Lab testing changes detailed in October's QST

In the upcoming October issue of QST, the ARRL Lab introduces a series of significant new receiver tests. These are the result of development work and discussions with professionals in the radio industry over a period of several
years.

One new test concerns receiver blocking gain compression and reciprocal mixing. The ARRL Lab describes Blocking Dynamic Range (BDR) as "a condition in which the weak [desired] signal is 'blocked' or suppressed" by a strong
interfering signal. This is also known as desense or overload.

In some cases, the cause is a reduction in receiver gain. In other cases ("noise limited" measurements), the cause is an increase in the receiver's internal noise due to the noise sidebands of the local oscillator mixing together with the
strong interferer (also known as "reciprocal mixing"). The level of the noise masks the gain reduction effect on the desired signal.

Instead of reporting the BDR as a noise limited measurement, the Lab will now be using narrow-band
measurement techniques to "dig out" the desired signal on noise-limited measurements and determine the point of gain reduction.

To distinguish from earlier measurements, the name will be changed to "Blocking Gain Compression." A separate reciprocal mixing test indicates the level of noise increase within the receiver caused by the interfering signal.
Together, these two measurements provide more information about how the receiver behaves with a single strong interfering signal.

From 1983 onward, ARRL Product Review published IP3 (third order intercept) figures for receivers based on a noise floor intermodulation distortion (IMD) response level. In 1993, the level was changed to a response that produced an S5 reading on the receiver's own S-meter.

The decision was based largely on this being a more typical average of signals that would be found on the bands. The drawbacks to this approach are that there is a great variation in S-meters from receiver to receiver (see Product Review, April 2005) and it overlooks the significant change that can often be observed in receivers at higher signal levels.

With this in mind, the ARRL Lab has decided to measure and report IP3 at three levels. The lowest level measurement is made at the noise floor, as has been done in the past. The middle level is done at a standard level of -97 dBm, defined as S5 in the IARU Region 1 standard for S-meters. For the
highest level, instead of selecting a particular receiver response, the maximum level of expected interferer will be set to 0 dBm (S9 + 73 dB by the IARU standard, a loud signal indeed!).

These three levels taken together should present the best overall view of a receiver's total performance, with each level being most useful in a particular context. For example, someone doing SSB or CW work on VHF would care most about the receiver performance at the noise floor, while on HF, an S5 level would be more useful.

According to ARRL Lab Test Manager Mike Tracy, KC1SX, members frequently ask why manufacturers' sensitivity specifications are given in microvolts but Product Review measurements are reported in dBm (decibels relative to a
milliwatt). He said, "The chief reason is that manufacturers typically do not include a bandwidth in their specification, and measurements in different bandwidths are not directly comparable. All other things being equal, there is more noise power in a 3 kHz bandwidth than a 2.4 kHz bandwidth. To overcome that limitation, the sensitivity testing is done with a 500 Hz bandwidth filter, or as close to that as is available. This permits reasonable comparisons of different receivers."

Although the filters that a receiver has cannot be changed, the variation in actual bandwidth can be determined by calculating the Equivalent Rectangular Bandwidth (ERBW). This is the width that the filter would have if it passed the same noise power and possessed the "ideal" shape of vertical sides and a flat passband response.

For more on the changes coming in future Product Reviews, be sure to check out the October issue of QST.

Wireless Institute of Australia