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

..