Friday, February 13, 2015

A Milliwatt to Kilowatt RF Power Meter using the Analog Devices AD8307

A friend of mine, Dick Benson, W1QG, built a pretty cool combined Forward and Reflected Power and SWR meter using two AD8307 Logarithmic Amplifier chips from Analog Devices.  He used a PIC to convert the dBm readings back into watts (and to calibrate the device, too).

N2PK also has an interesting design for a "Forward Power and Return Loss Meter."  It, too, uses two AD8307 ICs.

One of these days I'll build a "Power and SWR" meter, too, but for the lab bench I thought a basic Power Meter would suffice.  Almost all of my work on transmitters is either testing or repairing them, so I don't really need to measure SWR or Reflected Power.  With that thought in mind, why not design a 0 - 60 dBm (1 milliwatt to 1000 watt) power meter?

Here's a picture of the completed power meter.  It reads either 0-30 dBm (1 - 1000 milliwatts) or 0-30 dBW (1 - 1000 watts).  Yes, the scale is in milliamperes.  Someday I'll make an appropriate scale.  But meanwhile, it's easy enough to convert the 0-3 mA scale to 0-30 dBm/dBW.

(click on image to enlarge)

And here's the schematic:

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And here's a better version, drawn after I'd posted the one above...

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Choice of parts (apart from the AD8307) was pretty much dictated by what I had in my junkbox.  The case was originally from a Drake W4 wattmeter. (I no longer recall where I found the case -- I must have picked it up (sans meter and electronics) at a swapmeet sometime in the past).

The meter (0-3 mA) was chosen because it was in the junkbox and, most importantly, it fit the hole in the case's front panel!  I initially wanted to use a meter that was marked from 0 to 600 mA (actually a 100uA FS meter), because the scale would be perfect for a 0-60 dBm meter without a range switch.  But I really wanted to avoid cutting a meter hole, so the 0-3mA meter won out.  The 0-60 dBm overall range is now broken into two sub-ranges:  0-30 dBm and 0-30 dBW (i.e. 30-60 dBm).

I also wanted to used op-amps with rail-to-rail outputs (so that I could drive down to 0 volts).  Fortunately, I happened to have some TLC2272 op amps on hand.

Schematic Notes: 

The design uses 5VDC, provided by a 78L05 regulator that drops the input DC (e.g. 9V - 20V) down to 5V.  The 1N4148 is just there to provide circuit protection in case the wrong polarity DC is attached to the input.

The AD8307 has a 92 dB Dynamic Range (-75 to +17 dBm), but if you look at the charts in the datasheet you'll see that accuracy suffers a bit at the ends of this range -- I think a range from -60 to +10 dBm is more reasonable if you want to preserve accuracy.  So I decided to limit my top-end to +10 dBm, which would put my bottom end at -50 dBm.

50 dB of attenuation (externally applied) is required to drop the "application" power range of 0 - 60 dBm down to the meter's input range of -50 to +10 dBm.  Not a problem, as I'll explain later.

The 52.3 ohm resistor to ground at the BNC input parallels the AD8307 input impedance (1.1K || ~0.7pf) and brings it closer to 50 ohms.  (Note, the 0.7pF would be the differential input capacitance.  I.e. 1.4 pf to ground (for each input pin), in series).  Here's an S11 plot of the Input port's Return Loss:

Because of the lower power levels that the AD8307 would be working with, I decided to shield the AD8307 to (hopefully) prevent external RF fields from affecting the reading.  I built the sides of a small shielded box on the ground plane using copper-clad PC stock. Copper tape (soldered to the box sides) caps the box.

Power and the output signal from the AD8307 pass through the box via feedthrough caps (1nF, if I recall their value correctly).

I amplify the AD8307's output signal, whose slope is about 25mv/dB, by a gain of 4, which increases the signal's slope to 0.1V/dB (i.e. 1V/10dB).

For the op amp to generate 0-3V for each of the two ranges, given that the AD8307 output ranges from about 0.9V to 2.4V over an input range of -50 to +10 dBm, I have two switchable "offsets" that connect to the negative-input of the op-amp, thus shifting the AD8307's output down so that 0 dBm (or dBW) corresponds to 0V out of the op amp and a reading of 30 (dBm or dBW) corresponds to 3V.

