hello, i'm relatively new to ham radio and had a few technical
questions i was hoping to throw out...anyone that can answer these
would be very helpful!

1. i've seen transmitter schematics that were simple, and others that
were complex. as a general rule of thumb, are the more complex ones
trying to compensate for frequency drift, or maybe eliminate higher
harmonics? how efficient and/or stable are the simple transmitter
schematics?

2. other than frequency range, what characteristics are you concerned
about when trying to match a transmitter to an amp?

3. without an amp, couldn't you still run the signal to an antenna and
it would be a weak transmitter? all the amp does is increase the
voltage and current supplied to the antenna, correct?

4. for an FM transmitter, does the modulation occur to oscillator
directly, or is the oscillator's signal modulated after "leaving" the
oscillator? i guess what i'm asking is whether or not there is an input
to the oscillator, or is it just an "output only" frequency generator?


thanks!
jason

Hi Jason

Nice questions, let me try to answer a few ..

wrote in message
ups.com...

1. i've seen transmitter schematics that were simple, and others that
were complex. as a general rule of thumb, are the more complex ones
trying to compensate for frequency drift, or maybe eliminate higher
harmonics? how efficient and/or stable are the simple transmitter
schematics?

One obvious thing is that CW transmitters tend to be simple, SSB
transmitters complex. But there are a thousand design variables. One big
one is the complexity of the ICs employed. Today you can have a very stable
VFO with just a few parts. You tend to pay a little bit of a price in phase
noise, but frequency drift is not an issue. With an analog VFO, you can add
a lot of complexity trying to get around frequency drift, but phase noise is
never an issue. Years ago, all you had was analog. A few years ago, DDS
(direct digital synthesis) was complex and expensive. Today, analog VFOs
tend on the expensive side! It is similar with amplifiers. In many radios,
all, or most, of the PA is in a single brick, instead of a fistfull of
parts. Ditto with almost everything up and down the chain.

Frequency is also an issue and again that is changing with technology. A
few years ago, it was hard to get directly to VHF. You typically had
several oscillators getting mixed up, frequency multiplied, etc. This was
especially true if you had an analog VFO because it is very hard to get
stability at VHF, and multiplying the frequency also multiplies the drift in
an analog VFO. There are still reasons you might want to do some mixing up
to get to VHF with a DDS VFO, but DDS parts up into the gigahertz range are
now cheap parts.

It was only a few years ago that a DDS VFO cost hundreds of dollars. Today
you can buy a chip with a VHF synthesizer and amplifier and modulator for
less than the tuning capacitor in an analog VFO.

2. other than frequency range, what characteristics are you concerned
about when trying to match a transmitter to an amp?

If you are buying commercial, you are looking at price, of course, and
expected reliability, along with power consumption. For SSB, you need the
amplifier to be linear, which implies lower efficiency. For FM/CW you don't
need linear, so the amp can be a lot more efficient. If you are designing
the amp, then you are worrying about impedance mathcing, as well.

3. without an amp, couldn't you still run the signal to an antenna and
it would be a weak transmitter? all the amp does is increase the
voltage and current supplied to the antenna, correct?

Yes of course. There are times when you want a lot of power, but most of
the time it really isn't necessary. Also, it is a lot easier to get antenna
gain than power to the antenna, especially at higher frequencies. The need
for power depends a lot on what you do, what frequencies you operate, and to
a degree, what "floats your boat". If you are doing EME or HSMS, you need a
fair bit of power - hundreds of watts, anyway. If you are chatting on the
local repeater, typically a watt is as good as a kilowatt. Lots of folks
called QRPers like to use very low power. For them the "legal limit" is
five watts, but many try to see what they can do with milliwatts. The
current miles/watt record is held by a guy who operated 40 microwatts over a
500+ mile path.

4. for an FM transmitter, does the modulation occur to oscillator
directly, or is the oscillator's signal modulated after "leaving" the
oscillator? i guess what i'm asking is whether or not there is an input
to the oscillator, or is it just an "output only" frequency generator?

Typically you would modulate the oscillator, although these days, the audio
may well be simply data to the synthesizer. However, because FM is
typically done at VHF and higher, there may well be additional oscillators
mixed with the modulated signal to get up into the VHF/UHF range.

Hope this helps

...

xpyttl wrote:
Hi Jason

Nice questions, let me try to answer a few ..

wrote in message
ups.com...

1. i've seen transmitter schematics that were simple, and others
that
were complex. as a general rule of thumb, are the more complex ones
trying to compensate for frequency drift, or maybe eliminate higher
harmonics? how efficient and/or stable are the simple transmitter
schematics?

