Which Rig is Best - Operating Features and Performance Part 2 of 2 February 27, 1993 Derived from February 1993 QST "Lab Notes". Copyright 1993 American Radio Relay League, Inc. All rights reserved. Thank you for requesting the following information from the ARRL Information mail server. ARRL HQ is glad to provide this information free of charge as a service to League members and affiliated clubs. For your convenience, you may reproduce this information, electronically or on paper, and distribute it to anyone who needs it, provided that you reproduce it in its entirety and do so free of charge. Please note that you must reproduce the information as it appears in the original, including the League's copyright notice. If you have any questions concerning the reproduction or distribution of this material, please contact Mark Wilson, American Radio Relay League, 225 Main St., Newington, CT 06111 (mwilson@arrl.org). Q: Now, what about performance? I am not an engineer; how can I decide which rig works the best. Does this mean that more watts is the best, or something? A: Maybe, although more performance (like more watts) sometimes means more cost, more size, more weight, or some other factor that also must be considered. Rather than get into every single performance issue in detail (that would take a book), we'll concentrate on the most important performance issues. The performance characteristics that are the most important are subject to some disagreement, but most people will agree on the following: Receiving: Sensitivity, dynamic range and "cleanliness." Transmitting: Output power and spectral purity. Q: Could you review receiver characteristics for me? How about starting with sensitivity? A: Sure (that's what this column is for!). You may want to have your ARRL Handbook, or one of our other electronics books handy (I suggest our new one -- Understanding Basic Electronics) to help with some of the technical terms I may toss your way. Receiver performance specifications often starts with a measurement of sensitivity. This is a measurement of the ability of a receiver to hear weak signals. Sensitivity can be expressed in several different ways, some more common than others. (A receiver-design engineer usually selects the one that is most useful.) The two conventions most often seen in amateur receivers are microvolts into 50 ohms (for example -- 0.15 uV for a 10- dB signal-to-noise ratio) or dBm (decibels relative to 1 milliwatt into 50 ohms). Q: This is starting to sound complex. What's the bottom line? A: The bottom line, for the most part, is the lower the sensitivity number, the better. A sensitivity of 0.16 uV is better than a sensitivity of 0.2 uV. The larger then negative noise-floor number, the better. A noise floor of -140 dBm is better than a noise floor of -130 dBm. (Typical HF transceivers have noise floors between -135 and -140 dBm.) These two ways of expressing sensitivity are just two different ways of saying the same thing. If you would like to know more about the formulae for the relationship between the different ways of expressing sensitivity, the affect of bandwidth on sensitivity measurement, and other related factors, refer to the ARRL Handbook. Keep in mind, though, that more is not always better! The band noise floor sets the practical limit. Once you have reached that point, greater receiver sensitivity simply amplifies band noise. When considering sensitivity, remember the old adage -- On HF, the system sensitivity is usually limited by external noise -- increased receiver sensitivity on 75-meters in the summertime usually means you can hear more static crashes! Also, too much sensitivity may make a receiver more susceptible to overload. For the amplitude-detection modes (AM, CW and single sideband), there are two different methods of measuring and expressing sensitivity -- the first is to measure the level of desired signal that results in a specified amount of signal-to-noise ratio. The second is to measure the actual noise floor of the receiver. The latter is actually a special case of the first -- representing a signal-to-noise ratio of 0 decibels (dB). (Refer to the cited ARRL literature for more information about the decibel). Q: Now I understand all about sensitivity. I have heard a lot about dynamic range. Can you explain it? A: It sounds real high tech, but dynamic range is easy to understand. In a nutshell, dynamic range is a measurement of the range of signals that a receiver can listen to -- the difference between the weakest signal a receiver can hear and the loudest signal a receiver can simultaneously accommodate without undesirable effects. This could be expressed in many different ways, but the one usually seen is to express it in decibels. There are two ways that we measure dynamic range, resulting in two different kinds of dynamic range: blocking dynamic range and two- tone, third-order dynamic range. Q: The last one sounds like a real tongue twister, so I will start with the first one. What is blocking dynamic range? A: I'm glad you asked! Let me explain how we measure it. I will give you examples for how we test dynamic range using our ARRL "standard" spacing of 20 kHz for desired and undesired signals. We tune the receiver to a "desired" signal (supplied by one signal generator #1) and tune