duty cycle calcutation

japer

hello

i am a little confused about calculating duty cycle
should i measure duty cylce from inverse to inverse or should i measure at the inverse
also should the measurement be inside the frequency cycle

thanks for any help
japer

Jon Williams

Duty cycle is the ratio of on-time to overall cycle time.· If you have a square wave that is on for 10 milliseconds and off for 30 milliseconds, the duty cycle is 25%:

· 10 / (10 + 30) = 0.25 --> 25%

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Jon Williams
Applications Engineer, Parallax

japer

thanks jon

i though that was the way. to the inverse

and the frequecy would be the amount of time it repeats itself within 1 second ?

thanks alot
japer

Jon Williams

Yes. If you take the cycle time and divide it into 1 you will get the frequency in Hertz. Using my example above it would be.

1 / 0.04 (secs) = 2.5 Hz

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Jon Williams
Applications Engineer, Parallax

japer

whew sigh of relief
thanks jon, that exacty the figure i got
the timming seems to be very important and the next chapter
will proably expand on these inputs/outputs
thanks for your input
japer

japer

nope im wrong
i come up with 25 hz for a 40 ms cycle and 2.5 hz at a 400 ms cycle
more work needed on my part

japer

Jon Williams

Nope, you're right -- I missed the decimal point by one (should use a calculator instead of my old brain).

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Jon Williams
Applications Engineer, Parallax

T!

Also, since duty cycle is a ratio, it can be expressed in dB.· This expression is always negative (since it would be impossible to have a ratio of greater than 1) but the negative sign is often dropped.· Not that dropping the sign is correct, only that it is often done.

So, a -3 dB duty cycle might just be called "3 dB", and would indicate a ratio of 0.5 : 1.0, or a true square wave with on and off times being equal.· A 6 dB duty cycle would be the afore mentioned 25% duty cycle, while 10dB would be a ratio of 0.1 to 1, while 20 dB would be 0.01 : 1 and 30 dB would be 0.001 : 1.

Admittedly this representation is most often used in the world of radar, but it is applicable elsewhere also.· If one is not familiar with this application a fair introduction can be found here:· https://ewhdbks.mugu.navy.mil/dutycy.pdf

T!

steve_b

I work on Weather RAdars and we don't commonly use dB as a way to express duty cycle ratios....however, they are directly proportional (or indirectly?? haha)

Most birdwatt-type meters measure average power.
So lets say in our radar that's running a 1.6uS pulse width at a Pulse Repetition Frequency (PRF) of 600Hz we would get a power meter reading of about -3.18dBm.
IF we throw in our waveguide/couple losses (~56.24) we end up with a net power of 52.62dBm (-3.18 - 56.24). This is equal to 182Watts average power.

So if we use the following calculation (thank god for look-up tables): Duty Cycle Correction = 10LOG (1/(PRF x PW)) which, in this case, would work out to -30.18dBm.
We then subtract the DCC from the Average power (dB) measurement: 52.62-30.18 (ignore sign) = 82.8dBm which is equal to 190,427Watts peak.

When our pulse changes to 0.8uS @ 1200 PRF our DCC is still ~30.18....so when we change pulses we try to keep the same output power so we don't outgass our magnetron.

Sorry....I'm at work and am emersed in the jargon....sorry for rambling!

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·

Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

japer

wow
this is so above my comprehension
i just dont kow what to say


japer

T!

steve_b said...
I work on Weather RAdars and we don't commonly use dB as a way to express duty cycle ratios....however, they are directly proportional (or indirectly?? haha)

Most birdwatt-type meters measure average power.

True, many Birds do show power as Wattage, and they are (for the most part) not peak power meters, so they show average power.· But average power can also be expressed in dBm and some Birds do this native.· And try something like an HP 436 or a 437, then you can get the readings directly in dBm, making things so much easier.· Or Ergs, or Watts, or picoWatts, or whatever you want.· My switches always stay in dBm [noparse];)[/noparse]

So lets say in our radar that's running a 1.6uS pulse width at a Pulse Repetition Frequency (PRF) of 600Hz we would get a power meter reading of about -3.18dBm.

This PW and PRF would equal a duty cycle of -30.18 dB, as you point out later.

This means the peak power for this meter reading would be +27 dBm, or 500 mW.

