Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
The distortion in S-meter scale-shape at very small signal levels, below
about S5, could be corrected very easily just by changing scale markings. The meter scale would then end at about S2 instead of S-zero. And the meter would indicate correctly as a wattmeter over the whole length of its scale. But S-meter manufacturers are reluctant to do this. They consider it would spoil the linear appearance of the scale at the low end and so lose customers. How foolish they are! Keen radio amateurs can change their S-meters into accurate instruments over the whole scale just by scraping off existing scale markings and re-calibrating with a fine-nibbed pen and black indian ink. But on second thoughts, to save yourself the trouble, when using the meter it's even easier just to visualise and remember where S3 should be marked on the meter scale. --- Reg, G4FGQ |
#2
|
|||
|
|||
Nice tutorial, Reg I printed it out for ref.
....however. The 2M rigs I've measured (3) are WAY off. The +20 +40 and +6o are usually also around 6dB plus or minus 3 or 4 dB. I measured my TS-830s and it was not very close, and my 706, but don't recall the results and haven't done the TS 2000 yet. 73, Steve K9DCI "Reg Edwards" wrote in message ... At HF it's all very simple. To summarise - The S-meter is essentially a 50-ohm power or wattmeter. It is correctly calibrated with a 50-ohm signal generator with its open-circuit volts set to 100 microvolts. The S-meter should then read S9. Therefore, at the standard S9, the input to the receiver is 50 microvolts across 50 ohms, corresponding to a power input of 50 picowatts. The standard S-unit = 6 dB. So for each change of one S-unit the input voltage halves (or doubles). At S-zero the input voltage is 50/512 = 0.1 microvolts, which is roughly equal to the the internal noise level of a good receiver with a bandwidth of a few KHz.. Theoretically this is about the noise level you should get when the antenna is disconnected. At S9 + 40dB the input voltage is 50*100 microvolts = 5 millivolts. Some S-meters may indicate as high as S9 + 60dB. The input voltage is then 50 millivolts which is about the overload point of a very good receiver. So the whole scale is calibrated logarithmically, with S9 being about half-way along it, and with 54 dB below S9 and 40 or 60dB above S9. The input voltage range is from from 0.1 microvolts to 5 or 50 millivolts. ============================== However, all meters have indicating errors. The problem arises because of the difficulties and great expense in designing and manufacturing receivers with an agc meter system which can accommodate a signal level range of 54 + 60 = 114 dB. Economics invariably rules the roost. (It helps to have very high receiver gain and attenuators at or near the receiver input.) Fortunately modern receivers all tend to have errors of the same sort and sign. So amateurs using different manufacturer's receivers will exhange very similar signal strength reports. These errors congregate at the very small signal end of the range. Meter calibration begins to go wrong at around or below S4 or S5. Meter readings of, say, S1 or S2 may actually be appropriate to power levels of S3 or S4. A meter reading of S-zero may be appropriate to a power level of S2 or S3. That is, at very small signal levels the S-meter underestimates signal power level. But it's not of great consequence. At HF, to which the foregoing applies, signals are usually below the noise and QRM levels anyway. --- Reg. |
#3
|
|||
|
|||
"Steve Nosko" wrote in message ... Nice tutorial, Reg I printed it out for ref. ...however. The 2M rigs I've measured (3) are WAY off. The +20 +40 and +6o are usually also around 6dB plus or minus 3 or 4 dB. I measured my TS-830s and it was not very close, and my 706, but don't recall the results and haven't done the TS 2000 yet. 73, Steve K9DCI "Reg Edwards" wrote in message ... At HF it's all very simple. To summarise - The S-meter is essentially a 50-ohm power or wattmeter. It is correctly calibrated with a 50-ohm signal generator with its open-circuit volts set to 100 microvolts. The S-meter should then read S9. While I will not debate or dispute the origional S-meter meanings, I would have to say there is almost no rig that follows that calibration. I have measured a few low band rigs and several vhf/uhf rigs. The low band units seem to be somewhat more 'calibrated' than the VHF and above rigs but most are usually way off. From my observations and the write ups in QST the S-meter and the 50 uV and all the 6 db per S-unit should not even be spoken of . We need to kill the idea of the s-unit being 6 db or anything at all. They are just numbers on an uncalibrated meter on most receivers. You might just as well mark the meter from 0 to 100 and go by whatever number the meter is showing. The meters are not linear, log, or any combination that can be depended on from one brand to another brand. Even changing bands on the same receiver will usually change the 'calibration' of the s-meter. |
#4
|
|||
|
|||
Thierry, ON4SKY wrote:
"With -93 dBW for a S9+10 signal at receive, that`d mean that the popwer`d be only P (W) = 10^ (dBW/10) = 0.7 watt? I`m familiar with received carrier power expressed in dBm. -93 dBW is a number, 30 dB less if expressed in dBm, or -123. This is a strong signal for a narrow-band receiver. Across 50 ohms, 1 watt is sq rt PR, or 7.07 volts. 93 dB is about 45,000 times, as a voltage ratio, so a received voltagw of -93 dBw is about 0.00016 volts, or 0.16 millivolts, or 160 microvolts. S9+10dB is a strong signal. So is 160 microvolts. I`ve read that each "S-unit" is 6 dB. S-1 would be 18 dB less than S9+10 db, and that`s a voltage ratio of 7.94. S-1 would be about 20 microvolts, and well above the threshold of most receivers. Best regards, Richard Harrison, KB5WZI |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Flakey Ham IV Control Unit F-I-X-E-D !! | Antenna | |||
Flakey Ham IV Control Unit F-I-X-E-D !! | Antenna | |||
Poor quality low + High TV channels? How much dB in Preamp? | Antenna | |||
Traveling Waves, Power Waves,..., Any Waves,... | Antenna | |||
AM/FM Reception on a new RCA/Radio Shack All In One unit DVD/CD,Cable,Aux Inputs etc. | Antenna |