signal generator #2 20 kHz away to be the undesired signal. We then increase the level of the undesired signal until we see a 1 dB drop in the desired signal. This represents what will happen when you are listening to a station and a strong signal suddenly appears in another part of the band when the big signal makes your receiver go deaf. The difference between the noise floor, and the level of signal it took to cause the desired signal to drop in level by one dB, expressed in dB, is the blocking dynamic range of the receiver being tested. A typical receiver might have a blocking dynamic range between 120 dB and 150 dB. The larger this number -- the better. Q: That sounds easy enough. Now what is this two-tone, third-order dynamic range? A: In a nutshell, this is one way of measuring if a receiver will generate any false signal responses. We use the same basic test setup we used to test blocking dynamic range, but in this case we use two identical level signals spaced 20 kHz apart. On the 20- meter band, for example, the two signal generators are tuned to 14.020 and 14.040 MHz. We tune the receiver to 14.000 MHz, the frequency that is one of the two third-order products of the two generator frequencies. We then increase the signal levels of the two generators until the measured third-order product is equal to the noise floor of the receiver. We then repeat the process with the receiver tuned to 14.060, the other third-order product. The difference between the noise floor and the level of signals that caused the appearance of the third-order product, expressed in dB, is the two-tone, third-order dynamic range. A typical receiver might have a two-tone, third-order dynamic range between 90 and 105 dB. The larger this number -- the better. Q: I am with you so far. What else counts in a receiver? A: Well, there are many things that are important, but those are the "biggies." You may also want to consider things like audio frequency response, filter performance, how well the receiver's automatic gain control works (AGC), S meter performance and audio output power, to name just a few factors. As we put together our QST Product Reviews, the reviewers and the Product Review editor pay close attention to the performance of these things. If there is anything of note, it is often discussed in the running text of the Product Review. Q: What do you mean by receiver "cleanliness"? A: In modern, synthesized transceivers, a key receiver-performance issue is spurious-free, highly linear, low-noise signal reproduction. The introduction of frequency-synthesis techniques in Amateur Radio gear has created a new set of challenges for radio designers. This is because using these techniques created the potential for receiver spurs ("birdies"), internally-generated noise that rises with signal levels, and related effects that can mask or distort desired signals. An unrelated but important issue is freedom from excessive hiss outside the desired-signal passband in audio and IF amplifiers. You'll see these issues and their impact discussed in QST Product Reviews. For example, frequency-synthesizing circuitry is notorious for generating phase noise. Phase noise often manifests itself as broadband hiss caused by a phase-noisy oscillator chain in your transceiver. You'll hear it when you are tuned to a frequency adjacent to a strong signal. Phase noise can also be transmitted by your radio, causing interference to others. (Imagine having a phase-noisy receiver tuned to a frequency adjacent to a strong signal generated by a phase-noisy transmitter!) Phase noise has improved tremendously over the past five years, but can be an annoying problem in some older rigs. If you see the dynamic range measurements in QST Product Reviews reported as "noise limited", this is a strong clue that the transceiver may suffer from phase- noise problems. Refer to a recent QST Product Review column for an example of a composite-noise test photograph. When it comes to composite noise -- less is best -- the best transmitted phase noise that is typically seen is -120 dBc/Hz at 10 kHz offset. This subject was treated in detail in a two-part article that appeared in March and April 1988 QST. Q: Let's move to the transmitter. Are more watts always better? A: Usually having the capability to run more power is a good thing -- there are always times when one wants to have a bit more ability to punch through interference, noise or fading conditions. However, even this has its trade-offs. More power usually means increased cost, increased current consumption, and increased size and weight -- factors that might be strong minuses for some applications. Many hams find that output power in the 100-watt class is adequate for HF work. Most HF amateur transceivers are in the 100-watt class. SOme of them struggle to get there, or fall slightly short; others exceed 100 watts comfortably. Is the difference significant? The guy on the other end will never notice the difference between 95 and 110 watts -- this is only a small part of an S unit. (Many of the VHF transmitters and transceivers have output power ranging from 1 to 50 watts.) If you are going to operate RTTY, look for a rig that is rated for 100% duty-cycle operation. During an RTTY transmission, your transceiver is operating at maximum output