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QEX Article


 

Extremely Wideband QRP VSWR Meter

Logarithmic detectors and simple coupler work from 1- 500 MHz at 100mW

A very long time ago, I read an article in some amateur radio publication that explained how performance problems in VSWR meters relate to the nature of diode detectors at different power levels.  At low power levels, these diodes are square-law detectors, so that output voltage varies linearly with applied power, but at high levels these diodes are rectifiers, so that output voltage varies linearly with applied voltage or the square-root of applied power.  I wish I could find that article again. 

Most of us learned about logarithmic RF detectors from the 2001 QST article that featured the Analog Devices AD8307[1].  I immediately built several pieces of test equipment that included the AD8307 and similar devices, as did many other folks, as evidenced by the flurry of articles. 

It occurred to me in 2008, that a VSWR meter with a pair of logarithmic detectors in place of the diode detectors would overcome the problems inherent in the diode detectors.  As a bonus, a subtraction replaces the division in the calculation that compares the forward indicating voltage to the reflecting indicating voltage because we are now dealing with logarithms.  I actually wrote up an invention disclosure for such an instrument before discovering that someone else had already patented this apparently obvious idea in 2002.  Too bad.  Look for US patent 6486679[2]. 

A QST article by Kaune in 2011 featured dual logarithmic detectors in a VSWR Meter[3] and led me to check whether the 2002 patent was still in force.  If a patent proves to be commercially unsuccessful, patent owners often choose not to pay the renewal fees, which increase as the patent matures.  The first fee is due after year 7.  I went to the US Patent and Trademark Office Patent Application Information Retrieval web site[4] to learn the status of patent 6486679 and was gratified to see the message: “Patent Expired Due to Nonpayment of Maintenance Fees Under 37 CFR 1.362”.  That’s Chapter 37 of the Code of Federal Regulations, whatever that is.  This patent expired in December of 2010, the month before the Kaune article, so we are all free to do as we choose with this concept.  I was thrilled.  This led me to reconsider the approach and what I wanted in a VSWR meter. 

What I wanted was a VSWR Meter suitable for QRP that gives consistent measurements as power level varies.  Most instruments cannot handle the low power levels inherent in QRP operation, but the excellent sensitivity and huge dynamic range of the AD8307 logarithmic detector allows operation at low RF power levels, even with short couplers that have very small coupling coefficients at low frequencies.  I started by adding a pair of AD8307 logarithmic detectors to an existing coupler in an old VSWR meter and achieved an early success.  It turned out that simple transmission line couplers work up to very high frequency if they aren’t too long.  The limit is ¼ wavelengths at the highest frequency.  Such couplers are what you see in old Heathkit and citizen’s band VSWR meters.  They have no ferrites to impose frequency limitations.  They just have length, distance between coupled lines, and characteristic impedances of the main and coupled transmission lines. 

I decided to use printed circuit techniques in order to make couplers with balanced and controllable characteristics, so I could reproduce them.  This turned out to introduce interesting new problems, but it’s all about learning, isn’t it?  The lesson below is that simple microstrip couplers do not work well, because modes in the air above the board have different propagation characteristics than modes in the dielectric within the board.  Luckily, the fix is not difficult.  

The Directional Coupler

Figure 1 shows the bare printed circuit board that hosts the logarithmic detector circuits and constitutes the forward and reverse directional couplers.  The transmitter enters the main line in the center from an SMA connector on the left to the antenna or other load via an SMA connector on the right.  I designed the prototype to use SMA connectors to learn what frequency response I could achieve.  The results are truly remarkable! 

The main line conveys RF power from the transmitter on the left to the antenna on the right.  The main line couples a sample of the forward signal (from the transmitter to the antenna) to the upper line and then to the logarithmic detector at the upper left.  A matched termination at the other end in the upper right on the rear absorbs and does not reflect any contribution from the opposite direction.  Similarly, the main line couples a sample of the reverse signal (from the antenna back to the transmitter) to the lower line and then to the logarithmic detector at the lower right.  A matched termination at the other end in the lower left on the rear absorbs and does not reflect any contribution from the opposite direction. 

