Those articles are two of my favorites. But I also like this one because I have found that old signal level meters designed for analog signals are still useful for digital signals:
Originally Posted by holl_ands
Meters and Spectrum Analyzers will give different results, depending on the
bandwidth of the measurement filter, which is usually much less than 6 MHz:http://www.pi-usa.com/pdf/dtva.pdf
Originally Posted by ProjectSHO89
Also, keep in mind that TVfool and the FCC mapping tool provide estimates of signal POWER, not voltage. To convert from the projected power value to voltage (as measured with a dB voltmeter), add 48.8 to the dBm value to convert to dBmV (into a 75 ohm load).
As an example, if the forecast signal strength were -48.8 dBm, a unity gain (0 dB) antenna were used, and the circuit were perfect (no losses or mismatches), the dB voltmeter should read 0 dBmV, a -58.8 dBm forecast should read -10 dBmV, a -38.8 dBm forecast should read +10 dBmv, etc
Your conversion factor of 48.8 is correct when going from dBm to dBmV, but there is no such thing as a dB voltmeter. Decibels express a difference in power levels. If not, how could you use the constant conversion factor when going from dBm to dBmV?
Signal level meters that are calibrated in dBm and dBmV both measure signal power, not voltage. Each uses a different reference level to compare with the signal to be measured, but the difference between these two reference levels is constant. That is what makes it possible to use the conversion factor:
The 0 dBm reference level = 1 mW
Converting the 0 dBmV reference level also to power (E squared divided by R):
0 dBmV reference level = 1.33333E-05 mW
This could also be stated as 0.0000133333 mW, but using the powers-of-ten engineering notation as above makes it possible to retain more significant figures on the average handheld scientific calculator. (You can also say it as "1.33333 times ten to the minus five milliwatts.")
dB ratio = 10 log (P1/P2)
= 10 log (1/1.33333E-05)
= 48.7506 dB
The dB ratio could have also been calculated using the voltages from each reference level and the formula dB = 20 log (E1/E2), but the reference level for dBm is by definition 1 mW of power. If we convert the reference level for dBm to voltage the impedance must also be 75 ohms. The reference level voltage for 0 dBm for 75 ohms is 0.274 Vrms, but it is 0.224 Vrms for 50 ohms.
To quote the ARRL Handbook:
Sometimes there is confusion about whether the decibel was calculated using power, voltage or current. Since the current and voltage equations use 20 instead of 10 times the log term, some hams believe the "voltage" or "current" decibel is different than one calculated using power. This is not true, however. There is ony one decibel definition, and that is ten times the log of a power ratio.
I like to use these sites when making conversions:
Edited by rabbit73 - 6/14/12 at 12:45pm
Signals that are stronger than the reference level are assigned a positive value. Signals that are weaker than the reference level are assigned a negative value. Signals that are equal to the reference level are assigned a zero value (which doesn't mean no signal). The accuracy of the meter depends upon its calibration and the standards used for calibration.
Fortunately, we don't need to know the absolute value of the power measurement to a high degree of accuracy because most of our measurements are comparisons to find the strongest signal when aiming an antenna, finding the best location for an antenna, comparing antennas, and measuring loss in a distribution system. When making comparisons, meter linearity is important. This is easily checked with a built-in or external fixed attenuator.
A signal level meter (SLM) that uses a dB scale with a reference level of 1mV across 75 ohms for 0 dBmV is a relative power meter, not a voltmeter. It allows us to measure signal levels in dBmV, cable loss in dB, amplifier gain in dB, and make dB comparisons of antennas---all differences in power, not voltage. It's very convenient, because you can easily add or subtract decibel values. The "V" in dBmV is only to tell us what reference level is being used.
The older field strength meters (FSM) that came before the SLMs calibrated in dBmV used microvolts. They WERE signal voltmeters, and used the same reference level of 1000 microvolts (equals 1mV), but the scale was calibrated in microvolts. The later field strength meters included a dB scale along with the microvolt scale, which was the transition instrument between the early field strength meters and the present SLMs:
Notice that 10 times the voltage gives a 20 dB difference in power. This explains why some of the early field strength meters, calibrated in microvolts, had 20 dB attenuators marked with the label "10," for ten times the voltage.
A meter scale can be calibrated in any units that are appropriate for the application. Which scale you use determines what you name the meter. If you use the microvolts scale you can call it a voltmeter. If you use the dB scale, it becomes a relative power meter.
So, your meter to measure signals can be calibrated in microvolts, dB, dBmV, or as in the photo above with two scales: one for microvolts and one for dB (but not "dB microvolts"). I don't think there is such a thing as a "dB voltmeter" because I'm not aware of a measurement unit called "dB volt."
Many years ago I attended a seminar led by Gordon Gow, longtime president of McIntosh Laboratory (mfg of high-end high-fi equipment). Whenever he hired a new EE as an employee, he had to sit him down to talk about decibels and invariably had to tell him that "there is no such thing as a voltage decibel. Decibels indicate a POWER ratio."
A measurement unit cannot be an expression of power and voltage at the same time; it is not consistent with Ohm's Law. Given enough information, one can be calculated from the other, but they are not synonymous. Please look at the screen shot below of digital channel 31 from my Sadelco DisplayMax 800 and note that the reading of +13.4 dBmV is labeled PWR: