Looking Down

CHIRP transducers meet their match in sounder electronics, and the view below comes into sharp focus.

CHIRP-ready transducers like Airmar’s B265
are the key to getting clearer pictures in deep water.

Every winning fisherman depends, to some extent, on perovskite crystals. For that matter so do the rest of us, whether we know it or not.

Perovskite crystal isn’t a new kind of bait, nor is it that glittery stuff that makes lures sparkle. It’s the name given to a naturally occurring mineral and to a whole range of synthetic copies.

The vital property that makes perovskite crystal so important to fishermen and boat owners is that it is piezoelectric. That means that if a crystal is compressed or stretched, it produces an electrical voltage, and if it is subjected to an electric voltage, it expands or contracts. And that is exactly what is required in a sonar transducer.

A sonar amplifier in a sounder produces a short pulse of alternating current electricity—usually at 50kHz (50,000 cycles per second) or 200kHz—and sends it to the transducer. In the transducer, the alternating current makes the piezoelectric crystal expand and contract at the same frequency, producing an ultrasonic “ping.”

When the echo of the ping returns to the transducer, the same process happens in reverse: The crystal vibrates, creating a pulse of alternating current, which the sounder processes, creating either a picture, or a digital display of depth.

Natural perovskite might do the job, but the first refinement, even in a $100 Walmart fishfinder, is to use synthetic crystals, made by grinding the ores of such esoteric metals as zirconium and titanium into a fine powder, mixing them in exact proportions, and baking them in a highly controlled kiln to produce a high-tech ceramic disc or block with a layer of pure silver fused onto both its top and bottom surfaces to provide the electrical contact. 

To make an efficient transducer however, requires more. Most conventional pulsed sonars operate at one or two distinct frequencies, so the ceramic has to be tuned so that it resonates at the right frequency. But “resonating” doesn’t mean ringing like a bell.

Ringing—in this case the tendency for the ceramic to go on vibrating even after the electrical pulse has stopped—is a very undesirable characteristic because it produces indistinct pulses and correspondingly blurred and indistinct images on the sonar screen. Potting the ceramic in epoxy helps dampen this ringing effect and also physically protects the brittle crystal.

Over the years companies such as Airmar, which makes transducers for most of the major marine electronics companies, have worked wonders at achieving the two apparently contradictory targets of optimum resonance with minimum ring, to produce transducers that are very efficient at specific frequencies.

But then along came a new fishfinder technology called CHIRP (compressed high-intensity radar pulse)—also known as Broadband, Spread Spectrum, and Frequency Modulation. 

“CHIRP is a game-changer for professional and sportfishing enthusiasts and is revolutionizing the fishfinder industry,” says Airmar’s V.P. of sales and marketing Jennifer Matsis. She compares it with switching from analog television to HDTV. “Customers currently using CHIRP are reporting detection so exact that certain fish species can be identified at depths never before imaginable,” Matsis says.

The key feature of CHIRP is that instead of using very short pulses of ultrasound, it uses much longer-duration “chirps,” with each chirp sweeping across a wide range of frequencies. The long chirps put as much as 1,000 times the energy into the water when compared to a conventional fishfinder. They also produce clearer pictures in deeper water and at higher speeds, but demand a completely different kind of transducer—one that is designed to resonate across a wide range of frequencies, instead of being optimized for just one or two. (For an explanation of how CHIRP sounders show fish size, visit www.pmymag.com.)

One day, perhaps, piezoceramic technology will have advanced far enough to produce a single ceramic that can cope with the very wide range of frequencies that are used by chirping sonars. But at the moment, a multifrequency chirping sonar requires at least two separate ceramics: one to handle the high frequencies from about 130 to 210kHz, while another—which is more often an array of several matched ceramics—to handle low frequencies of 42 to 65kHz.

Airmar’s  B265LH, for instance, includes eight ceramics: A large one, nearly three inches in diameter, handles the high frequency range while a cluster of seven one-inch ceramics handles the lower frequencies. 

But there’s more to a CHIRP transducer than high-tech ceramics. It’s a simple fact of life that whenever you pass any kind of energy through anything it gets turned into heat, and a transducer is no different. One old problem that has been brought back to the surface by CHIRP is that the impedance of a transducer—its tendency to oppose the flow of electricity—varies depending on the frequency at which it’s operating. That’s important because if the impedance drops, more power flows through the transducer, making it more likely to overheat and more likely, in the long term, to degrade the piezoelectric characteristics of the ceramic. 

For a single-frequency transducer it’s relatively easy to match the impedance of the transducer to the sonar. But matching the sonar to the changing impedance of a CHIRP transducer calls for some far more sophisticated technologies within the transducer itself, including the ability to communicate technical data about the transducer back to the sonar that is pumping power into it.

Another considerable challenge thrown down by the long chirps is the fact that the transducers have less time to cool than their predecessors. The pulses of conventional sonar are so short that a conventional transducer spends less than one percent of its time actually transmitting, compared with about ten percent for a CHIRP transducer. More power flowing for a longer duration makes more heat, which somehow has to be dissipated.

For a product that is surrounded by sea water, getting rid of heat shouldn’t be a problem—unless, of course, the heat is sealed into the nice protective waterproof epoxy potting or shoot-through-hull box.

The problems aren’t insoluble but none of this technology comes cheap, particularly when it is packaged in a guaranteed 100-percent waterproof housing and 40 pounds of bronze casting or a custom-made in-hull tank. The recommended retail price of Airmar’s B265LH through-hull, for instance, is $1,700, while the R509LM is $4,495. Compare those with typical retail prices of less than $200 for the popular dual-frequency but non-chirping B45.

But the B265 is up to 1,000 times more sensitive than the B45, and the R509 is more than 6,000 times more sensitive than the most basic version. That’s a lot of extra bang for your extra bucks.   PMY

Airmar Technology Corp.
(603) 673-9570.

This article originally appeared in the March 2012 issue of Power & Motoryacht magazine.