Transmission line couplers should be embedded in a uniform dielectric such as air or FR-4 printed circuit material, so the Heathkit style coupler works and stripline couplers work.  Stripline is printed conductors embedded in dielectric between two ground planes; whereas microstrip is printed conductors above a dielectric with a ground plane below the dielectric only.  Microstrip does not make good transmission line couplers. 

Wikipedia[5] explains:  “The λ/4 coupled line design is good for coaxial and stripline implementations but does not work so well in the now popular microstrip format, although designs do exist. The reason for this is that microstrip is not a homogeneous medium – there are two different mediums above and below the transmission strip. This leads to transmission modes other than the usual TEM mode found in conductive circuits.”  

A 2nd reference[6], with minor edits, says:  “Planar structures (unless they are stripline) have notoriously bad directivity. Directivity (Isolation minus coupling) is determined in these types of structures by the difference between the even and odd mode phase velocities with the best coming when these are equal. In microstrip, the odd mode is mostly in dielectric, and the even mode is mostly in air. To equalize the phase velocities, you need to slow down the even mode which can be accomplished using a dielectric overlay over the lines (microstrip case).” 

A 3rd reference[7] also suggests the method of adding dielectric material above a microstrip coupler to make it work, and that’s what I did.  The dielectric above the microstrip should be the same as the material below for the even and odd modes to propagate at the same speed.  We call this configuration of microstrip within a thicker layer of dielectric “embedded microstrip.”  Figure 2 shows that it looks like stripline without the top ground plane. 

It is very critical to control the impedances of the main and coupled lines in order to minimize degradations to directivity.  The coupled lines can be of any impedance as long as they terminate in their characteristic impedance.  The main line must be the characteristic impedance of the transmission line between your transmitter and antenna or tuner, usually 50Ω.  I tried to make all three be 50Ω. 

I found four impedance calculators for embedded microstrip transmission lines on the web.  Two agree closely, and the other two disagree dramatically.  Unfortunately, I started with a bad one, but I got excellent results with one from the Swedish company Multi-Teknik[8].  I designed and had ExpressPCB fabricate the board using 0.080 inch traces with 0.070 inch gaps between the main and coupled lines using the ExpressPCB parameters for dielectric constant and trace thickness. 

Figure 3 shows the first working QRP VSWR Meter.  Note that the dielectric overlay converts the ordinary microstrip to embedded microstrip.  SMA connectors that connect to the edge of a printed circuit board prove to be a convenient way to attach to the center main line trace for this prototype.   

The Logarithmic RF Detectors

The Analog Devices AD8307 data sheet[9] describes the operation and use of this logarithmic detector.  I built a test jig following figure 37 in that data sheet to characterize these devices to assure that forward and reverse measurements are on the same basis.  My method sets all devices to a slope of 0.025 Volts/dB and an intercept of -86 dBm.  My settings are only as accurate as my power measurement, but they are consistent across devices and allow good matching.  I could measure the characteristics and compensate in software as I did in my DDS project[10], but this time I measured and installed the shunt resistances necessary to achieve the desired slope and used trim-pots to set and match the intercepts. 

The curved lines in Figure 4 show how well 8 sample AD8307s agree following the calibration process and indicate that their usable dynamic range is from +10 dBm down to -74 dBm.  Measurement results within this range are very reproducible.  It is this huge dynamic range that allows the VSWR Meter to accommodate the low coupling coefficients and corresponding low signal levels at low frequencies as well as the high coupling coefficients and high signal levels at high frequencies.  The straight lines in this plot are the best fits to the linear portions of the real curved line data between 0 dBm on the high side and -60 dBm on the low side, well within the linear dynamic range.  These straight lines serve to determine the slopes and intercepts that best fit the data.  

The circuit of data sheet figure 37 also serves as the circuit for the two logarithmic detectors on the printed card except that I add a 0.1 μF capacitor from pin 3 to ground in case this instrument ever finds use at frequencies below 100 kHz.  Figure 5 shows the schematic of the bi-directional coupler/logarithmic detector board.  In later versions, the ‘select in test’ resistors R5 and R12 became 100kΩ trimpots that set the slope of the logarithmic detector response to 0.025 V/dB.  Similarly, resistors R4 and R6, and trimpots R7 and R14 became 100kΩ trimpots that set the zero power input intercept to -86dBm.  It is critical that both logarithmic detectors have precisely the same slope and intercept values.    

