|   Interview with David Marshall, Chief Science Officer of Marshall Radio Telemetry -- "Development of the New Field Marshall"Interviewer: "Tell us about this totally new Marshall receiver, and
what you set out to achieve."
DLM: "We started out as a project just to replace the current Field
Marshall with one that has a much wider frequency selection. Yet as
things went on, we kept finding new possibilities, "Why don't we
change this, why don't we add that?" Suddenly, last year we were
spending a lot of time on it, suddenly we were getting very picky
about what we wanted to improve. We began thinking it's a new receiver
and it's got to be really good. And we got lucky in a few instances.
We happened to strike a rich vein of potential that will keep it
viable for a long time to come. So what you get in the Field
Marshall is a brand new technology, state of the art, and if I do
say so myself, a bit of a precision job of engineering.
As it turned out, the biggest change was to improve the filtering on this receiver, to make it so much more responsive to a weak signal, so much more sensitive and so much more free of the noise which obscures your signal. Those were breakthroughs."
Interviewer: "Why do you say Breakthrough? That's a strong word for an
improvement."
DLM: "Well, receiver technology has been around for a long time, and
the same techniques have been used for decades and decades. But with
the latest generation of electronic RF [radio frequency] devices,
there are substantially new, profoundly new approaches to issues and
it was just natural to take advantage of these. And we used what we
think are some novel or clever ways that gave a result that even
surprised us."
Interviewer: "Describe for us the concept of 'Signal-to-Noise Ratio'
and how you improved it."
DLM: "The Signal-to-Noise Ratio concept sounds kind of daunting and
maybe not so easy to understand, but it's the essence of a receiver if
you're trying to pick up a weak signal as we do in telemetry a lot.
Only one thing stops you from picking up the weakest signal, and that
is the amount of noise introduced by the first stages in the receiver.
Think about it: if you have a weak signal out there and it's coming
through a good antenna, and suppose you had no noise introduced in
your receiver, then it's simply a matter of amplifying and amplifying
until you can hear that weak signal, and that's very easy to do. But
unfortunately there's always a white noise, that hiss you hear from
the receiver, caused by thermal vibrations in the very semiconductors
and metals themselves and that hiss is your enemy. It gets amplified
along with the signal and ends up obscuring it. You try to amplify the
weak signal to no avail. But if you can reduce that noise, then you
can amplify and hear weaker signals, so the Signal-to-Noise Ratio is
the only measure of how good a receiver is at picking up a weak
signal.
Also, in a programmable receiver where you can select your own
frequency, you need to have a special kind of oscillator, and inherent
to those oscillators is that they generate a lot of noise, which is
called 'phase noise.'"
Interviewer: "So this limitation is found in all other receivers, yet
you've been able to overcome this somehow?"
DLM: "Yes, by using new semiconductors, more expensive ones, and the
right design parameters and filtering mechanisms, which is all why I
used the word breakthrough to describe what we've done. With the phase
noise, we've also taken exhaustive steps to 'corral' that and contain
it in its own private little place so it cannot spread out and affect
channels that are getting amplified. Any receivers on the market that
are described as 'synthesized receivers' are limited by this problem."
Interviewer: "So is this as sensitive as anything available in the market?"
DLM: "Well, we compare it to our Stealth. The Stealth receiver that
we've been producing for the last 8 years or so has always come out on
top of any test for sensitivity that we've made against any other
receiver. 'Sensitivity' is another term for the Signal-to-Noise Ratio,
or the ability to detect a weak signal. And this new receiver is the
same as the Stealth in terms of sensitivity but at a much lower cost.
That's one of the things I should have mentioned earlier that by
modernizing and updating a lot of the circuitry, we've been able to
substantially lower the cost and make it more affordable than the
Stealth was."
Interviewer: "How is this different from what some call a noise-blanker?"
DLM: "A noise blanker is kind of a simple, more primitive approach to
just cutting out certain types of noise in the form of big spikes and
sudden jumps in the signal level, like noise from car ignitions. But
here we're talking about the more subtle and insidious type of noise,
and that's much harder to overcome than by just using a noise blanker.