A "Range" switch (0-30dBm or 0-30 dBW) selects a "course" DC offset, which then can be fine-tuned with the "ZERO" pot.  The pot spans the same amount of voltage, but shifted, when the range switch is toggled (because the total resistance in the voltage divider does not change).  Note that changing ranges requires re-zeroing the meter. 

Two more op amps round out the design.  The first drives a 10 uF cap, which acts as a peak-hold (with its "slow" decay determined by a parallel 1 Meg ohm resistor. This feature is useful when looking at peak-power.  The op amp drives a 2N3904 transistor which is in the feedback loop.  This transistor serves two purposes -- it provides adequate current to charge up the 10 uF cap (the TLC2272 is a bit wimpy), and its base-emitter junction blocks the cap from discharging through the op amp when the output of the op amp drops down below the capacitor's voltage.

Note that the 2N3904's V(BR)EBO is 6V (min), which is greater than the 5V powering the TLC2272 driving the 2N3904's base.  So there's no danger to the transistor's Base-Emitter junction when the 10uF cap's voltage is high and the op amp's output is low.  By the way, I used 2N3904 transistors because I have a bunch on hand.  2N2222  transistors or any other garden-variety NPN would be fine, just as long as it has a minimum Beta of at least, say, 20, and a V(BR)EBO of at least 5V.

A switch allows selection of "slow" or "fast" decay of the peak-hold cap by switching in a 10K resistor to parallel the 1 Meg "slow" decay.

The other op amp drives the meter and isolates the peak-hold cap from the relatively low resistance represented by the meter.  The 500-ohm pot acts as a "Gain" control and it ensures that the reading of the analog front-panel meter correctly corresponds with the input power.  Initial setup requires a bit of tweaking between this control and the "Zero" control to get the meter's needle to read correctly from 0 to full-scale, but once it's calibrated, one shouldn't need to touch the "Gain" control again (thus, it's on the back panel).

A second 2N3904, in the feedback loop of this meter-drive op amp, provides current gain for the its TLC2272.  These op amps are a bit anemic with respect to current-drive, and their output voltage can drop appreciably with loading.  The transistor's current amplification keeps my 3 mA meter from loading down the op amp's output.

If better accuracy is desired, I can read power via an external DVM rather than use the front-panel's analog meter.  A separate BNC connects a DVM to the output of the meter-drive op amp output -- remember, this output goes from 0 to 3V with a slope of 1V/10dB.  There's a series 10K just to limit current in case ESD somehow hits this signal, but it has no effect on the DVM reading, due to the DVM's much higher input impedance.

Additional thoughts:
I used TLC2272 op amps because that's what I had on hand, but these devices cannot source much current (e.g. the 3 mA required for my meter) without experiencing significant voltage drop at their outputs.  A better choice would be the LMC662 family used by N2PK in his Power Meter.  This device would allow you to eliminate both 2N3904 transistors, with the 2N3904 used for the peak detector replaced with a simple diode, e.g. 1N4148.
Something else to try would be to replace the single "Zero" pot with two pots, each selected by the Range switch.  I'd tried to select my voltage divider so that the single pot wouldn't need much tweaking (ideally: none) when flipping between the range switch, but I wasn't successful.  If it turns out that the zero pot, over time, needs no retweaking for a given range, then there's no reason why the single pot couldn't be replaced with two pots mounted, say, on the back panel, instead of on the front panel.

Notes on Construction:

I like to build on copper-clad PC stock because it provides a great ground plane for the circuitry (and I have quite a bit of it in my junk box).  When mounting IC's, I'll mount them "top-up" so that I can see their part numbers.  Pins going to ground are bent down and soldered to the copper plane.  All other pins are bent out so that they are straight out from the sides of the IC (like the wings of an airplane).

To give the device stability (in case, say, only one pin goes to ground), I'll solder a power-bypass cap to the board so that it's lying on its side and then solder the IC power pin to the other lead of the cap.  Or I'll solder a high-value resistor (e.g. 1 Meg) to the board next to a pin.  The resistor sticks up straight and will support the side of an IC that has a pin soldered to to the top of the resistor.

Here's the start of the build...

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Bending pins is great if you're using DIP packages, but often I'll be using SOIC parts.  What I try to do for these is to purchase little prototyping boards designed to adapt specific SOIC packages (e.g. SOIC-8) to DIP spacing.  I'll then stand these proto boards off the copper plane using either leaded 1 Meg resistors or, where appropriate (e.g. Ground), stiff wire.

