The ARS QRP Lab Reviews the MiniR2 Receiver
The ARS Sojourner
From 1992 to 1995, Rick Campbell, KK7B, published a series of QST articles on phasing receivers and transmitters. These designs stirred up a great deal of interest, attracting some of the best minds in amateur radio. To this day, KK7B's work, along with the work of Wes Hayward, W7ZOI, sets the standard for excellence. For an interesting compilation of some of their contributions to the radio art, see QRP Power, published by the ARRL.
The subject of this review is the MiniR2 receiver, which is the receiver component of a transceiver described in an article entitled "A Small High Performance CW Transceiver," appearing in the November, 1995 issue of QST. You can buy a kit version of the MiniR2 from Kanga USA.
The MiniR2 is a direct conversion receiver, using the phasing method to reject the unwanted sideband. Our sample was the kit sold by Kanga USA. This board includes the core circuits of the receiver, but does not include the local oscillator, phasing network, or input filter. We constructed those components using exactly the same circuits as the project in QST, housing them in separate die cast aluminum boxes. See the photograph of our sample, which we refer to as the "disintegrated receiver."
This review focussed our attention on an interesting and important issue in testing receivers. A number of the tests the ARRL Lab uses for receivers are impacted by a receiver's bandwidth. Up to now, we have been following the ARRL's lead (with some misgivings). However, our tests of the MiniR2 dramatized the problem for us. Because of its wide bandwidth, using the standard battery of tests for the MiniR2 would lead to highly misleading results.
The ARRL Lab has a reasonably successful way of dealing with this problem, because most of the radios it reviews are commercial transceivers that include a "CW" filter, with a bandwidth of 500 Hz (more or less). However, the bandwidths of the radios we review vary all over the map. It is impossible for us to use a "standardized" bandwidth.
For this reason, we have decided to emphasize the tests that are "bandwidth invariant." For assessing sensitivity, we will use noise figure. For assessing 2nd and 3rd order IMD performance, we will use intercept points.
This review eliminates all tests that are affected by bandwidth, such as blocking dynamic range, MDS, and second and third order dynamic range. We may introduce altered forms of those tests in our next review, but for now, we felt it was imperative to introduce "apples to apples" testing.
Our review also omits several tests that we will continue to use in the future, but which don't apply in this case. Two examples are phase noise and stability. These performance factors are purely a result of the type of local oscillator the builder decides to use with the MiniR2.
Feeling a little tangled up with knotty technical questions? Many of our graphics have this portrait of a fellow electronics adventurer. Just click, and you're on your way to helpful background material.
Introduction to Receiver Tests
Many of our receiver test reports use bar charts that compare the unit being reviewed to a number of other HF transceivers. Please read this brief explanation of the purpose and layout of these charts.
Receiver Tests—When No External Signals are Present
Spurious signals
There were no birdies in our sample.
Power Requirements on Receive
Our sample drew 53 mA on receive with a 13.8 volt power supply, and the same with a 12 volt power supply.
Receiver Tests—When One External Signal is Present
Sensitivity
We measured a noise figure of 14.4 dB for our MiniR2, which would generally be regarded as appropriate for general operating on 14 MHz. A "contest grade" receiver should be capable of a 14 MHz noise figure of about 12 dB.
In our next review, we will have a new chart that compares noise figures of a number of radios.
Rejection of Opposite Sideband
An ordinary direct conversion receiver produces two AF responses --the "desired signal," and a second signal known variously as the "opposite sideband," the "unwanted sideband," or the "audio image." The purpose of the phasing network in the MiniR2 is to minimize the opposite sideband.
Our sample rejected the unwanted response by 42 dB, which we consider quite adequate. To put this in context, the specs for 15 meter Cub we reviewed last month state that the rejection of the unwanted sideband is at least 35 dB. The specs for an FT-1000MP state that the rejection of the unwanted sideband is at least 50 dB.
Audio Output
Most people think that the main point of a DC receiver is its excellent audio. When we measured total harmonic distortion (at high volume) with our HP 3334A distortion analyzer, total harmonic distortion was 0.8 percent. This is excellent audio performance.
Receiver Tests—When Multiple Signals are Present
Selectivity
We measured a 6 dB AF response of 2330 Hz. You will find interesting information about the width and shape of the MiniR2's AF response in this AF spectrum analyzer graph.
KK7B has told us that his intent was to create a response just a bit wider than a Collins mechanical filter. He also mentioned that the MiniR2 can roll off a little more gently on the filter skirts for good impulse response, since the receiver doesn't depend on the shape of the bandpass for opposite sideband suppression.
Third Order IMD Intercept Point
In our revised approach to receiver testing, this has become one of our most important figures of merit, because it is bandwidth invariant. We calculated a third order intercept point for our sample of +2.5 dBm. Although this is a respectable performance for third order IMD, it was not quite as good as we expected.
KK7B has told us that the diplexers on the MiniR2 mixer IF ports roll-off at about 10 kHz (for good amplitude and phase balance below 4 kHz with off-the-shelf, 5% tolerance components). Since the tones in IMD testing are separated by only 20 kHz, quite a bit more of these tones get into the common-base stages than in the MiniR2's "big sister," the R2.
Please follow this link for comparisons with the third order intercept points of other HF radios.
Second Order IMD Intercept Point
In this bandwidth invariant test, we calculated an excellent second order IMD intercept point of 91 dBm. Please follow this link for comparisons with the second order IMD intercept points of other HF radios.
In-band IMD
The MiniR2's in-band IMD performance was outstanding--exactly the outcome one hopes for in a high-quality DC receiver. This test allows you to visualize the audio "presence" for which DC receivers are famous.
Here is the link to an AF spectrum analyzer graph. For purposes of comparison, here are links to the AF spectrum analyzer graphs for two much more expensive and complex radios, the Yaesu FT-1000 MP and the Elecraft K-2.
Conclusion
Examining the MiniR2 was refreshing and fascinating. And listening to this receiver, with a big antenna in quiet conditions, is pure joy. Although the MiniR2 would probably be an inappropriate choice for contesting, for ordinary operating it would be hard to beat.
KK7B is continuing to work on the next generation of phasing receivers. For example, this year's "Four Days in May" QRP symposium included a paper describing the "R2Pro", which is loaded with intriguing new ideas. On top of that, KK7B and W7ZOI are co-authoring a new book that will probably be as important to our hobby as Solid State Design for the Radio Amateur. We have much to look forward to!
Here are some important links:
The lab's goals and equipment.
