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#1
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IMD products from VSWR meters
Because active SWR meters do not usually suffer
from this effect, they can normally be left in without causing such problems Can anyone clearly explain what is an "active" SWR meter? I would presume that the line is decopled from the diode by means of an amplifier. Is that what they mean? 73 Tony I0JX |
#2
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IMD products from VSWR meters
Antonio Vernucci wrote: Because active SWR meters do not usually suffer from this effect, they can normally be left in without causing such problems Can anyone clearly explain what is an "active" SWR meter? I would presume that the line is decopled from the diode by means of an amplifier. Is that what they mean? 73 Tony I0JX In this neck of the woods "active" usually means the meter has peak reading circuitry that requires power to operate. In general, as far as ordinary peak reading meters, this is the most accurate type as far as trying to read voice peaks. The better ones use fast acting LED's as the meter. A "passive" meter is one that uses capacitors to give the meter a "hang time" to sort of read peaks. But it's not as accurate, and almost always reads low. "20-25% worth" for the average meter. The cross needle meter in my MFJ-989C is an example of a passive meter circuit. I call it "psuedo peak reading". :/ I also built a passive circuit into my old green cube heathkit meter. It has a switch on the back to switch the caps in or out. Naturally, the passive meters require no power to operate. MK |
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IMD products from VSWR meters
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#4
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IMD products from VSWR meters
In this neck of the woods "active" usually means the meter has
peak reading circuitry that requires power to operate. In general, as far as ordinary peak reading meters, this is the most accurate type as far as trying to read voice peaks. The better ones use fast acting LED's as the meter. If by "active" one just means that the meter requires power, then I would see little difference between an active and a passive circuit as far as intermodulation is concerned. As a matter of fact if, in an "active" meter, the amplifier (or the active display) follows the diodes, there would be no difference at all (as to intermodulation) with respect to a passive meter. If the amplifier is instead put before the diodes, the intermodulation products generated by the diodes (if any) can get coupled to the line. In summary the distinction is between a circuit in which the diodes are directly coupled to the line and a circuit in which there is an isolating stage between the line and the diodes. And not between a passive and an active circuit. 73 Tony I0JX |
#5
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IMD products from VSWR meters
In message , Antonio
Vernucci writes In this neck of the woods "active" usually means the meter has peak reading circuitry that requires power to operate. In general, as far as ordinary peak reading meters, this is the most accurate type as far as trying to read voice peaks. The better ones use fast acting LED's as the meter. If by "active" one just means that the meter requires power, then I would see little difference between an active and a passive circuit as far as intermodulation is concerned. As a matter of fact if, in an "active" meter, the amplifier (or the active display) follows the diodes, there would be no difference at all (as to intermodulation) with respect to a passive meter. If the amplifier is instead put before the diodes, the intermodulation products generated by the diodes (if any) can get coupled to the line. In summary the distinction is between a circuit in which the diodes are directly coupled to the line and a circuit in which there is an isolating stage between the line and the diodes. And not between a passive and an active circuit. 73 Tony I0JX The belief that SWR meters produce harmful harmonics probably dates from the 1950s, when TV coverage was low, and many viewers had to drag in weak signals from a considerable distance. I'm in the UK, and the signal level on UK Channel 5 (around 65MHz) was only about 30 microvolts. Oh yes, the picture was snowy! Obviously, the 2nd and 3rd harmonics from transmitters operating on the higher HF bands could easily wipe out TV reception, especially lowband (our 'Band 1'). As well as using the usual 30MHz lowpass filter, some amateurs went to considerable lengths to suppress harmonics emanating from the transmitter itself (with particular attention to the screening and bonding of the metalwork, and the filtering of all leads (mic, key, power). Without this, there was little point in even thinking of fitting a highpass filter etc to the TV set. I believe there were one or two cases where the final vestiges of interference were traced to harmonics being generated in the SWR meter. This was cured by ensuring that the line-up between the transmitter and the antenna was TX - SWR Meter - LPF - ATU (if used) - Antenna. Doing it this way ensured that any harmonics from the meter were removed in the LPF and (to some degree) the ATU. The line-up above is the one which I always recommend should be used if possible. Generally, there is no reason not to. It is good engineering practice. But as to whether it is always essential? I suspect that often it is not. Ian. -- |
#6
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IMD products from VSWR meters
"Owen Duffy" wrote in message ... I see in the Wikipeadia entry for SWR meters the following: "When not actually measuring VSWR, it is best to remove the ordinary type of passive SWR meter from the line. This is because the internal diodes of such meters can generate harmonics when transmitting, and intermodulation products when receiving. Because active SWR meters do not usually suffer from this effect, they can normally be left in without causing such problems." I dispute part of that. On receive, the diodes are not conducting, so they won't generate IMI products. If they were a problem, then every receiver front end in the world would be a worse problem. |
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