a 5-ohm resistor, and add the weak signal through a 500-ohm resistor. A directional coupler or even a T-connector followed by a 6-dB pad to restore 50 ohms impedance will be fine. For the measurement, just select a frequency offset and adjust the strong signal for a 3 dB loss of S/N. DR2 is given by equation (3), where PGEN is the level of the strong signal at the antenna connector of the test object. If the signal generator is calibrated, one only has to subtract the loss because of the cables and the signal-combining network. If the generator is not calibrated (it could be a homemade low-noise crystal oscillator), it is still possible to determine the power level from the noise figure as described in conjunction with figure 3, although one would then need calibrated attenuators to bring the power down for the saturation level to be somewhere around –130 dBm. Signal leakage is the critical point here. Click here to return to Winter 2005 highlights Click here to subscribe to VHF _________________ © Copyright 2005, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.
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