QRZ! Ham Radio 1/QRZ Ham Radio Callsign Database - December 1993.iso/arrl/best_rig.2

Text File | 1993-11-21 | 22KB | 428 lines

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.

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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