The Math - With thanks to Glenn Pederson WB9QIQ for bringing the previous error to my attention

 

The outputs of the coupler board are two voltages that indicate the logarithms of the forward and reflected powers from the two couplers. 

 

The linearized logarithmic detector response from Figure 4 is

 

V = (0.025 volts/decibel x 10 log Power in dBm) + 2.15V                          (1)

 

And the reflection coefficient traditionally represented as the Greek character rho is a ratio of reflected signal level compared to forward signal level.  In terms of reflected and forward power, and remembering that dividing numbers is the same as subtracting their logarithms,  

 

            ρpower =  Pr/Pf            or           10 log ρpower = 10 log Pr - 10log Pf             (2)

 

VSWR meters measure voltages corresponding to the reflected and forward powers to calculate reflection coefficient and then VSWR.  The simplest way I can explain is to start with the difference between these voltages which is a ratio of the reflected power to the forward power, which is in turn, the power reflection coefficient ρpower.  So,

 

(Vr – Vf) = [0.025 x (10 log Pr) + 2.15 - 0.025 x (10 log Pf ) - 2.15]           (3)

 

This conveniently simplifies to:

 

(Vr – Vf) = (10 log Pr – 10 log Pf) / 40                                                        (4)

 

(Vr – Vf) = (log Pr/ Pf) / 4 = (log ρpower)/4   or                                            (5)

 

log (ρpower) = 4 x (Vr – Vf)                                                                            (6)

 

Take the square root of the power reflection coefficient to get the voltage reflection coefficient ρvoltage, because we ultimately want the Voltage Standing Wave Ratio.  That’s the “V” in VSWR.  Since we’re still dealing with logarithms, we obtain the square root by dividing the logarithm of the power reflection coefficient by 2. 

 

log (ρvoltage) = log (ρpower) / 2  = 2 x (Vr – Vf)                                               (7)

 

So far, we’ve taken the two voltages, subtracted them, and multiplied by 2.  Then we obtain ρ by raising 10 to the power log ρvoltage

 

Finally we obtain the VSWR from the formula VSWR = (1 + ρvoltage)/(1 - ρvoltage).  All this is just a few lines of code in a high level language, as we’ll see. 

The Arduino

I did all the early development work with a pair of digital multimeters and a calculator to prove the principle.  When I finally laid out this board and ordered three from ExpressPCB[11] for about $63 including shipping, progress came rapidly.  I had a working board and nothing else ready and just the idea that I should use a computer and some analog to digital converters. 

At this point, I first considered the Arduino.  It was sitting on top of my oscilloscope, waiting to do something useful.  I had never done anything with it except to change the default ‘BLINK’ program that blinks the LED on and off to instead send CQ on the LED.  In short order, I learned that the Arduino has several analog inputs and that the programming language supports raising one number to the power of another.  That was all I needed besides a display.  Doug and Ben, the experts at Gateway Electronics[12], explained that a sample 2 line by 16 character black on green LCD display in my junk box would serve.  I am very lucky to live near to Gateway Electronics and usually visit daily. 

The Arduino does an amazing job of unburdening your tasks.  I expected to spend a week interfacing some kind of display and learning to twiddle the right bits to make it work.  It turns out that the Arduino directs you to references that show you how to connect displays it understands and already has the programming library built in.  Similarly, the analog to digital converters operate with simple program commands.  Figure 6 shows the simple code that performs the basic functions of the VSWR Meter.  This is all the code from the original working VSWR Meter, not just an excerpt. 

More bits of resolution would be nice, but 9 to 10 is adequate.  I reduced the analog to digital converter reference voltage AREF from 5 V to 2.4 V to get that last bit of resolution.  See the sidebar on Arduino Uno ADC References. 

If this were a commercial product, I might select another processor that consumes less power or has higher A/D converter resolution, but the Arduino is excellent for rapid prototyping.  I ordered several more. 

Power Considerations for Portable Operation

The prototype in Figure 3 draws 70 mA from a 5V supply for the Arduino, the display, and the logarithmic detectors.  The backlight for my junk-box display draws another 160 mA, but this display works fine without the backlight in suitable ambient illumination. 

