Tuesday, June 22, 2010

I am looking, like most hams, for points of comparison when I read QST Product Reviews ...

Q Dan Marriott, VE7CTN, asks, “I am looking, like most hams, for points of comparison when I read QST Product Reviews. And, while I think I understand what some of the tests mean, their labels or abbreviations are not always intuitive.For example, when evaluating receive performance I see references to MDS, IMD, IMD dynamic range, blocking dynamic range, third-order products and third-order intercept points. Can you clarify?”

These terms, and many others, are described in great detail in the ARRL Handbook’s Test Procedures and Projects chapter. A collection of QST articles that explain the Product Review process and tests can be downloaded from the ARRL Web page at http://www.arrl.org/tis/info/bestrig.html. Here is a very brief summary that will get you started.

The MDS (minimum discernible signal, or “noise floor”) is a measure of receiver sensitivity. It describes the amount of receiver input noise. A receiver should be able to just detect a real signal at the level of the noise floor, thus the term, minimum discernible signal. MDS is usually expressed in dBm—decibels relative to a milliwatt. A typical receiver might have an MDS of –135 dBm, or 135 dB less than a milliwatt. That is an extremely small amount of signal power. The MDS numbers give you a reasonable idea of a receiver’s sensitivity. Useful SSB sensitivity usually falls in a range of about –120 dBm to –135 dBm.

Intermodulation distortion (IMD), whether transmit or receive, is the mixing of two frequencies to produce additional frequencies. Recall that the chief property of a mixer is to produce sum and difference frequencies by mixing signals from the input with those from the local oscillator. Living in an imperfect world as we do, electronic circuits are not perfectly linear so additional mixing takes place between these original frequencies and the ones that are intended to appear.

Assume that your original frequencies are F1 and F2 with the difference between them being N Hz. The combination of all the above mentioned mixing creates undesirable signals at 2F1 – F2 and 2F2 – F1 (among others—these are just the ones that usually affect transmitter or receiver performance). These end up appearing at N Hz above and below F1 and F2. They are known as third order products. There are other signals that appear as well (on a transmit IMD graph, you typically see third, fifth, seventh and ninth-order products or more), but these are not as high in amplitude as the third-order ones.

How does this affect real-world operating? On receive, if your radio is tuned to 14.020 MHz and two strong signals appear at 14.040 and 14.060 MHz, assuming nothing else is on the band, you will get a false signal appearing on 14.020 where you are tuned. Under test conditions, the two off-channel signals are identical in strength, at a level that gives an IMD response equal to the MDS level. The difference in strength between the on-channel signal “ghost” and the two off-channel signals is the IMD dynamic range, expressed in dB.

This test indicates the general intermodulation behavior of a receiver. If, for instance, you were slugging it out with the DX crowd trying to work a rare one, you would have signals of many different strengths appearing at many different frequencies inside and outside the passband of your receiver’s filters. When the signals are strong enough, intermixing will produce false signals within the receiver’s passband. The higher the IMD dynamic range of your receiver, the weaker (and less annoying!) these false signals will be. Typical receivers will have an IMD dynamic range of from 80 to about 105 dB.

Blocking dynamic range is basically a measure of how strong an off-channel signal must be to produce either an increase in noise in the receiver passband or a decrease in receiver gain, otherwise known as “desense.” In ARRL Lab tests we use a signal that is 20 kHz away from where the receiver is tuned. A signal within the normal operating amplitude of the receiver is added on frequency and then the level of the off-channel signal is increased until the on-channel signal decreases by 1 dB (blocking is occurring) or the output increases by 1 dB because of receiver noise. (In this case, the measurement is reported as being “noise limited.”) The difference between the off-channel signal and the receiver’s noise floor (MDS) is the blocking dynamic range expressed in dB. Again, different combinations of frequencies and signal strengths will produce different blocking behavior, but a higher blocking dynamic range number at 20 kHz indicates better general blocking performance. Typical receivers will have a blocking dynamic range of from 90 to over 150 dB.

Third-order intercept is related, of course, to two-tone, third order IMD. One characteristic of third-order products is that they increase/decrease three times faster than the on-channel products (if the input tones are of equal level). These responses can be plotted on a graph, but the two lines never actually intersect because the receiver always goes into gain compression well before that could happen. So, as signals keep getting stronger, both the on-channel and third-order responses “roll off.” Third-order intercept is the theoretical point at which these two lines would cross. It gives a relative indication of a receiver’s strong signal performance.

If two receivers have the same IMD dynamic range and one has an MDS of –125 dBm and the other –135 dBm, the third-order intercept of the –125 one will be higher. Read that again. Yes, the radio that doesn’t hear as well will give a higher third-order intercept point. That doesn’t mean that you want to always look for a lower third-order intercept point. Third-order intercept should always be evaluated in conjunction with MDS and dynamic range—they are all related.

From QST February 1999