08 March 2014

An RF Power Meter based on the Analog Devices AD8307 log/linear chip

On studying the RF Workbench series published on VE3BPO's brilliant website, QRP Homebuilder http://www.qrp.pops.net/ Todd, VE3BPO advises that an excellent 50Ohm RF workbench can be built inexpensively with a sensitive RF Power meter forming the core of the test environment. On RF Workbench page 5 http://www.qrp.pops.net/RF-workbench-5.asp, VE3BPO outlines a list of basic tools needed to get started as an RF experimenter. Based on this I built a version of the Bare Bones RF Power Meter. I chose the Bare Bones version since I am currently only working on HF projects. I want to keep it simple.

This power meter was designed by W7ZOI/W7PUA around the impressive Analog Devices chip AD8307. The design and build were originally published in QST June 2001, p38-p43. Subsequent to that W7ZOI published some 'essential notes'  http://w7zoi.net/Power%20meter%20updates.pdf. Update 10Mar07, 5June07. Included in the QST article and the 'essential notes' are a number of very interesting applications and experiments that can be performed. EMRFD also covers this power meter, Ref Chapter 7 paragraph 7.3 on RF Power Measurement. Refer to the schematic Fig 7.13.

Essentially this sensitive power meter is based on a logarithmic amplifier integrated circuit from Analog Devices AD8307. This circuit functions as a logarithmic detector, accepting signals from audio up to 500MHz over a power range from around -80dBm up to over +10dBm. The output is then a DC signal that tracks with spectacular accuracy, changing by 25mV for each dB input change.

It is not the intention here to repeat the details of the published material but rather to relate my build experiences and measurements.

A very interesting and clear description of the AD8307 can be found here: http://www.analog.com/static/imported-files/data_sheets/AD8307.pdf

I built my version using ugly construction as per the original article. Since I have a large amount of PC board on hand I elected to make the enclosure from this material. I soldered together PC boards to make an RF tight enclosure. Since I have used a large analog meter I also tried to provide shielding around the meter space using more PC boards. Since I have a very basic set of hand tools (plus an electric drill) I found it a challenge to build this enclosure. Eventually I came up with an incredibly ugly looking method to attach the rear cover. Using pieces of tin plate from A/C venting pipes obtained from the local hardware store, I bent this plate into small angle pieces which I then soldered to the enclosure. I then bolted the rear cover to these fixtures. Refer to pics below. I then connected the meter wire via a feed-through capacitor to the circuitry. I have not performed any measurable tests to confirm how well the shielding works. However to date I have seen no anomalies in the meter performance. I have transmitted a 10 watt signal into my end-fed wire located atop the shack roof with no affect on the meter. Of course if I put a small piece of wire about 8 inches long into the RF input I certainly then detect RF (as would be expected) at -48dBm level.

Subsequent blogs record a series of measurements made that progressed towards achieving the calibration of this power meter.

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