Jon NØWL suggested I optimize for battery operation by turning the Arduino off between measurements rather than using a delay loop to set the timing.  This didn’t help as much as I expected, because there is other circuitry on the Arduino including a second processor to handle USB communication to the PC, and this other circuitry stays active and consumes over 30 mA. 

The solution is to remove the ATMega328P-PU chip from the Arduino and build suitable circuitry around it.  Figure 7 shows the prototype CPU board is similar in layout to the Arduino.  Some folks call this a “Pseuduino.”  Along the way, I purchased more such chips from DigiKey and learned to program them with a bootloader[13],[14].  Then I needed to program them remotely from the Arduino board.  Ben, one of the experts at Gateway Electronics, taught me how to program an ATMega328P-PU chip remotely from an Arduino Uno with the chip removed.  I find only one reference[15] on-line with only verbal instruction, so I provide a second sidebar to illustrate this method of remote programming.  Elimination of the excess circuitry reduced the current drain of the ATMega328P-P, which still ran full time, to about 16 mA.  That means the USB interface and other circuitry drew over 30 mA. 

I built a first Pseuduino to replace the Arduino in the VSWR Meter and a second for use as a fixture to aid in assembling and calibrating these instruments in a production environment.  I eventually turned an Arduino into a Pseuduino so that I wouldn’t have to build any more.  I describe this procedure in a third sidebar below. 

I found an excellent tutorial on how to put the CPU of an Arduino into various power saving modes when it isn’t necessary to be active[16].  The ATMega328P-PU chip draws about 2.5 mA in the lowest power sleep mode, including 1 mA for the reference diode.  With the display energized full time, CPU and display together consume 5 mA in sleep mode, which lasts for about 90% of the measurement cycle.  

I added circuitry to turn on the remaining high current circuitry when necessary.  The Logarithmic Detectors consume about 15 mA but only need to be on for 30 ms prior to the two measurements by the A/D converter.  The backlight is optional, but I made it light up for about 100 ms after each display update with a repeating period of 500 ms or for 20% of the time for battery operation.  Under these conditions, current drain is under 10 mA without the backlight and 33 mA with the backlight pulsing or blinking after each measurement. 

Finally, I found a white on blue LCD display at Gateway Electronics with a very much lower backlight current, but which doesn’t operate at all without a backlight.  This backlight requires much less current than the ATMega328P-PU chip, so it made sense to run the backlight constantly rather than running the CPU to blink the backlight. 

Figure 8 shows the supply current as a function of time for the high and low backlight current displays.  The bottom line in each is the zero reference.  Sensitivity is 10 mA per division in Figure 8a (1 mV per division divided by the 0.1Ω current sense resistor).  Current rises from 8 mA in sleep to 37 mA when the logarithmic detectors turn on and 22 mA when they turn off and the CPU chip performs the measurement and calculations.  Finally the backlight blinks on raising the peak current to 70 mA for 100 ms.  Average current was under 25 mA for the display with the high current backlight in this energy saving mode, but the blinking was annoying.  Sensitivity is 5 mA per division in Figure 8b (500 µV per division divided by 0.1Ω).  Current rises from 5 mA in sleep to 30mA when the logarithmic detectors turn on and the CPU chip performs the measurement and calculations.  Final current drain with the backlight is now 7.1 mA, well suited to battery operation.  The 5 mA sleep current includes the ATMega328P-PU chip, the display, the backlight, and current through the reference diode that supplies AREF.  Yes, I could save another 1 mA by applying power to the reference diode only during measurements.  Perhaps I will. 

Figure 9 shows the final prototype with the backlit 2 line by 16 character white on blue LCD display. 

Figure 10 shows the innards of the final prototype.  I have since added nickel- metal hydride AAA cells that fill some open space in the cover and charging circuitry.  A relay disconnects the battery when power is available from the wall-wart.  This proved simpler that the alternative semiconductor circuits I tried.  The wall-wart charges the nickel metal hydride cells through a constant current circuit. 

I also tried a 2 line by 40 character black on green LCD display provided by Herb AF4JF.  It has no backlight and requires suitable ambient illumination.  This enabled sufficient characters to implement a bar graph for analog display.  OK1DX used this approach[17], and it looks neat. 

