From: jab@hpuerca.atl.hp.com (Alan Barrow)
Subject: Re: What fish finder
Date: Wed, 16 Dec 1992 02:18:02 GMT
Organization: Hewlett-Packard NARC Atlanta

In <1992Dec14.172352.17968@ncsu.edu> plheron@eos.ncsu.edu (Pat L. Heron) writes:

>Allan, thanks for the refresher course in rudimentary arithmatic. 
>The specifications for sonars describe power consumption by the 
>device, NOT power into a matched load such as in audio amplifiers. 

My Interphase Sportsman power rating includes both a peak and rms value
for output power. This has nothing to do with power consumption. The
current drain they report in Amps. Before purchasing the unit, I made
a matrix of my top contenders and compared them in several areas.
Of the Hummingbird, eagle, interphase, and Lowrance units I considered,
all reported *output* power in both rms and peak watts. Power drain was
reported in amps. I believe you are off base here!

>The sonar transmits a FIXED pulse width FIXED modulation frequency 
>FIXED pulse rate signal into the water. (using a wider pulse would 
>increase the ambiguity in determining depth because time-in-flight 
>is used to determine depth, so pulse width is not changed).  To put 

Bzzzzt!!!! Wrong answer. My Interphase sportsman varies the pulse width
as well as amplitude (power) at increased depths. The explanation I rcd
was that at deeper depths it allows for better noise immunity.

>more power into the water, AMPLITUDE is increased.  The actual width
>of the pulse is quite narrow 

Amplitude is increased, but so is the pulse width. All the pulses are
"quite narrow".

>(narrower in time than required for the sound wave to 
>travel 1/2 the minimum distance that can be discriminated by the unit).

>The bottom line (me pun?) is that, since the unit's internal electronics
>can only consume an inifinitesimal quantity of energy during the short 
>time that the transduces is transmitting.  Thus 

>PEAK POWER IS AN EXCELLENT INDICATION OF THE AMOUNT OF 
>ACCOUSTICAL ENERGY A UNIT CAN PRODUCE.  In fact, it is the only 

I agree with this point. I did in my earlier posting. But a sonar that
had a 1kw peak pulse that occured less than once a minute would be
useless. Thus the rms value is reported, but not usually needed.

A fantastic peak with a low rms value would indicate a lower duty cycle
(less pulse on time VS off time). This would impact display update rate,
etc. I agree, not usually an issue on main brand finders, but it *is*
reported to be accurate.

>measure that the consumr has. Implicit in this reasoning is 
>the assumption that all unit's efficiencies are similar.

Invalid assumption. There is a big difference in performance between
units oriented towards fresh water use, and those for salt water.

Salt water attenuates the pulse more, and usually needs higher power,
better processing, etc. You will rarely see a hummingbird bass finder
on serious saltwater boats!

>|> You could have very high peak output, but a very low duty cycle.
>|> Likewise a high duty cycle (lots of pulses that were low power.)

>nope,

Yes you can. For the same rms rating, there is an infinite combination
of peak power, duty cycle, and waveform. You will have to do more than
say nope. 

>then you are ignorant of sonar specifications, unlike home audio 
>equipment, output power is not specified.

Output power is the "ACCOUSTICAL ENERGY" (your terminology) a unit
produces. It is reported in peak and rms watts.

>|>The frequency also has something to do with it, balancing
>|> reduced attenuation VS higher resolution. (Shorter wavelengths from
>|>higher frequencies allow higher resolution.)

>and this statement proves it, the modulation frequency is fixed.

Different units have different pulse frequencies. Some have more than
one. Lower frequencies work better at deeper depths, but allow less
resolution. (Higher frequencies attenuate faster) 

(Hummingbird make a hi freq unit that works fine for bass that sucks for
saltwater. My interphase lo freq unit works great for 300' saltwater but
is not as crisp at 20' as the hummingbird on the lake.)

Interphase make a dual freq unit to take advantage of both.

The "frequency" we are referring to is the modulation freq of the pulse.
The transponder is tuned to this freq. 

The pulse rate is a much lower number and is the number of pings /
second. As the depth increases, the pulse rate can be reduced, to allow
for return time.

So the transponder frequency is fixed, but the pulse rate and width (and
thus duty cycle) can vary.

Let me confuse you more. Conservation of power dictates that the power
emited by a device cannot be more than the power that goes into the
device. (Averaged over time.) There are no 100% efficient devices, so in
reality, the average output power is much lower. (Often 50% ) It also has to do
with how much distortion you are willing to live with. Given that we are
dealing with square waves, most finders probably use Class C amps. We
will be liberal, and assume they are 75% efficient.

Thus:
  12v x 3A= 36 watts   
            36 w * 75%= 27w average power output *max*

Using an 800w peak, the duty cycle can be determined by:
            27 / 800 = .0338  or 3.38% 

Thus the total of all pulses are only for 3.38% percent of the time. A
more efficient unit could have a higher rms output with the exact same
peak output. It would just have a higher duty cycle.

All that said, peak power is a pretty good measure, but rms *does* tell
you something useful. (And it is not the DC power consumption of the
unit!  :->  )


 Alan Barrow  km4ba | I've seen things you people wouldn't believe. Attack
 jab@atl.hp.com     | ships on fire off the shoulder of Orion. I watched
                    | C-beams glitter in the dark near the Tannhauser gate.
 ..!gatech!kd4nc!   | All those moments will be lost in time -
         km4ba!alan | like tears in rain. Time to die.          Roy Batty
Newsgroups: alt.fishing