wrote in message
ps.com...

i guess what i meant was more along these lines: you need to make sure
the peak-to-peak input voltage and current capabilities of the input is
matched with the amp, right? or does the amp have a lot of leeway in
those regards?

Actually, no. Typically the amp doesn't have a lot of flexibility there,
but in general the exciter does.

In amateur practice, the impedance of almost anything that connects together
at RF is 50 ohms or converted to 50 ohms. Given that, the power level then
converts directly to voltage and current.

Typically, HF exciters have a lot of range in output power. The FCC
prohibits the sale of HF power amplifiers having a required drive of less
than 80 watts. Since most HF exciters have 100 watts out, the required
input power of HF amps is rarely specified, since it will almost always
match almost all exciters. I don't think other countries have the same
rules, but the FCC also requires that the amps cannot easily be modified to
accept lower drive, so it is expensive for manufacturers to make amps
differently for different markets, since that typically would require a
complete redesign.

On VHF, it's a little different. Most VHF exciters only have a few output
power settings. VHF amps generally specify the input power, and typically,
it will be a fairly wide range. But still, if you want, say, 200 watts at 2
meters, you would buy a different amp for a 5 watt HT than you would for a
50 watt mobile.

just to make sure i have it straight, if i were to transmit a sine wave
at 146mhz, anyone listening in on 146 mhz wouldn't hear anything
(except maybe less noise than usual). if i were to vary the frequency
between 146.0001 and 145.9999 at a rate of 100hz, then anyone recieving
would hear a quiet 100hz tone. now if i were to vary the frequency
between 146.001 and 145.999 (holding all previous listeners constant),
then anyone listening would hear a much louder tone...correct? or am i
not understanding it yet? ;-)

That is exactly right.

There is a weird behavior of FM receivers called the "capture effect". A
signal of sufficient amplitude at the input of an FM receiver cose to where
the receiver is tuned will totally quiet the receiver. In an AM, CW or SSB
signal, the amplitude is converted to audio almost directly. As a result,
noise on the input appears as noise on the output.

FM is different, though. In FM, we want to hear the frequency modulation.
When there is no signal, we hear the detector randomly wandering around
trying to interpret the noise as signal, but the amplitude of the noise
really has no effect. Once a signal is detected that is strong enough for
the detector to follow, the detector follows the signal and there is no more
noise. To exaggerate this effect, most FM receivers amplify the signal so
much that the amplifiers become saturated and amplitude variation in the
input signal is clipped (well, softly clipped), so that the later stages of
the receiver see the same amplitude. This is different than AM/SSB/CW where
it is the amplitude changes you are looking for.

This is one of the reasons that FM signals have so much higher quality ...
any noise is actually a result of noise in the receiver (or transmitter)
rather than the atmosphere. With other modes, not only can you hear the
atmospheric noise, but at lower frequencies, the atmospheric noise is many
times higher than the noise inherent in the receiver.

The other reason is bandwidth. In amateur practice, FM signals are 5 kHz
wide, compared to about 2.5 for SSB. In commercial practice, FM signals are
wider still.

There is another "gotcha" in your description above. If you vary the
carrier at, say, 1000 Hz, from say, 146.999 to 147.001, the actual bandwidth
will be somewhat wider than you expect, and it will be dependent on the
frequency of the modulation. I know this doesn't make sense, it has to do
with some weird math. If you studied Fourier series back in school it was
some abstract mathematical thing that had nothing to do with the real world.
Well, guess what. Fourier has everything to do with radio! The result is
that to stay within the 5 kHz bandwidth, the highest modulating frequency
has to be somewhat lower than 5 kHz. This is one reason why the FCC
prohibits amateurs from broadcasting music; reasonable fidelity of music
requires higher bandwidth than voice.

...

Look at the specs of your VHF FM Ham rig. It says bandwidth is
16K0F3. That's 16 kHz for 5 kHz of Dev. at 1 kHz modulation
frequency.

So to stay within 5 kHz bandwidth you need to reduce the audio
bandwidth and dev A LOT less than 5 kHz dev.

Consider 900 MHz and other 12.5 kHz channel radios that are 10K0F3
and sound like dirt.

There is another "gotcha" in your description above. If you vary
the
carrier at, say, 1000 Hz, from say, 146.999 to 147.001, the actual
bandwidth
will be somewhat wider than you expect, and it will be dependent on
the
frequency of the modulation. I know this doesn't make sense, it
has to do
with some weird math. If you studied Fourier series back in school
it was
some abstract mathematical thing that had nothing to do with the
real world.
Well, guess what. Fourier has everything to do with radio! The
result is
that to stay within the 5 kHz bandwidth, the highest modulating
frequency
has to be somewhat lower than 5 kHz. This is one reason why the
FCC
prohibits amateurs from broadcasting music; reasonable fidelity of
music
requires higher bandwidth than voice.