power continuously. THis is hard on the final amplifier stage and power supply! Many rigs perk along fine during low duty-cycle operating (CW or SSB) but overheat quickly on RTTY. For RTTY, a transceiver should be capable of tolerating continuous, 100% duty-cycle operation for at least 10 minutes. Do remember that there are many QRP (low-power) hams doing world- wide communication with power in the milliwatt range, so a ham transmitter and station success is not determined by power alone. What if you want to crank down the power and see what you can do with a watt or two? Will the transceiver allow you to adjust the minimum output to whatever level you desire? Some rigs have a minimum output level as high as 5 to 10 watts. Q: Interesting. What is "spectral purity?" A: Part 97 (the FCC regulations that govern the Amateur Radio service) requires that all transmitters meet standards for the purity of their signals. Of course, no system is perfect, and all transmitters inadvertently transmit some signals outside of their intended frequency range or channel. These signals are called "spurious emissions" -- a term that includes all types of signals that are not the fundamental and its desired modulation. Amateurs must be concerned with the level of these spurious emissions, both to ensure that the transmitted signal is in compliance with Part 97, and to ensure that the transmitter doesn't interfere with other services (including their own or their neighbor's television reception). Spectral purity is measured using a spectrum analyzer. Refer to the test-result table in each Product Review for a list of spectral-purity test results. Any transmitter advertised in QST must meet the minimum FCC requirements. Q: I have seen two-tone IMD photographs in QST Product Reviews -- what are those all about? A: These are the results of a two-tone transmit IMD test. One very real concern to hams is that of adjacent-channel splatter on single-sideband phone operation. (This can also be found in full- carrier, double-sideband, amplitude-modulation (AM) operation.) I have often heard badly distorted audio QRM and discovered that it was coming from a loud station 50 kHz down the band. (This adjacent-channel spatter has made my life miserable in nearly every VHF contest I have operated in.) This splatter is caused by non-linearities in the radio-frequency amplifiers used in the transmitter. The most common effect is third-order intermodulation. This can result when there is more than one frequency present in the modulation (always the case when a voice is the modulation source) these multiple frequencies will mix together in the same third-order relationship that was discussed earlier under receiver testing (along with similar higher-order products). The result is a fair amount of unwanted energy clustered around the desired frequency channel. These unwanted IMD signals can cause QRM to stations located many kHz away. Many Product Review columns contain an example of the published two-tone test results. Although a voice contains many frequency components that will mix together in a way that is difficult to predict, a real good indication of the IMD performance of a transmitter can be obtained using only two tones of equal amplitude at the microphone input. These two tones are the two "tallest" pips in the center of the photograph. When he spectrum analyzer is adjusted so that these two tones are 6 dB below the reference line at the top of the spectrum-analyzer screen, the reference line represents the peak-envelope power (PEP) of the SSB signal. The other pips clustered above and below these two tones are the IMD products. Their value, referenced to PEP, can be read directly from the spectrum analyzer screen's vertical scale. Less is best! The smaller the IMD pips are on the spectrum analyzer screen, the less IMD the transmitter is generating. Let me put this in perspective, though. All transmitters (even those designed by Zack in the ARRL Lab) will generate some IMD. The higher-order products (those the farthest away from the two tones in the center of the photograph) are the ones that are the most troublesome. Most of us are not surprised when we experience interference from a station that is transmitting 3 kHz away -- we almost expect it. Our receiver passband is not usually sharp enough to completely eliminate the station "next door." We usually strongly object to interference from a station 20 kHz away, though. On the other hand, if the station 20 kHz away is 40 dB over S9, a whopping signal, but not all that rare, and the IMD products are as good as - 50 dB PEP (a good amplifier by any standard!), the resultant IMD spatter will still be S7, assuming 6 dB per S unit - - the old Collins standard. This would be horrible interference if the rare DX station you were listening to was only S5. Q: Well, you certainly rattled off all of the answers. I want to know as much as you do about radios so I can make the best decision for me. What should I do next? A: Well, experience in anything doesn't come overnight! We assume you have been looking at advertisements and dealer displays. You can learn a lot about a piece of equipment from the way it is advertised and from the marketing information supplied by the manufacturer or dealer. You can probably dismiss a few out