I am thinking of building the "Advanced VHF Wattmeter" by
Bob Kopski K3NHI published in QEX May/June 2002, and was
wondering if anyone who subscribes to the NG understands
the function of IC2a.

This is a voltage follower with a diode in the feedback
path, which as I recall is a precision rectifier. It is
then followed by a biasing circuit, which seems to fight
against the low impedance output of IC2a, which again
I don't understand the need for.

+
------|\
| \----||----+----+-------+----- to DVM
+---|/ | | |
| - | 10k |
+-----------------+ | |
- 0v - -0.7v

IC2a is interposed between the output of the AD8307 log amp
and the input of the DVM.

The AD8307 is runs between ground and +5v, and so
will only produce an output of between 0 (-80dbm) and 2.5v
(+20dbm). I don't therefore see the purpose of the stage
of the biasing arrangement.

Surely all that was needed was to take the output of the
AD8307 into the potential divider to set mv/dbm and
the to set the bias on the -ve side of the DVM to get
the 0dbm offset.

Any ideas?

Thanks

Stewart G3YSX

The circuit should be

+
------|\
| \----||----+----+-------+----- to DVM
+---|/ | | |
| - | 10k 3k3
+-----------------+ | |
- 0v - -0.7v

Sorry for the error

Stewart G3YSX

Stewart Bryant wrote:


I am thinking of building the "Advanced VHF Wattmeter" by
Bob Kopski K3NHI published in QEX May/June 2002, and was
wondering if anyone who subscribes to the NG understands
the function of IC2a.

This is a voltage follower with a diode in the feedback
path, which as I recall is a precision rectifier. It is
then followed by a biasing circuit, which seems to fight
against the low impedance output of IC2a, which again
I don't understand the need for.

+
------|\
| \----||----+----+-------+----- to DVM
+---|/ | | |
| - | 10k |
+-----------------+ | |
- 0v - -0.7v

IC2a is interposed between the output of the AD8307 log amp
and the input of the DVM.

The AD8307 is runs between ground and +5v, and so
will only produce an output of between 0 (-80dbm) and 2.5v
(+20dbm). I don't therefore see the purpose of the stage
of the biasing arrangement.

Surely all that was needed was to take the output of the
AD8307 into the potential divider to set mv/dbm and
the to set the bias on the -ve side of the DVM to get
the 0dbm offset.

Any ideas?

Thanks

Stewart G3YSX