Transmitter maximum deviation question

[ke7vvt]

Most all mobile radios have this specification for the transmitter maximum deviation, +/-5kHz. I want to find out the range of frequencies that a transmitter will cover in addition to the tuned frequency. What information in the datasheets says how much that deviation is at a given power output?

For example, national calling frequency is 146.520 MHz. Tuning the transmitter to that frequency causes the main power output to peak at that 146.520 MHz, and it falls off exponentially (I assume) at nearby frequencies. At +/- 0.005 MHz, 146.515 and 146.525 MHz, power = ?. At +/- 0.01, 146.510 and 146.530 MHz, power = ?. And of course, a higher transmit power causes a greater deviation than a lower power, and hopefully the shape of the peak will be nearly the same just with amplitudes differing.

Maybe "maximum deviation" is not what I am looking for, it might be called another word.

TRANSMITTER
RF Power Output:
55/25/10/5 Watts
Modulation Type:
Variable Reactance
Maximum Deviation:
±5 kHz / ±2.5 kHz
Spurious Radiation:
Better than -60dB

With this information, assuming it is the maximum deviation, what that means to me is that at maximum power (55W) there is a maximum deviation of +/- 5kHz, which happens at 0W. So I could make a graph Power vs Frequency (x) and put a dot at (x,y) (146.520Mhz, 55W) and dots on the sides (146.515MHz, 0W) and (146.525MHz, 0W). If that is correct (that the +/- 5kHz deviation is relative to 0W), then I could continue fit the power output curve as a function of frequency to a Gaussian curve to find out what other frequencies and at what power I would be spilling over to. As opposed to just a square fit, I doubt the power out deviates at full power (55W) across the +/- 5kHz and go straight to 0W there after.

Edit: Sorry, another specific question. Is that maximum deviation relative to 0W or -3dB, -6dB, -60dB or some other "common" reference point? Thanks!

Specifically the radio I am interested in is the Yaesu FT-1900R, but most any other radio has this same "maximum deviation" specification.
Yaesu FT-1900R Specifications

Thanks!
john

 

[Captain Kilowatt]

I believe that you have no idea whatsoever what the term deviation means as it pertains to a transmitter. :blink:

The term deviation means how far from the carrier frequency an FM transmitter will deviate, or change, when modulation is applied.It has NOTHING to do with power output. +/- 5 KHz is considered normal FM while +/- 2.5 KHz is called Narrow FM. If a carrier was on 146.520 MHz and the deviation was +/- 5 KHz then the frequency of the transmitted signal at 100% modulation would constantly vary with modulation from 146.515-146.525 MHz.

 

[ke7vvt]

Ah, thanks Captain. That that makes sense. I must be looking for something else then. Do you know what power output vs "carrier" frequency is so I can find that specification?

 

[mackmobile43]

The more deviation to a point dictates how loud/distorted the tx is.

 

[hookedon6]

...For example, national calling frequency is 146.520 MHz. Tuning the transmitter to that frequency causes the main power output to peak at that 146.520 MHz, and it falls off exponentially (I assume) at nearby frequencies. At +/- 0.005 MHz, 146.515 and 146.525 MHz,...

it's not a function of frequency.
as long as the rig "sees" 50 ohms, it really doesn't care what freq its on.

max power transfer will occur @ 50 ohms

 

[Captain Kilowatt]

Ah, thanks Captain. That that makes sense. I must be looking for something else then. Do you know what power output vs "carrier" frequency is so I can find that specification?

The only thing I can think of that fits that is the term "power bandwidth" and you will never find that specification quoted in a piece of ham gear or most anything else either. I am beginning to wonder if you are thinking about "occupied bandwidth" which is how the transmitter's RF power is spread out from the center carrier frequency. Usually the specs simply state the bandwidth of the transmitted signal and give a reference as being "X" number of dB down at "Y" KHz away from the main carrier freq. This is most often quoted in the RX section to indicate how well a receiver will reject signals close to the desired signal.It is also applicable to transmitters as well as transmitters also have a specific bandwidth of the transmitted signal but it usually is not quoted as it depends on mode. As long as the transmitter meets FCC guidlines for transmitted bandwidth for the mode it is set for things are fine. That is something the manufacturer has to worry about unless the operator gets out the golden screwdriver and plugs into a rack full of audio processing equipment. It could also happen if something were to go bad with the radio but it is something that would be readily apparent as people on adjacent frequencies would be quick to let you know you are QRM'ing them.

 

[ke7vvt]

The only thing I can think of that fits that is the term "power bandwidth" and you will never find that specification quoted in a piece of ham gear or most anything else either. I am beginning to wonder if you are thinking about "occupied bandwidth" which is how the transmitter's RF power is spread out from the center carrier frequency. Usually the specs simply state the bandwidth of the transmitted signal and give a reference as being "X" number of dB down at "Y" KHz away from the main carrier freq.

Yes, exactly what I want to know.

