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#1
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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) |
#2
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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 .. |
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