of hand based on price, performance or features, leaving you with only a handful of choices. One good way to learn about radios is to read as much information as you can about radios, and listen to as many opinions as you can find about what is good, bad and indifferent about the way that they work. A good place to start is in the QST Product Review columns that we have done over the past several years. If you have the back issues, start reading. Seeing how each feature was described, and reading the reviewers' likes and dislikes will help you quite a bit as you think about how features work and how important they might be to your purchase decision. Now, if you don't have lots of back issues of QST (I know you are all League members, but some new members don't have issues going back to 1953) pick up a copy of The ARRL Radio Buyer's Sourcebook. The Sourcebook offers reprints of the most popular QST Product Review columns from the past several years, plus a few "golden oldies" from years past, to help you with used equipment. You might be able to find back issues of QST at a large local library, or through one of the members of your local radio club. Reprints of QSTs, or back issues, are available from ARRL Headquarters. Q: (Several weeks later . . .) I have read every QST Product Review ever done. (I even found out they used to call the column "New Products" many years ago!) and now I really know all about rigs from every era! There are still a zillion radios to choose from -- how do I decide which one is really best for me? A: Well, if your really read every QST, you probably know enough about all of the available rigs to know exactly what you want. If not, it is time to do some narrowing down. Now, talk to people that own (or have owned) the rigs you are considering. You may find a wealth of advice through your local radio club, local repeater group or through on-the-air contacts. Look around the bands for people using the rig you are considering. Someone who has actually used one of the rigs will be the best person to answer questions. Their specific descriptions of the features, and a summary of their likes and dislikes, will tell you much about what you want to know. The list is getting smaller. You are probably considering spending anywhere from several hundred to several thousand dollars -- a large investment for most people. A real inexpensive expenditure that will help you spend your money wisely is the owner's manual for the unit you are considering. THere is no printed material that can tell you more about how the equipment performs and how it does it. You can see diagrams about how the front and back panels are laid out, how to hook the radio up to other equipment and page after page of instructions on how to use sometimes-complicated features. Service can be important, too. Service is often a local issue, so ask around locally to find out about the service offered by local dealers and equipment manufacturers. Most hams will have a tale to tell (of happiness or woe) about their service experiences. You will probably find that a private conversation will result in a lot more frankness. Q: Well, thanks! I think I know which radio I want -- I at least have it narrowed down to two different possibilities. But I want to be sure. Is it time to just bite the bullet and plunk down the dough? A: We recommend one other step. There is no substitute for hands- on experience with the radio of your dreams. Find a local ham or dealer who has one. With luck, you'll be able to secure an invitation to visit and operate it for a while. This will give you an opportunity to really put the radio through its paces. Considering the amount of money you might spend, this is a worthwhile step even if you have to drive a fair distance. If you have a local ham-radio dealer, you may be able to use the demo model in the store. This is not quite as good as spending an evening with the radio, but it will give you a chance to compare all of your choices in one place. Q: I sure never would have thought that there were so many factors to consider. I sure do thank you. I will mull it all over and get cracking right away. What kind of radio do you own? A: Well, we will keep that a secret, but one of our Laboratory Engineers, Zack Lau, KH6CP, believes that the most popular rig among the headquarters staff might be the Heathkit HW-9. It seems that many of us have owned one at one time or another. Of course, our theory is that it is just the same one that has changed hands several times! But remember, what we like is not what really matters -- it is what you like that is important. There is no one who knows your needs better than you, so it really wouldn't be fair to anyone if we were to impose our personal preferences on you. We have given you some of the information you need to pick the best rig for you. When you do, remember that the main purpose of any fun is to have fun with Amateur Radio. Good luck, and enjoy your new radio! \0/ For the curious, third-order intermodulation products will appear at frequencies of (2F1-F2) and (2F2-F1), where F1 and F2 are the two generator frequencies. \1/ "Phase Noise and its Effects on Amateur Communication?", John Grebenkemper, KI6WX, QST March and April 1988. \2/ Radio Buyer's Sourcebook - available from ARRL Headquarters