From: "Tracy, Michael, KC1SX" Date: Fri, 12 Jan 2001 09:41:25 -0500 Cc: "'topband at contesting.com'" W5PR: > I read this with interest, but I would like to know what it means > more in layman's terms. What is "BDR" and "IMD DR"? Chuck, That's a good point! :-) Since these things are quite significant to 160, I'll try to explain: Dynamic range in general is the difference between the weakest signal that can be perceived and the strongest signal that can be present without adversely effecting the weakest signal. In the case of ham rigs, the weakest signal is generally considered to be one that is equal in power to the receiver's internal noise, although most folks can pick out weaker signals using the "filter" between their ears. BDR stands for Blocking Dynamic Range and it refers to a condition where the weak signal is "blocked" or suppressed. In layman's terms, you'll often hear this described as "desense" because it reduces the effective sensitivity of the receiver. BDR as a lab measurement normally refers to the point where the weak (presumed desired) signal is reduced by one dB by the presence of a strong (presumed undesired) nearby signal. The frequency difference between the two is the "spacing". The blocking dynamic range is a measure of the difference between the receiver's noise floor and the level of the signal that caused the blocking condition. A "noise-limited" measurement means that the undesired "blocking" signal caused an increase in receiver noise output before the "desense" effect was observed. Usually this is caused by interaction of the signal with the phase noise of the internal oscillators (phase noise being energy that every oscillator has close to its desired output frequency) - that's why rigs with a high transmit composite noise (most of which is phase noise usually) often also have noise-limited dynamic range measurements. Some folks consider this to mean that a "real" BDR measurement can not be made for that rig, but in the ARRL Lab we believe that the _effective_ blocking dynamic range on a noise-limited measurement is where the noise increases by 1 dB, which is the same change in the signal to noise ratio that you get when the signal decreases by 1 dB. So if you see a rig where the sensitivity (aka MDS) is -140 dBm (which is dB relative to a mW, in case anyone didn't know) and it has a 20-kHz spacing blocking dynamic range of 125 dB, that tells you the level of signal that caused blocking 20-kHz away was -15 dBm. Now, all S-meters are different, but it is convenient to use the Collins standard for discussion purposes. In that standard, S9 = -73 dBm (or 50 microvolts, assuming a 50-ohm system). Therefore, -15 dBm would be about S9+60. Quite a strong signal, but certainly not unheard of and many rigs don't have a BDR that high. Intermodulation is when two or more signals mix, creating false signals that aren't really there. These false signals are referred to as intermodulation distortion (IMD). For lab testing purposes, it is most convenient to use only two signals (requires the fewest expensive generators) for this test. IMD dynamic range is the difference between the receiver's noise floor and the level of two nearby signals that caused a false signal to appear right where we are trying to listen. The process of mixing makes the highest such false signals appear at a frequency spacing equal to the difference of the two signals. If you are listening on 1820 kHz and you have strong signals at 1840 and 1860 kHz, you will hear a false signal, since the two are spaced 20 kHz apart and you are 20 kHz below the "lower" one. Someone listening on 1880 will also hear the same false signals. As with BDR, IMD DR can also be noise-limited. In this case, the effect on the frequency you are trying to listen to is being created entirely by the interaction of the nearest strong signal and the receiver's phase noise and oscillators. Sometimes the result is just an increase in noise and sometimes it can be a signal. However, in both cases, the more "distant" generator is turned off when the IMD is observed and you would find the IMD is still there. In the case of non-noise limited measurements, if you remove either signal, the IMD goes away. Now, the important thing to note is that these lab measurements don't exactly duplicate real-world conditions because only unmodulated carriers are used and you usually find more than two signals near the one you are trying to listen to. However, they are a good yardstick for comparing real-world performance of different radios. 73, Michael, KC1SX