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newbie technical questions
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. ... |
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) |
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 |
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