The ARS QRP Lab--Progress Report and Request for Input

Introduction

We decided that our first project should be the diagnosis of a radio that has already been reviewed by the ARRL Lab. We wanted to compare our results with theirs and to learn from the similarities and differences. We used the equipment described in the July issue of the Sojourner.

We selected the QRP Plus as our first transceiver. It has been around for a long time now, and many people know it well. Furthermore, the manufacturer, Index Laboratories, is no longer in business, so we can have a frank discussion without harming any company's commercial interests. Still, we ask you to remember that personal reputations may be an issue here, and we all need to avoid any commentary that could be harmful to an individual.

This report does not use our snazzy new format. We'd like that format to be user friendly, graphically oriented, and a useful educational tool. We also want the reader to able to tell, at a glance, how the radio compares to other radios tested by ARRL and ARS. We're still working on the format and hope it will be ready for our next review. In the meantime, we'll use some plain Jane tables and text to get the information across.

And Now for an Important Issue.

The ARRL's report on the QRP Plus, in the September, 1996 issue of QST, stated that the first sample radio failed to meet several receiver specifications claimed by the manufacturer. In addition, the report said that the radio's transmitter could not meet spectral purity requirements at a power level of five watts. Index sent additional samples to the ARRL Lab, and ultimately the lab ended up testing four of them. At least one of those radios had a better receiver, but none of them had a legal five watt transmitter.

Here is where we need your input. The ARRL's review of the QRP Plus appear to be a fairly dramatic example of "tweaking," in which a manufacturer is permitted, and even encouraged, to supply the Lab with optimized versions of its products. We don't feel comfortable with that approach, and would prefer to test radios "off the street," which we've borrowed from friends and ARS members.

But we need to be sensitive to some of the drawbacks to our preferred approach. Sometimes, a kit radio isn't assembled correctly. Poor performance might be attributable to faulty construction, rather than the radio's design. And many (if not most) lower power transceivers are modified by their owners. (In fact, the "mods" are often encouraged by the designers themselves.) It is often hard to tell whether a given radio is the original design, or some permutation.

To guard against these problems, we are suggesting a two stage approach. First, we test a radio "off the street." If the manufacturer or designer is concerned that our original test may have been flawed, then he will be encouraged to provide us with his own version of the radio, but only if he fully documents any steps he may have taken to optimize the radio and gives us permission to share that information with our readership.

Once again, we need your thoughts on this question. We would also like your comments on our testing methods, and whether we have omitted any tests that are important to the QRP community. If any designers or manufacturers have read this article, we encourage their comments as well.

Review of the QRP Plus

We will move now to our testing of the QRP Plus. We remind you that the purpose of this first review was to serve as a kind of "quality assurance" for the ARS Lab. There are some issues that we did not investigate for the QRP Plus, but would like to explore in future reviews. For example, we would like to find a reasonable way to assess AGC. In addition, we'd like to work with the concept of noise figure.

Our report of the transmitter functions of the QRP Plus will essentially follow the logic of the ARRL. However, we have used a slightly different logic for the receiver.

The receiver part of the review will be divided into three environmental settings:

A. What happens with this receiver when there are no external signals at all?

B. What happens when there is just one signal?

C. What happens when there are multiple signals?

One more introductory comment. In reality, the ARRL tested four versions of the QRP Plus, which were all different from each other. When we started this project, we weren't sure which one should be compared against our QRP Plus. But the results of the test made it reasonably clear that our QRP-Plus closely resembles the first sample the ARRL received. So from now on, that is the radio against which we will compare our results.

Transmitter Tests

Power Output

CW and SSB power output from our sample ranged from 5.2 watts at 1.8 MHz to 5.5 watts at 28.0 MHz. The ARRL stated that its sample(s) had a "typical" power output of 5 watts.

Spectral Purity

Spectral Purity was an important issue for our QRP Plus. For a transmitter with an output of 5 watts or more, FCC regulations require the suppression of spurious emissions to be 40 dB or more. Our sample failed to meet this requirement on 30, 17, 15, 12 and 10 meters. All four of the ARRL samples failed on the same bands except 30 meters.

Our sample, and all of the ARRL samples, would have been legal at any power less than 5 watts. However, even lower power requires suppression of 30 dB or better. Our sample was surprisingly close to the bare minimum on 30 and 15 meters.

Here are the hyperlinks to the Spectral Purity graphs for our sample: 1.8, 3.5, 7.0, 10.1, 14.0, 18.07, 21.0, 24.9, and 28.0.

Two Tone Transmit IMD

Our chart for two tone IMD transmit testing looks almost idential to the ARRL's chart.

CW Keying

We tested our sample at 60 wpm. Here is the hyperlink to our chart, which shows the first two dits. It is difficult to compare our results with the ARRL, because the ARRL doesn't include any calculations with its chart. For what it's worth, our results seem to indicate satisfactory keying characteristics, because the rise and fall times are similar, and the length of the first two dits is roughly the same.

Receiver Tests--when no external signals are present

Spurious signals

The ARRL Lab does not test for spurious signals ("birdies"). But we think it's an important issue, especially with low power radios that are increasingly using simple and inexpensive synthesizers.

We made no quantitative attempt to characterize birdies by comparing them to a reference signal. Instead, we asked a simple question. "Would this birdie interfere with a weak CW signal." In reality, our test was pretty rigorous.