One obvious thing is that CW transmitters tend to be simple, SSB
transmitters complex. But there are a thousand design variables.
One big
one is the complexity of the ICs employed. Today you can have a very
stable
VFO with just a few parts. You tend to pay a little bit of a price
in phase
noise, but frequency drift is not an issue. With an analog VFO, you
can add
a lot of complexity trying to get around frequency drift, but phase
noise is
never an issue. Years ago, all you had was analog. A few years ago,
DDS
(direct digital synthesis) was complex and expensive. Today, analog
VFOs
tend on the expensive side! It is similar with amplifiers. In many
radios,
all, or most, of the PA is in a single brick, instead of a fistfull
of
parts. Ditto with almost everything up and down the chain.

Frequency is also an issue and again that is changing with
technology. A
few years ago, it was hard to get directly to VHF. You typically had
several oscillators getting mixed up, frequency multiplied, etc.
This was
especially true if you had an analog VFO because it is very hard to
get
stability at VHF, and multiplying the frequency also multiplies the
drift in
an analog VFO. There are still reasons you might want to do some
mixing up
to get to VHF with a DDS VFO, but DDS parts up into the gigahertz
range are
now cheap parts.

It was only a few years ago that a DDS VFO cost hundreds of dollars.
Today
you can buy a chip with a VHF synthesizer and amplifier and modulator
for

Good info to know. I was kinda looking around to make a (mostly) IC
transmitter like that... (anything 70cm and under). Are you aware of
any chip PN's or schematics I could dive into to learn?

Thanks,
Dave

"dave.harper" wrote in message
ups.com...

Good info to know. I was kinda looking around to make a (mostly) IC
transmitter like that... (anything 70cm and under). Are you aware of
any chip PN's or schematics I could dive into to learn?

Analog Devices' parts are favored by hams. The greatest number of projects
out there use the AD9850, but that part is a little older, and does not
provide modulation input. Also, it is really only good up to about 30 MHz,
you can push a bit past that but the power requirements explode and the
noise goes up. Without any component changes it will go down to 1 Hz.
Newer AD98xx parts are starting to get more attention, and some of those are
useful up into the low 100's of MHz. The 99xx parts have more features yet.

TI has a TRF4400 which is a 440 MHz synthesizer with modulation and an amp
(not much of an amp, tho!), and the almost identical 4900 for 900 MHz. Some
folks have used these down to 6 meters. The TI parts have greater channel
spacing than the AD parts, but that also reduces the frequency of the
crystal or oscillator, which has a big effect on the power consumption. TI
also has a 2050/1/2 which are synthesizers (I don't recall if they have
modulation) up into the gigahertz range. I've been toying with using one of
the TI parts as a VFO for an HF rig by using a prescaler to divide down the
frequency (and consequsntly the spacing).

The AD parts are pricey, but AD is very good about samples. The TI parts
are all in the five buck neighborhood. There are a zillion projects/kits
out there based on the AD parts.

If you've ever built an analog VFO, these DDS parts are like black magic.
They take little in the way of support circuitry, no fretting about layout,
no spending weeks trying to work out the temperature compensation, no moving
around because you breathed on it, you just dial in the frequency you want
and you are spot on.

...

hey, thanks for the info. that was very helpful! as you probably
expected, i would like to ask a couple of follow-ups on 2 of the
questions:

xpyttl wrote:

2. other than frequency range, what characteristics are you
concerned
about when trying to match a transmitter to an amp?

If you are buying commercial, you are looking at price, of course,
and
expected reliability, along with power consumption. For SSB, you
need the
amplifier to be linear, which implies lower efficiency. For FM/CW
you don't
need linear, so the amp can be a lot more efficient. If you are
designing
the amp, then you are worrying about impedance mathcing, as well.

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?

4. for an FM transmitter, does the modulation occur to oscillator
directly, or is the oscillator's signal modulated after "leaving"
the
oscillator? i guess what i'm asking is whether or not there is an
input
to the oscillator, or is it just an "output only" frequency
generator?

Typically you would modulate the oscillator, although these days, the
audio
may well be simply data to the synthesizer. However, because FM is
typically done at VHF and higher, there may well be additional
oscillators
mixed with the modulated signal to get up into the VHF/UHF range.

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? ;-)

thanks again for the help!
jason

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.

...

Perhaps you didn't get it but what the newbie really wants is to hang
a KW amp on to his kit FM broadcast wireless mic. Maybe my
assumption is wrong.

For FM, audio is applied directly to the oscillator, not after.
Cheap transmitters are usually low power and drifty. They can sound
great though. Expensive transmitters can be low power and stabile
and can sound great. Stabile and RF clean are the first design
features, next comes power. The major problem with hanging an amp on
anything, is that is tends to amplify not only the RF signal, but any
other spurious or broadband noise from the transmitter. This is the
bad part, so what you have to do then is design the transmitter for
low noise and bandpass filter it before going to the amp then Low
pass filter (well).

Some of more complex transmitters are to allow it to be rock steady
and channelized.
Simple transmitters must be crystal controlled with temperature
compensation (where most of the cost will be) but may be actually
cleaner and more noise free than a synthesized transmitter. A simple
tunable transmitter will usually walk up and down faster than you can
tune the radio.