I placed an actual analog meter on a PWM output for a true analog display.  This could lower the cost of parts, but current drain is much higher, because the CPU chip must stay active and never enter a low power sleep mode.  Still, this enables a familiar interface except that the operator no longer needs to set the full scale reading in the forward direction before switching to obtain a reading in the reverse direction.  The correct reading just appears at the meter with no switching and no complex cross-needle indicator.   

Figure 11 shows the accuracy of this instrument in measuring VSWR over a wide range of load resistance at a sub-QRP power level.  I performed these measurements as best I could by placing small resistors with the shortest possible leads within a coax connector.  With a 7Ω termination, the ideal VSWR should be 7 and the instrument measures between 5 and 6 over the entire frequency range.  Similarly, with a load resistor of 500Ω, the ideal VSWR should be 10 and the instrument measures between 5.7 and 9.1 over the entire frequency range. 

Recent Efforts

Recent attempts to make this unit more readily manufacturable led me to simulate a true stripline version by replacing the unclad FR-4 overlay with one that uses a single sided copper clad FR-4 overlay.  This required adjusting the trace widths.  I wasn’t as lucky this time, and the mainline impedance measures 56Ω.  This limits the minimum measureable VSWR in a 50Ω system to about 1.2, but the exercise shows that stripline works, and we are presently trying to find someone to manufacture such boards inexpensively. 

Finally, I received several requests to incorporate power measurement.  When I finally lashed a third logarithmic detector onto the simulated stripline board, this proved to be a surprisingly simple upgrade.  The first try at power measurement was flat within ± 0.3 dB from 1 to 30 MHz.  With some attention to equalization, the third try was flat within ± 0.3 dB to above 150 MHz.   

Drawbacks

Are there any drawbacks or problems with measuring VSWR at such low power levels?  You bet!  It turns out that when you connect to a real antenna instead of a dummy termination, the reverse log detector senses signals from the antenna that may dominate the small return from a low VSWR antenna and erroneously raise the apparent VSWR.  For this reason, I will consider shortening future versions of the sampler to make them less sensitive.  Slightly increasing the spacing between the main transmission line and the sampling transmission lines will yield the same result.  For now, it is best to use this instrument well above the lowest power levels. 

Availability

Lee NØVI and I are trying to turn this into a product or an SLQS[18] club project, if we can produce accurate couplers at low cost.  The main transmission line impedance is critical to performance.  An air gap under the FR-4 layer would degrade performance, so we want to make the coupler in stripline rather than embedded microstrip. 

Assembly of a kit would require installation of at least four surface mount chip resistors and six surface mount chip capacitors, more if we include power measurement.  I characterize and match the logarithmic detectors to optimize performance and recently developed a simpler method to perform this previously burdensome task.  For these reasons, the coupler should probably be provided as a complete calibrated assembly. 

Check Lee’s website[19] to learn whether a product became available. 

Conclusion

This instrument enables fairly accurate VSWR measurements for really low power operation over an incredible frequency range from below 160 meters to above ¾ meters.  Accuracy degrades gracefully as power decreases.  Results are acceptable at the lowest HF frequencies down to 30 mW!  With slightly more power, this instrument proves useful at 100 kHz and below. 

Measurements directly to the SMA connectors on the prototype coupler are accurate up through UHF.  With added adapters and coaxial cable to convert to BNC connectors, performance is still very accurate up through VHF and more than usable to 500 MHz. 

The prototype works over the frequency range from 1-500 MHz at less than 100 mW and accommodates 100 Watts up to 15 meters, 70 Watts up to 10 Meters, and less with increasingly higher frequencies as coupler efficiency increases.  Increasing the spacing between the main transmission line and the sampling transmission lines or shortening these lines would allow higher power at higher frequency at the expense of less sensitivity at the lowest frequencies. 

This was an incredibly enjoyable and rewarding project.  I have used personal computers to control circuits in other recent projects that appeared in QEX, but this was my first attempt at embedded programming since I spent nearly 2 years programming a DSP in assembly language in 1992-1993.  I had the Arduino programmed and running in 2 days.  What a kick! 

