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

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