This white noise is pervasive and it's mathematically intractable if
you're trying to use simple approaches to getting rid of it.
Interviewer: "Can you increase that sensitivity by adding a
'preamplifier' on your antenna?"
DLM: "Yes, you can. With a receiver that has a poor front-end, you can
improve the sensitivity by adding a low noise pre-amplifier. But it
isn't needed with the Field Marshall. It's kind of a hassle to add an
extra piece of equipment, which is why we built it in."
Interviewer: "How much of this receiver, then, is completely new?"
DLM: "Let me think, I need to do some math... I'd have to say...
100%. Starting at the antenna, we took the basic idea of our
collapsible yagi antenna which has been very successful over the last
ten years and has been very reliable in the field and made it that
much more robust and stronger, and more attractive as well. So we've
gotten rid of the large springs, the pulleys and the cables and just
streamlined everything a lot. So you'll see those differences
right away if you're familiar with our previous antenna.
Inside the receiver, we compressed everything, made the case more
rounded and ergonomic, taking advantage of the smaller size and the
placement of all the parts inside. We completely changed all the
electronics inside the receiver. It looks somewhat similar from the
front but inside are all new circuit boards with all brand new parts,
all parts that will be available a long time into the future since
they're fairly new designs. And of course, in the process we did a few
of these magical things to the circuit to improve the performance,
improve the range, the filtering, the pleasantness of the sound and so
forth. Those high tones sound sweet as honey."
Interviewer: "What makes your yagi so directional? Tell us how you
designed your yagi antenna."
DLM: "Well, anyone can throw together a yagi and you can come up with a
usable antenna. 'Homemade' is the norm among HAMs (Amateur Radio
enthusiasts). But if you really want to get a high-performance
antenna, you need to spend some extensive time optimizing things. By
that I mean the length of every element, the spacing between the
elements, and the tuning of the matching circuitry and a bunch of
things. But the catch is that if you change one of those things, then
you have to go back and redo it for all the other ones. It's just very
tedious to get it just right, and that's why people don't typically do
this. I personally spent weeks outdoors, in lots of hayfields in
Malad, Idaho, because the radio noise was quiet there. I remember
having bees and grasshoppers on me for days on end."
Interviewer: "Why do you have to be out in a field for so many days
doing this?"
DLM: "Well, we did a lot of computer modeling in advance to kind of get
a general idea of the directions we wanted to go, but in the end you
don't trust anything but field trials."
Interviewer: "How much do can you really achieve with all this antenna tuning?"
DLM: "There's a trade off between several basic things in antenna tuning. A key one is the front-to-back ratio. That is, how strong the signal is picking up in the forward direction, the direction you want to go
versus the unwanted response you get in the reverse direction.
Unfortunately in all antennas there's an unwanted signal from the back
that throws people off sometimes to go 180 degrees from where the
target really is. But if you minimize that then you also degrade the
gain or the magnifying ability of the antenna. So we try to find a
good compromise and we try to avoid the side lobes and to get the
sharpest beam, which gives you your pinpoint accuracy."
Interviewer: "Why is your antenna bigger than some others?"
DLM: "If you want to be able to pinpoint and if you want performance,
there's no way you're going to get that with shortened elements.
That's why we've always stuck with the idea of having full length
elements, but then mechanically make it as small and convenient as
possible rather the sacrificing the performance through just making
them small. We realize that convenience is a great thing but
ultimately there are those times when you want the absolute best
performance and that's always been our commitment, to have that
available right there with you at all times."
Interviewer: "I see here you refer to this new receiver as 'The Quiet
One.' Wouldn't most people say that a receiver needs to be loud?"
DLM: "Well, you do hear your signal very loud. What you don't hear is a
lot of background noise and this is because of our special filtering.
You'll find that this receiver is especially good at lowering the
noise picked up from sources, like TV broadcasting stations which is a
terrible source of interference. We've been able to use this special
filtering technology, the polyphase filters, to lower all that noise
and the end result, what you hear, is a clean, loud signal and that's
the ideal thing to be hearing."
