EMRFD Message Archive 4939

Message Date From Subject
4939 2010-08-29 09:05:04 ha5rxz June 2001 QST Power Meter

The existing circuit has a nominal full scale value of 10 dBm. If I want to build a meter that has a 20 dBm FSD figure is it best to increase the meter series resistor R6 or decrease the gain of the Op Amp by altering R4 or R5?

I know that I can add a 10dB attenuator to the input but this only introduces another source of errors.

HA5RXZ

4940 2010-08-29 10:04:12 w7zoi Re: June 2001 QST Power Meter

Hello OM,

No, I'm afraid not. The op-amp is nothing more than a meter driver. The feedback resistors just set up the way the meter is driven. It allows you to use whatever meter you might have available. But the full scale level is determined by the AD8307 and by the input network. If you really want a +20 dBm for the max input, a 10 dB pad really is the best way to go.

Good luck with the project.

73, Wes
w7zoi

4939	2010-08-29 09:05:04	ha5rxz	June 2001 QST Power Meter
Message	Date	From	Subject
The existing circuit has a nominal full scale value of 10 dBm. If I want to build a meter that has a 20 dBm FSD figure is it best to increase the meter series resistor R6 or decrease the gain of the Op Amp by altering R4 or R5? I know that I can add a 10dB attenuator to the input but this only introduces another source of errors. HA5RXZ
4940	2010-08-29 10:04:12	w7zoi	Re: June 2001 QST Power Meter
Hello OM, No, I'm afraid not. The op-amp is nothing more than a meter driver. The feedback resistors just set up the way the meter is driven. It allows you to use whatever meter you might have available. But the full scale level is determined by the AD8307 and by the input network. If you really want a +20 dBm for the max input, a 10 dB pad really is the best way to go. Good luck with the project. 73, Wes w7zoi