EMRFD Message Archive 3056

3056 2009-05-19 01:41:28 ha5rxz Conversion Between 75 Ohm and 50 Ohm Systems Message Date From Subject I'm sorry but after trying hard with this particular problem I've had to ask for help, my math skills are just not good enough. I have an amplifier block which is intended for 75 ohm cable TV systems and the specification is as follows: Maximum RF Input (Single Tone) 50 dBmV Input Impedance 75 Ohms Gain 35 dB Output Voltage 62 dBmV (I think this is a maximum figure) So, if I fit a 1:1.5 transformer on the input and drive the assembly with a 50 ohm source what is the maximum signal generator output in dBm before I let the magic smoke out? I only have one of these so I need to get this right first time. HA5RXZ Hi might I ask "Why worry?" just slot it in a 50ohm system it will work. If you are interested in fractional dB levels for test purposes then it might be important. The best wideband solution then is to use a minimum-loss resistive pad. These can usually be found in profusion at flea markets. A wideband transformer with better characteristics might be quite difficult to achieve. If getting all the gain is important, will the input/output mismatch matter that much?? Alan G3NYK "Hi might I ask "Why worry?"" Because I only have one of these units and the manufacturer has told me that overdriving it WILL kill the module stone dead. HA5RXZ "Maximum RF Input (Single Tone) 50 dBmV". dBmV is a rather unusual unit; is the above statement correct? 50dBmV means 50db (100000 times) more than a millivolt; 100 volts!!! In a 75-ohm system that is 130 watts. Perhaps it's really "dBuV"; a more-common unit. In that event, 50dBuV is 100 millivolts; a much-more-likely figure. Can you clarify this point before we proceed? When talking about voltages, +50dB means 316 times the voltage (the same as 100000 times the power). 73 Johan SM6LKM kerrypwr wrote: > 50dBmV means 50db (100000 times) more than a millivolt; 100 volts!!! Oops; brain fade!!!!!! I never use dBV/uV; always dBm. I dislike what I perceive as the manipulation that makes dB-voltage line-up with dB-power. Sorry to have confused things (and myself!!). Confirmed, maximum input to the module is 50 dBmV HA5RXZ