Thursday, 5 August 2010

dBm Meters using the AD8307 logarithic amplifier

I'm having a go at building a couple of dBm RF Power Meter from AD8307

The first is W7ZOI design having found a nice 500uA meter at w/e there is a nice construction website here. Also note W&ZOI own notes on callibration here. There are plenty of variations on this design and the daa sheet and application notes from Analogue Devces are worth a read. Some like this one use an AF amp to provide an audible feedback too

Part of the circuit from the design from N2PON (QST August 2008) uses temperature compensation. Basically he divides the pin 4 output of the AD8307 that is fed to a DVM through a resistor network containing a 10K pot that adjusts the GAIN to achive c1mV/dB with an NTC temperature compensating thermistor (c1K at 25'C) connected to ground so if there is any temperature changes the instrument retains a level of thermal callibration.

In many places this is not necessary but I know from experience my own shack does vary from -10'C in deep winter to +30'C in summer if it is unheated. Any meter is likely to be built into a die-cast box that will take a while to equilibrate too... so I am trying to use a thermistor salvaged from a PC motherboard (as had a few) with a 2k7 value at 25'C and c4K close to freezing and 500ohms when about 100'C (basically put it in boiling water and into ice water and measured). So I think could use it but would adjust the value of by placing resistors in parallel and series with to obtain 1K at 25'C.

2 comments:

Eric Söderman said...

Here are two watt meter designs:

http://hem.passagen.se/communication/meny.html

Eric SA5BKE

M1KTA said...

Nice digital options on the AD8307 design, thank Eric.

If anyone follows his link and cannot find them try

http://hem.passagen.se/communication/wattm.html
and
http://hem.passagen.se/communication/watt2.html