Our QRP Plus had serious problems with birdies. Here is the hyperlink to our full report, which shows the frequencies at which we found spurious signals, and following is a summary, showing, in parentheses, the number of birdies we found on each band:

1.8 (10)

3.5 (20)

7.0 (12)

10.1 (2)

14.0 (11)

18.07 (2)

21.0 (13)

24.5 (1)

28.0 (61)

Bandwidth

We are toying with the idea of documenting the shape of each available bandwidth. However, for this report, we kept it simple. We measured the 6 dB bandwidth of our QRP Plus, when the "500 Hz" SCAF filter was being used. The ARRL also uses a 6 dB bandwidth in its test called "IF/Audio Response." Both the ARRL and we measured a bandwidth of about 700 Hz.

As a footnote, we assumed that the noise bandwidth was also 700 Hz. According to the literature, this is a reasonable simplification when the slope of the applicable filters is steep (as it is in the SCAF filter).

Receiver Tests--when one external signal is present

Minimum Discernible Signal

We measured an MDS of -130 with the 500 Hz filter. The ARRL measurement was -132. Given the variation in QRP Plus samples, and the inherent inaccuracies of measuring noise, we think we are in the same ballpark.

Phase Noise

The ARRL doesn't normally measure the phase noise, or reciprocal mixing, of a receiver. It does measure "composite noise" in its transmitter testing.

We think that measuring receiver phase noise is much more relevant for low power operators. However, we're still looking for the best way to approach it. We tried the method described in Chapter 14 of the ARRL Handbook, but encountered hopeless problems with receiver overload. For now, we are using the method described by Wes Hayward, W7ZOI, which uses a single measurement spaced 10 kHz from the signal of the test generator. We are using our HP 8640B signal generator without any crystal filtering, because its own phase noise is remarkably low.

We measured a carrier to noise ratio of 107 dBc/Hz for our QRP Plus. Yet another hint that the synthesizer in this radio is problematic.

IF Rejection

This is a simple test. We measured IF rejection of 43 dB. The ARRL measured IF rejection of 42 dB.

Image Rejection

Another simple test. Our sample had an image rejection of 51 dB. The ARRL measured image rejection of 48 dB.

Audio Output

At a robust listening volume, we measured power output of 0.5 watts, and total harmonic distortion of 1.2 percent. The ARRL found 0.9 percent total harmonic distortion with an output of 0.2 watts. These appear to be compatible findings.

Receiver Tests--when multiple signals are present

Blocking Dynamic Range

We were unable to measure a true blocking dynamic range, because noise from reciprocal mixing dominated our measurement. The ARRL's polite term for this is "noise limited."

On 14.0 MHz, we measured a noise limited blocking dynamic range of 99 dB. This compares to the ARRL's measurement of 95 dB (relating to its first, non-optimized sample).

Third Order IMD Dynamic Range

Again, we were unable to measure true third order IMD dynamic range, because noise from reciprocal mixing was stronger than the IMD products. Our "noise limited" measurement on 14.0 MHz was 82 dB, which compares to the ARRL measurement of 83 dB (relating to its first, non-optimized sample).

We should mention that we were unable to make even a noise limited measurement of third order dynamic range on 3.5 MHz (which is one of ARRL's standard testing frequencies). On that band, noise and other spurious products were so severe and random that we were unable to get a reading from our AC voltmeter.

Third Order Intercept

Third order intercept is just a calculated figure, based on previously measured numbers for MDS and IMD dynamic range. We're not sure if there is any point in making the calculation if one or both of these tests is unsuccessful. For what it's worth, the calculated third order intercept from our measurements was -7 dB. The ARRL did not mention the third order intercept for the non-optimized sample.

Conclusions

The main point of this review was to take the ARS QRP Lab on a trial run. The results were reassuring, since our results were close the ARRL results in almost all cases. Given the inherent lack of accuracy in some measurements (particularly those involving noise), and the huge variation in QRP-Plus samples, we were surprised that the results were as close as they were.

Another important purpose was to pose issues on which we could receive your comments. Here is a reminder of the questions on which we would welcome your thinking:

• Do you agree with our approach to "off the street" samples?
• Have you detected any errors in our testing methods?
• Have we omitted any tests that you feel are important for the QRP community?

Originally, we didn't intended to include any editorial comments in this review. However, some of the implications of our testing were more dramatic than we expected. So, here is a very brief summary of the thoughts that this process has stirred up for us:

1. The synthesizer in our radio caused severe problems. We can anticipate that synthesizers in other inexpensive radios will also be problematic. Therefore, we need to develop better methods for assessing their performance. This would be in contrast to ARRL, which doesn't measure receiver phase noise at all.

2. Some important measurements depend on the bandwidth being set at 500 Hz, and the AGC being turned off. However, in many simple radios, including the QRP Plus, the filter settings are far from 500 Hz, and the AGC cannot be turned off. We need better methods for dealing with these limitations. We may turn more in the direction of noise figure and some of its derivitives, even though this will move us away from the ARRL's testing methodologies.

3. Although we don't have direct access to the advertizing material Index was using in 1996, we infer from the ARRL report that Index was claiming receiver performance far better than the non-optimized sample (or the ARS sample) delivered. And we assume that the Index advertizing stated or implied that the radio contained a legal five watt transmitter, even though Index was never able to provide a sample to ARRL that met FCC requirements. If any of these assumptions are wrong, then we welcome comments that will correct them. If these assumptions are correct, they remind that we might need to be cautious about what we read in promotional literature and advertizements.

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Russ Carpenter, AA7QU, is co-founder of Adventure Radio Society. Russ is an active QRP operator and a devoted outdoorsman. He lives on the McKenzie River in Oregon.

russ@natworld.com