DLS DLS 400A, Operating Manual

Page: 208 / 226

Appendix A
Page 196
P (load) = V * V / R watts
= (3.00E–7) / 135 watts
= 2.22E–9 watts
P (ref) = 1E–3 watts
dBm = 10*LOG [P(load)/P(ref)] dBm
= 10*LOG [(2.22E–9)/(1E–3)] dBm
= –56.5 dBm
2) dBm/Hz to dBm
Here we will assume that the bandwidth is 3000 Hz and the setting is –70 dBm/Hz.
dBm = dBm/Hz + 10*LOG (bandwidth) dBm
= –70.0 + 10*LOG (3000) dBm
= –70.0 + 34.8 dBm
= –35.2 dBm
Notes:
A) To change a level in
µ
V/
√
Hz by a certain number of dB use the following formula:
(Assume x is the amount to change the
µ
V/
√
Hz setting by, and the dB change
required is –6 dB.)
x = 10** ( y / 20 )
= 10** ( –6 / 20 )
= 0.50
Therefore the original setting in
µ
V/
√
Hz should be multiplied by 0.50 to give a change of
–6 dB.
B) To change a level in dBm/Hz by a certain number of dB simply change the dBm/Hz
setting by the required amount. An examination of the formula on the previous sec-
tion will bear this out.
LOADING
As can be seen from the above discussion the choice of loads plays a large part in the level
that the meter will read.