If you really are talking about ham radio, Ramsey makes some cool
inexpensive kits that really work great for a base station. If you
have a decent location or antenna, low power is fine. Most will want
more channels and other features, but might not make it sound any
better.

Note: FM Ham and Land-Mobile radio uses a much lower level of
modulation and different equalization than broadcast FM radio, so the
oscillator and audio designs are very different in that respect.


"xpyttl" wrote in message
...
Hi Jason

Nice questions, let me try to answer a few ..

wrote in message
ups.com...

1. i've seen transmitter schematics that were simple, and others
that
were complex. as a general rule of thumb, are the more complex
ones
trying to compensate for frequency drift, or maybe eliminate
higher
harmonics? how efficient and/or stable are the simple
transmitter
schematics?

One obvious thing is that CW transmitters tend to be simple, SSB
transmitters complex. But there are a thousand design variables.
One big
one is the complexity of the ICs employed. Today you can have a
very stable
VFO with just a few parts. You tend to pay a little bit of a price
in phase
noise, but frequency drift is not an issue. With an analog VFO,
you can add
a lot of complexity trying to get around frequency drift, but phase
noise is
never an issue. Years ago, all you had was analog. A few years
ago, DDS
(direct digital synthesis) was complex and expensive. Today,
analog VFOs
tend on the expensive side! It is similar with amplifiers. In
many radios,
all, or most, of the PA is in a single brick, instead of a fistfull
of
parts. Ditto with almost everything up and down the chain.

Frequency is also an issue and again that is changing with
technology. A
few years ago, it was hard to get directly to VHF. You typically
had
several oscillators getting mixed up, frequency multiplied, etc.
This was
especially true if you had an analog VFO because it is very hard to
get
stability at VHF, and multiplying the frequency also multiplies the
drift in
an analog VFO. There are still reasons you might want to do some
mixing up
to get to VHF with a DDS VFO, but DDS parts up into the gigahertz
range are
now cheap parts.

It was only a few years ago that a DDS VFO cost hundreds of
dollars. Today
you can buy a chip with a VHF synthesizer and amplifier and
modulator for
less than the tuning capacitor in an analog VFO.

2. other than frequency range, what characteristics are you
concerned
about when trying to match a transmitter to an amp?

If you are buying commercial, you are looking at price, of course,
and
expected reliability, along with power consumption. For SSB, you
need the
amplifier to be linear, which implies lower efficiency. For FM/CW
you don't
need linear, so the amp can be a lot more efficient. If you are
designing
the amp, then you are worrying about impedance mathcing, as well.

3. without an amp, couldn't you still run the signal to an
antenna and
it would be a weak transmitter? all the amp does is increase the
voltage and current supplied to the antenna, correct?

Yes of course. There are times when you want a lot of power, but
most of
the time it really isn't necessary. Also, it is a lot easier to
get antenna
gain than power to the antenna, especially at higher frequencies.
The need
for power depends a lot on what you do, what frequencies you
operate, and to
a degree, what "floats your boat". If you are doing EME or HSMS,
you need a
fair bit of power - hundreds of watts, anyway. If you are chatting
on the
local repeater, typically a watt is as good as a kilowatt. Lots of
folks
called QRPers like to use very low power. For them the "legal
limit" is
five watts, but many try to see what they can do with milliwatts.
The
current miles/watt record is held by a guy who operated 40
microwatts over a
500+ mile path.

4. for an FM transmitter, does the modulation occur to oscillator
directly, or is the oscillator's signal modulated after "leaving"
the
oscillator? i guess what i'm asking is whether or not there is an
input
to the oscillator, or is it just an "output only" frequency
generator?

Typically you would modulate the oscillator, although these days,
the audio
may well be simply data to the synthesizer. However, because FM is
typically done at VHF and higher, there may well be additional
oscillators
mixed with the modulated signal to get up into the VHF/UHF range.

Hope this helps

..

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.

..

Does anyone here remember the simple CW transmitter that used a 6146 as a
keyed ocillator? It would put out about 60 watts. There was a very low
current light bulb in series with the crystal to limit the current that
could flow in the crystal circuit. I think it was in the 1959 or 60
HANDBOOK. I used a gaseous regulator tube on the screen. It didn't chirp too
badly.........Analog VFO's yes now there is a fun project!


"xpyttl" wrote in message
...
"dave.harper" wrote in message
ups.com...


If you've ever built an analog VFO, these DDS parts are like black magic.
They take little in the way of support circuitry, no fretting about
layout,
no spending weeks trying to work out the temperature compensation, no
moving
around because you breathed on it, you just dial in the frequency you want
and you are spot on.

..

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
....[snip]....

CW is NOT dead on two meters; anyone listening in CW mode would hear you!
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)