..

thanks again for the help...your answers led me to a couple other
questions, if i may ask?

xpyttl wrote:

In amateur practice, the impedance of almost anything that connects
together
at RF is 50 ohms or converted to 50 ohms. Given that, the power
level then
converts directly to voltage and current.

so if i were to design a transmitter around an COTS amp, how would i
determine what i needed to "drive" it at, in terms of voltage? if i
have 3 variables (V, I, and R) then i'd need to know 2 of the 3, then i
can solve for the third... as it is, i only know R...?

This is one of the reasons that FM signals have so much higher
quality ...
any noise is actually a result of noise in the receiver (or
transmitter)
rather than the atmosphere. With other modes, not only can you hear
the
atmospheric noise, but at lower frequencies, the atmospheric noise is
many
times higher than the noise inherent in the receiver.

so i imagine people can do CW over FM, but does it usually occur on (I
guess what would be called) a simplified AM transceiver? either a
signal's present or not?

also, when people refer to ASK and FSK, is ASK basically an audio
output that's been sent over FM? and FSK is more like CW, only to a
receiver that can monitor both mark and space (0 and 1) frequencies?

thanks again!
jason

On 13 Mar 2005 15:18:03 -0800, wrote:



also, when people refer to ASK and FSK, is ASK basically an audio
output that's been sent over FM? and FSK is more like CW, only to a
receiver that can monitor both mark and space (0 and 1) frequencies?

thanks again!
jason

always thought amplitude-shift-keying was mainly used for morse
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

"J M Noeding" wrote in message
...
On 13 Mar 2005 15:18:03 -0800, wrote:



also, when people refer to ASK and FSK, is ASK basically an audio
output that's been sent over FM? and FSK is more like CW, only to a
receiver that can monitor both mark and space (0 and 1) frequencies?

thanks again!
jason

always thought amplitude-shift-keying was mainly used for morse
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

I believe he meant AFSK (audio frequency shift keying) vs FSK (frequency
shift keying).

Dee Flint wrote:
"J M Noeding" wrote in message
...
On 13 Mar 2005 15:18:03 -0800, wrote:



also, when people refer to ASK and FSK, is ASK basically an audio
output that's been sent over FM? and FSK is more like CW, only to a
receiver that can monitor both mark and space (0 and 1)
frequencies?

thanks again!
jason

always thought amplitude-shift-keying was mainly used for morse
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

I believe he meant AFSK (audio frequency shift keying) vs FSK
(frequency
shift keying).

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

jason

In article .com,
wrote:

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

That's pretty close, but not exactly on target.

AFSK can be used on top of either AM or FM. In amateur systems it's
most commonly used with FM.

FSK shifts the carrier frequency... a process which counts as a
modulation. The amount of frequency shift (in Hz) and the rate at
which you shift it (also in Hz) vary a lot from one usage to another.
There are both narrow-band and wide-band FSK modulations in use.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
In article .com,
wrote:

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

That's pretty close, but not exactly on target.

AFSK can be used on top of either AM or FM. In amateur systems it's
most commonly used with FM.

FSK shifts the carrier frequency... a process which counts as a
modulation. The amount of frequency shift (in Hz) and the rate at
which you shift it (also in Hz) vary a lot from one usage to another.
There are both narrow-band and wide-band FSK modulations in use.

thanks! also, i was just reading about mixers... and getting confused.
i've run some matlab programs to figure out how you can take something
at frequency x and frequency y, and end up with an output of frequency
(x-y). one transmitter's mixer i saw took a 160mhz and 14mhz input to
get a 146mhz output. adding or multiplying them together certainly does
not yeild 146mhz. so how does that work?

thanks for the help!
jason

"Dave Platt" wrote in message
...
In article .com,
wrote:

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

That's pretty close, but not exactly on target.

AFSK can be used on top of either AM or FM. In amateur systems it's
most commonly used with FM.

That is not correct either. AFSK is used extensively with Amateur Radio
digital systems, such as PSK-31, RTTY, even CW can be and is generated
using AFSK. And the modulation METHOD is not important. It can be either
Amplitude or Frequency Modulation.

Dan/W4NTI


FSK shifts the carrier frequency... a process which counts as a
modulation. The amount of frequency shift (in Hz) and the rate at
which you shift it (also in Hz) vary a lot from one usage to another.
There are both narrow-band and wide-band FSK modulations in use.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

I believe he meant AFSK (audio frequency shift keying) vs FSK
(frequency
shift keying).

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

jason


You are partly correct. If audio tones are fed into the microphone input
(audio stages) of an AM or FM transmitter you are generating AFSK. If an
audio tone is fed into a SSB transmitter it will generate a single output
frequency. If the tone is switched off and on , it will be received as a CW
transmitter being keyed off and on. Some newer transceivers generate CW
this way. If the tone is shifted in frequency , the output will be a
carrier shifted the same ammount. This is received the same as a FSK
transmitter. You can get the same effect if you have a crystal controled
transmiter and switch a capacitor across the crystal to change the
frequency.
There is almost no way to tell the differance between the two. If 100% pure
tones are used and everything else it 100% correct then there is no
differance in the transmitted signals. As always nothing is 100% and you
can tell the differance with very close and strong signals. This is still
close enough it does not matter.