Using a dBm Meter (Load Question)

I just got a dBm meter(HP 400EL), & I'm messing with a mixer input channel. The manual says the meter is calibrated to dBm, and will only be accurate with a 600 ohm load. So...I put in a test signal to the input transformer, which is 600:600, should I be reading dBm at the output of the xformer? If I test the output of the module unbalanced, do I need to add a 600 ohm load to get a correct reading? If I check levels at the input of one of the amp cards on the module, which all have a 604R from input to ground, I'm guessing I'll get a good reading, yes?

Thanks,

hodad said:

I just got a dBm meter(HP 400EL), & I'm messing with a mixer input channel.  The manual says the meter is calibrated to dBm, and will only be accurate with a 600 ohm load.  So...I put in a test signal to the input transformer, which is 600:600, should I be reading dBm at the output of the xformer?  If I test the output of the module unbalanced, do I need to add a 600 ohm load to get a correct reading?  If I check levels at the input of one of the amp cards on the module, which all have a 604R from input to ground, I'm guessing I'll get a good reading, yes?

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The meaning of the "m" in "dBm" is that the unit is "decibels referenced to 1 milliwatt."  A reading of 0 dBm is then 1 milliwatt.

Of course, power doesn't exist by itself, so a specific load is required, in this case 600 ohms. 

A 600:600 transformer doesn't present a 600-ohm load, though. It is a 1:1 transformer optimized for use at 600 Ω impedance.  You need a 600 Ω load on the meter input as the load, and then the reading should be accurate.

NB that the meter may have a low-frequency measurement limit.