Interviewer: "Someone new to this receiver might be alarmed if they
turn it on, then turn up the volume expecting to hear the loud hissing
sound, to gauge how loud the speakers are."
DLM: "What you want to do is first turn on a transmitter and then turn
it up all the way and see how it sounds. What you'll hear is a very
loud signal. Again, noise is the number one enemy of a receiver in
terms of being able to pick up over a long range. So, it's a beautiful
thing, having a quiet receiver."
Interviewer: "What is a polyphase filter?"
DLM: "I really can't explain that, since it would give away our whole secret."
Interviewer: "All tracking receivers on the market today expressly
warn about getting wet or even having any exposure to moisture. One
company even suggests putting their receiver inside a plastic baggie
if there's any chance of rain getting on the receiver. What have you
done to this new design to be able to be used outdoors?"
DLM: "The biggest risk area with regards to water is getting it into
the speaker, as most speakers are made of a certain kind of cloth type
of cone. We went to a mylar cone which is entirely water resistant,
and we covered it with water resistant speaker cloth as well, so we
prevented water from getting through the speakers. But it's not
waterproof, and if you submerge it or if you hold it up so water can
accumulate in the front panel area, water will go inside and
eventually may stop it from working. You can freely use it in rain,
but just keep the front panel shielded from the rain and hold the
front down, so water doesn't accumulate there."
Interviewer: "If you happened to fall in a creek or drop it into a
stream and totally submerge the receiver, then what?"
DLM: "In the user manual we point out that you'd first want to turn it
off, then take out the batteries because the batteries are going to
swell up if they're wet. And that's a bad thing. Then set it down in a
warm place, control panel end down, and let it dry out."
Interviewer: "It seems like a lot of time, cost and effort to develop
a new receiver. Why not just use one that's already available in the
market, like a Tracker or Wildlife Materials or some other and then do
some modification to it?"
DLM: "We've gone down that road before. We used to sell the Wildlife
Materials receivers, the TRX1000, the TRX3 and 16, and they were fine
receivers. We'd re-package them so they looked attractive, added our
collapsible antenna and the pistol grip and so forth. But in the end
it was always compromising one part or the other. We've now been
producing our own receivers for eight or nine years and have learned a
lot. And we just found the opportunity right now to start over yet
again knowing that there are many, many new and spectacular
developments in terms of the core technologies that are here and
available now these days. Wireless technology has just bloomed and
taken off in the last ten years.
It seems like it should be straightforward to design a receiver, and
the basic idea is. But there are so many subtleties involved, little
intricate things. We find out that if we move a certain component over
one half inch we get a dramatic different response because of the
proximity to this other part that's way over there. A wire going this
direction instead of that means it doesn't pick up the magnetic field
of that trace on the circuit board. I couldn't even begin to list all
the different trade-offs. We literally do hundreds and hundreds of
tests on a product like this to find these little things. It takes us
a lot of time, actually.
Designing a new receiver is not something I'd want to go through very
often. We have hundreds of pages of intricate notes and experimental
results of ideas that we tried, and it just takes a huge part of your
life and your brain capacity to go through a project like that. So we
hope to make this new receiver design a core technology to which we'll
add new features in later versions."
Interviewer: "We've talked about the insides, but it's also clear a
lot of thought went into packaging and reducing the size and clever
ways for the user to re-configure it."
DLM: "You know that Marshall Radio Telemetry has always held packaging
very high in importance, and this receiver exemplifies that. It's a
delight to hold, it's a delight to handle and a pleasure to use. And,
to look at. We could have gotten by with a cheaper case with bent
sheet metal and screws sticking out and a much cheaper antenna. But
just take a look around at cars for example. Do you see cars that have
square, metal box shapes? With screws poking out, without paint? Every
car's got paint on it, because that's the minimum acceptable level
that people will even buy. We just hope that's the way customers will
feel about packaging. And then they'll quickly discover that what's on
the inside is just as great..." |
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