IF we throw in our waveguide/couple losses (~56.24) we end up with a net power of 52.62dBm (-3.18 - 56.24). This is equal to 182Watts average power.

This loss, as are most losses, is expressed in dB.· Staying dB to dB makes it all easy, and that is why duty cycle is in dB for many radar applications.· Otherwise the average power in the system ( not counting couplers or WG loss ) would have to be expressed as something like 420.73 Watts and the raw ratios ( .00096 : 1·)multiplied against that to find the other answer.· Much easier to add and subtract than multiply and divide.

So if we use the following calculation (thank god for look-up tables): Duty Cycle Correction = 10LOG (1/(PRF x PW)) which, in this case, would work out to -30.18dBm.
We then subtract the DCC from the Average power (dB) measurement: 52.62-30.18 (ignore sign) = 82.8dBm which is equal to 190,427Watts peak.

Exactly, Average meter reading (in dB) of -3.18, added to systemic losses of -56.24, plus duty cycle 30.18 dB yields your peak power in the waveguide.· Your 82.8 dBm.· Now, you can convert to whatever format required, dBW, Watts, etc.· Working without duty cycle in dB would have cause more math.· Combine that with working with your losses in a ratio, not dB, and it gets more complex.· Now add in the antenna gain to get to ERP, again, easy to do in dB, more complex in ratios.· Now, an antenna gain of 36 dB plus the peak power of 82.8 dBm would yield an ERP of 118.8 dBm, or about 756 MW.· Almost real power.
·
When our pulse changes to 0.8uS @ 1200 PRF our DCC is still ~30.18....so when we change pulses we try to keep the same output power so we don't outgass our magnetron.

Naturally, half the pulse width and twice the PRF will yield the same duty cycle.· However, the magnetron filament derate voltage may not be the same due to how the magnetron reacts to the extended PW.· In this particular case I would bet they are the same, but if you went to something like 6.4uS and 150 Hz may require some adjustments in filament programming.

Sorry....I'm at work and am emersed in the jargon....sorry for rambling!

In the majority of radar work I have seen almost all values are expressed in dB for ease of transport.· I also have worked with weather radars, as well as precision tracking instrumentation systems, fire control, and acquisition / EW.· Its all good ;-)· ANd I also tend to get carried away, and we have taken this thread pretty far afield.

T!

Bean

Correct me if I wrong, but I think we are confusing people because expressing duty cycle as (dB) is completely different than measuring output power in (dBm).

(dB) is a ratio of (high_time to total_time) example -3dB = 50%
(dBm) is a ratio of (power to 1milliwatt) example 0dBm = 1milliwatt

Bean.

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·

T!

Bean (Hitt Consulting) said...
Correct me if I wrong, but I think we are confusing people because expressing duty cycle as (dB) is completely different than measuring output power in (dBm).

This is true, and it was also the core of my original posting....that duty cycle can easily be expressed in dB, as can any ratio.· I also said in that post that this is most commonly done in the world of radar, although you do find it in other applications.

(dB) is a ratio of (high_time to total_time) example -3dB = 50%
(dBm) is a ratio of (power to 1milliwatt) example 0dBm = 1milliwatt

Bean.

Absolutely correct, they are both ratios, but without the reference (the m in your 'dBm') it is not defined as a ratio to anything specific.· This is useful for any ratio, all you have to do is identify the units involved, be they Volts (dBV), Watts (dBW), milliVolt (dBmV) or anything else you need.· As an example some stereo radio sensitivity is expressed in dBf, or dB femtoWatts, so that something rated at 3 dDf would have a sensitivity of 2 femtoWatts, or 0.000000000000002 Watts, or 0.2739 micro Volts into a 75 Ohm load.· It can be seen that 3 dBf is far easier to express.· One of my favorites is dB·Dollars (dB$).· I have actually used this in papers before, gotta have some fun with it.

It could be said that the $54 Basic Stamp 2e cost 17.324 dB$, while a $35,000 car cost 45.441 dB$, and a Million Dollar ($1,000,000) home would be 60 dB$.· As is evident the log function of dB makes large numbers easier to express and possibly (depending on circumstances and application) easier to handle.

Being rather new to these forums I hope I have not unintentionally committed a faux pa by taking the discussion on a slight tangent....I am sure someone will let me know if I did

T!