In the picture below the two small green boards are the prototyping boards for the two TLC2272 packages.  (You can often find these on eBay).

And here' the completed unit!

To make the front panel overlay I used the same technique that I describe here.

In Operation:

In the photo below the meter is reading 10 dBW (i.e. 40 dBm, or 10 watts), and the DVM is connected to the meter's "To DVM" port, whose output slope is 1V per 10 dB (thus the 1V reading for a 10 dBW signal).

By the way, in the photo above I'm not really driving the meter with a 40 dBm signal -- if no external attenuation is used, the meter's "0-3 mA" scale can be spanned by an input signal ranging from -20 to +10 dBm (when the "Range" switch is in its "High" position (30 dBW)).  Only with an external 50 dB attenuator will the scale accurately reflect the actual power (e.g. 0-30 dBW or 0-30 dBm).  So, in the photo above I'm actually using no attenuation and I'm driving the meter with a -10 dBm signal from my RF Generator.

So it's worthwhile noting that other values of attenuation can be used in lieu of 50 dB.  The table below shows how the measurement range would shift with different values of input attenuation.  For example, if I used 20 dB of external attenuation and set the Range switch to its Low (30 dBm) position, then I could directly measure (using the meter's scale) 0-30 dBu (dB microwatt).

External 50 dBm Attenuator:

As I mentioned earlier, this design requires some form of external attenuation for the 0-60 dBm measurement range.  Fortunately, the shack/lab here has a variety of ways to attenuate RF signals:

(Back:  Bird 200 watt, 30 dB attenuator.  Front-left: 50 watt dummy load with 50 dB attenuator.  Front-right: homebrew -24 dB directional coupler.)

When testing and repairing transmitters, I usually use this setup:

Although the dummy load (military DA-437/GRC-103(V)) is rated at 50 watts, it seems to work fine with 100 watt transmitters (admittedly I minimize "key-down" time).  The 40 dB attenuator is just a voltage divider that divides the voltage at the dummy load (e.g. 70.7 volts @ 100 watts) by a factor of 100 (40 dB).  I chose this scheme of attenuation because it doesn't require the attenuator to dissipate large amounts of power and the 2.5K ohms has little impact on the 50 ohm impedance seen by the transmitter.

Here's what the 40 dB attenuator looks like:

(click on image to enlarge)

The series 2.5K resistor was tweaked (by paralleling another resistor) to give 40 dB +/- 0.15 dB of attenuation (while attached to the dummy load) over the range of 1 to 54 MHz.  (To keep the frequency response flat over that range I had to add a capacitive "gimmick" to ground next to the series resistors -- essentially some copper tape tied to ground that I wrapped around the resistor bodies). 

Note, too, that if driving the dummy load with 100 watts of power, the 2.5K resistor will need to dissipate about 2 watts, so select values accordingly. And to get 40 dB of attenuation there needs to be a 50 ohm load (e.g. the input of a spectrum analyzer) connected to the output (attenuated) port. 

Converting dBm to Watts:

To convert dBm to Watts, you can use the formula:

Power(watts) = (0.001)*10(dBm/10) 

But the table below might be simpler to use:

These readings can be easily scaled to other powers.  For a given dB range, multiply the milliwatt reading by the appropriate power of 10.  For example:
  • For the range 0-10 dBm, multiply "milliwatts" by 1
  • For the range 10-20 dBm, multiply "milliwatts" by 10
  • For the range 20-30 dBm, multiply "milliwatts" by 100
So, if a reading is 11 dBm, the power would be 13 milliwatts.  Similarly, 21 dBm would be 130 milliwatts.

If the Range switch is set to 30 dBW, then "milliwatts" is replace by "watts", and a meter reading of, say, 11 dBW would correspond to 13 watts.


N2PK Forward Power and Return Loss Meter  Design using two AD8307 Logarithmic Amplifiers.

Analog Devices AD8307 92 dB Logarithmic Amplifier, Datasheet

TI TLC2272 Dual Rail-to-Rail Op Amp, Datasheet

Links to my Directional Coupler blog posts:

Notes on the Bruene Coupler, Part 2

Notes on the Bruene Coupler, Part 1

Notes on HF Directional Couplers

Building an HF Directional Coupler

Notes on the Bird Wattmeter

Notes on the Monimatch

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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