To obtain a dBm reading of input level on the Agilent 8903A, it is necessary only to start from this basic identity:

 
 
 V2
 
 10 log(--- x 1000) = dBm
 
 R
 

From this, you need to find the value for V which sets the value for dBm to zero, for the various values of R (impedance). Expressing the above identity in terms of V:

 
 
 
 R x 10(dbm/10)
 
 V = ( ------------ )1/2
 
 1000
 

then setting dBm equal to zero, we have:

 
 
 R 
 
 V = ( ---- )1/2
 
 1000
 

Using this formula, the voltage can be determined for any value of impedance. Once you have this, the steps are as follows:

1. Be sure the 8903A is reading in volts.

2. Key in the voltage value determined by the formula.

3. Press RATIO key. Display now reads %.

4. Press LOG/LIN key. Display now reads input in dBm, referenced to whatever inpedance was used.

For example, if source being measured has 600 ohms impedance:

 
 
 600 
 
 V = ( ------ )1/2 = 0.61/2 = 0.7746 V
 
 1000
 

The dB ratio to 0.7746 V is the same as dBm into 600 ohms.