![]() The volts out your wall are about 120 volts 60 Hz (usa). Last edited by gmcjetpilot on May 6:59 pm, edited 2 times in total. Tube: AM/FM Zeniths, RCA, TrueTone table tops Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780 ![]() DC millivolts is easier to measure than millivolts AC or RF. To sum it up measuring millivolts is more of a challenge. So often you will see RF millivolt or AC meter in one title. This is where oscilloscopes or RF millivolt meters come in. To measure RF, high speed wave forms adds more challenge that a basic volt meter can't handle. Most of the time micro volts is associated with RF. Measuring small resistance and volts accurately has always been a challenge. If you are trying to measure 0.010 volts and a DMM only can measure +/- 0.010 volt you are out of luck. If you measure 1000 volts, one volt off is still accurate. The smaller the volts, the tighter the tolerance is needed. though someone out there will no doubt refute that!)Others state dBm to be 1 mW across 600 ohm, as do the telecom industry.A VU unit is also stated to be 1 mW across 600 ohm (for complex waveforms)As Leigh points out so succintly, its all so relative to impedance50 ohm would seems the norm in the RF industry, but 52 ohm, 72 and and 300 ohm also figure high in the list, with 75 ohm prevalent in the video industry.The decibel isn't an absolute value, just a way of expressing the ratio of a level to a given reference level.You take your given impedance, usually the one your stuck with, and go on from there.Most digital volt meters or DMM's can measure micro volts, just not that accurately due to tolerances. (add that to Leighs listof 'standard' impedance values, though this one isn't commonly used now. Well done, Leigh !As they say in the trade, A text book response!But i Don't blame folks for the confusion, particularly with dBmIt confused the hell out of me at the time.Some text books state a dBm (with no reference level quoted), to be 6mV across a 500 ohm impedance. The voltage correlation is different for each of these.-73 de Leigh W3NLB | | | | You can define a dBm scale for any load, with 600, 135, 75, and 50 being commonly used. The dBm scale requires a stated load impedance (50 ohms for the table above). The 0 references are shown in boldface.The dBm reference is 223.607 millivolts which equals 1 milliwatt into 50 ohms. ![]() If you want dBuV, just add 60 to the dBmV numbers. Between dBV and dBmV there's a constant offset of 60. And, Dave requested the voltage-referenced formula.Here's a table showing the dBm, dBV, and dBuV values for voltages from 1 volt to 1 microvolt: Note that between dBm and dBV there's a constant offset of 13. By using a voltage reference, you eliminate this extraneous step. #MICROVOLTS TO DBM GENERATOR#I gave the values for dBV and DBmV in my earlier response.While the dBm scale is commonly used on signal generator dials, it's not as easy to compute this value from measured parameters because you have to know the load impedance and do the power calculation. ![]() The dBV, dBmV, and dBuV scales are referenced to 1 volt, 1 millivolt, and 1 microvolt, respectively, and are independent of load impedance. ![]() The dBm scale is referenced to 1 milliwatt power into a defined load impedance (50 ohms here). Quote:Originally posted by Peter Bertini.According to my Boonton, around -77dBm should be closeI was trying to avoid this area of confusion.Decibels are always relative to something. ![]()
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