This is most often quoted in the RX section to indicate how well a receiver will reject signals close to the desired signal

I got that part for the signal isolation on the receiver, using this spec for wide fm,

RECEIVER
Circuit Type:
Double-Conversion Superheterodyne
Intermediate Frequencies:
21.7 MHz and 450 kHz
Sensitivity (12dB SINAD):
Better than 0.2uV
Selectivity: (-6/-60 dB):
12 kHz/28 kHz

What I was hoping for was a very similar specification but for the transmitter, like you said.

It is also applicable to transmitters as well as transmitters also have a specific bandwidth of the transmitted signal but it usually is not quoted as it depends on mode. As long as the transmitter meets FCC guidlines for transmitted bandwidth for the mode it is set for things are fine. That is something the manufacturer has to worry about unless the operator gets out the golden screwdriver and plugs into a rack full of audio processing equipment.

OK, so I am not going to find the power bandwidth specification in any manual, and if I do want to figure it out I have to test it with a bunch of expensive equipment (golden screwdriver and audio processing equipment)?

Thanks Capt.

 

[Captain Kilowatt]

What I am saying is that there is no reason to quote power bandwidth in any specs for radio gear. The specs spell out what the power output is and that power should remain pretty much constant across the entire band unless something is drastically wrong.If the radio is opened up to allow out of band TX then the power output generally drops off on the high end of that range and it varies from one radio to the next. The specs are guarenteed for the ham band freqs. ONLY.

 

[W5LZ]

And what wasn't said is that unless you have the required equipment to do that 'measuring' (probably the same price range as the purchase price of your house) there is no way of determining, or of 'improving', those 'specs' within reason for the average 'user'. You certainly can determine if those 'specs' are 'true', but it typically requires having a very good banker friend. (One reason why all this stuff is so expensive.)
- 'Doc

 

[dudmuck]

What I was hoping for was a very similar specification but for the transmitter, like you said.



OK, so I am not going to find the power bandwidth specification in any manual, and if I do want to figure it out I have to test it with a bunch of expensive equipment (golden screwdriver and audio processing equipment)?

Thanks Capt.

What you are asking concerns modulation index and bessel functions.
You can google around for fm bessel sideband and whatnot.
You will find that FM actually has an infinite number of sidebands, but only a limited number of them are significant. Precisely what the bandwidth is depends on the maximum audio frequency and the FM deviation, which is called modulation index.
To save you from math headache: It is widely regarded that amateur vhf/uhf transmitters occupy 16KHz of significant bandwidth, 8KHz on each side.

 

[C2]

Some of the terms I can think of that you may be interested in are:

Phase noise
Harmonically related spurs (level relative to the carrier, dBc)
Non-harmonically related

Deviation
Bandwidth (usually 3dB)
Flatness
Accuracy
Harmonic Distortion
Sensitivity
Incidental AM

related to bandwidth is also frequency response.

 

[ke7vvt]

Thank you for all of the helpful responses. I will investigate these new words.

I have another question, is there something special about 3dB? I understand that it is "half", but is it a common reference for any of these terms? I am wondering if the selectivity specification is +/- 5kHz at 3dB instead of 0dB...

Thanks again.

 

[ke7vvt]

It is widely regarded that amateur vhf/uhf transmitters occupy 16KHz of significant bandwidth, 8KHz on each side.

After +/- 8kHz is the power effectively zero?

 

[heyu]

Thank you for all of the helpful responses. I will investigate these new words.

I have another question, is there something special about 3dB? I understand that it is "half", but is it a common reference for any of these terms? I am wondering if the selectivity specification is +/- 5kHz at 3dB instead of 0dB...

Thanks again.

+/- 5kHz looks like the Deviation spec.

I think you said that the Selectivity was:

-6/-60 dB
12 kHz/28 kHz

This is for the receiver and means that at 12kHz (+/- 6kHz from center carrier frequency) wide, the received signal is attenuated by 6dB from what is received within the pass band. at 28kHz, the signal is attenuated by 60dB. This is fairly standard for selectivity and 3dB is not generally used. These numbers describe the "shape" of your RF receiver's filters. You can also calculate the shape factors using the number: 28/12 or 2.33:1 (-6/-60dB).

3dB is used often for items such as AM rolloff, frequency response, filter bandwidth, etc. Half power is sort of a special circumstance for a variety of reasons, not just because it is half. But that works well too.

 

[heyu]

After +/- 8kHz is the power effectively zero?

No. But, practically yes.

What is being spec'd is basically the result of Carson's rule.

Carson's rule can be used to estimate the occupied bandwidth of an FM signal. It states that the bandwidth is twice the sum of the peak deviation plus the highest modulating frequency.

BW = 2 * (Dmax + Fmax)

Dmax is the peak frequency deviation

Fmax is the highest modulating frequency

The occupied bandwidth for voice (typically ranging from 400Hz to 3kHz) with a peak deviation of 5kHz would be 16kHz.

In reality, a good receiver can detect signals all the way down to around -157dBm, but practically most radios will have too much noise and signal filtering for any signals outside this range to matter.