Dr Sam Green, WØPCE, is a retired aero­space engineer. Sam lives in Saint Louis, Missouri. He holds degrees in Electronic Engineering from Northwestern University and the University of Illinois at Urbana. Sam specialized in free space optical and fiber optical data communications and photonics. He became KN9KEQ and K9KEQ in 1957, while a high school freshman in Skokie, Illinois, where he was a Skokie Six Meter Indian. Sam held a Technician class license for 36 years before finally upgrading to Amateur Extra Class in 1993. He is a member of ARRL, a member of the Boeing Employees Amateur Radio Society (BEARS), a member of the Saint Louis QRP Society (SLQS), and breakfasts with the Saint Louis Area Microwave Society (SLAMS). Sam is a Registered Professional Engineer in Missouri and a life senior member of IEEE. Sam holds seventeen pat­ents, with one more patent application pend­ing. Contact Sam at w0pce@arrl.net.

 


Figure Captions

Figure 1 Bare printed circuit board with couplers shows main and coupled lines

Figure 2 Cross sections of planar transmission line options

Figure 3 Working W0PCE QRP VSWR Meter

Figure 4 Consistency of 8 AD8307 logarithmic detectors calibrated to 25 mV/dB slope and -86 dBm intercept

Figure 5 Schematic diagram of bi-directional coupler and dual logarithmic detector board

Figure 6 Arduino code performs W0PCE QRP VSWR Meter functions

Figure 7 Early version of Pseuduino copies Arduino functionality without USB interface

Figure 8a Supply current profile shows 8 mA during sleep with pulses to 70mA for 100ms for the backlight

Figure 8b Supply current profile shows 5mA during sleep with pulses to 30mA during measurements

Figure 9 White on blue LCD display from Gateway Electronics operates at low backlight current

Figure 10 Innards of recent prototype shows assembly details.  Battery fits in open space of cover.  

Figure 11 Accurate measurement of VSWR versus load resistance at low power over wide frequency range

 

Sidebar Captions

Figure Sidebar 1 External 2.40 Volt Analog Reference Supply

Figure Sidebar 2 Remove ATMega328P-PU Chip and connect as shown to program remote ATMega328P-PU Chip

Figure Sidebar 3 Remove surface mount ATmega16U2-MU and two LEDs and NCP1117ST50T3 LDO regulator to reduce supply current


SIDEBAR 1 - Arduino Uno ADC References

There seems to be some confusion in how to use the various voltage reference options for the analog to digital converter in the Arduino Uno. 

The best guidance I found is: http://arduino.cc/en/Reference/AnalogReference?from=Reference.AREF

Place the statement

analogReference (type) ; 

within the curly braces of the void setup() {  }  statement. 

Options for “type” must be all upper case and are:

a)      DEFAULT, which provides an internal reference of about 5V.  Mine measures 4.74V. 

b)      INTERNAL, which provides an internal reference of about 1.1V.  Mine measures 1.087V. 

c)      EXTERNAL, which uses any voltage between 0 and 5V applied to the AREF pin. 

If the user applies a voltage to AREF for use in the EXTERNAL mode, that voltage struggles against the internal source when the DEFAULT or INTERNAL mode is active. 

To illustrate the problem, set the Arduino Uno to default mode and plug an LED into the pins AREF and Ground.  Unless the LED is in backwards, it will glow brightly, indicating a current flow of tens of milliamps out of the AREF pin.  If you place a voltage reference diode across these pins, a similar current will flow through it in DEFAULT mode. 

The Arduino site above suggests a resistor between the AREF pin and the external reference voltage as in Figure S1.  This resistor decreases the actual reference voltage, so accuracy suffers, but this also provides a means to trim the external reference voltage to a lower desired value. 

I used a 2.46 V reference diode fed with about 1 mA from Vcc through a 2.7 kΩ resistor.  Then I connected the reference diode to AREF through a potentiometer.  At about 1.5kΩ, the reference voltage at AREF reduces to 2.40 V, the value I desired. 

 

 

 

 


SIDEBAR 2 - Program CPU remotely from Arduino Uno per Ben at Gateway Electronics

Connect the TX, RX, RESET, and GND lines of the remote ATMega328P-PU chip and the Arduino Uno without the ATMega328P-PU chip installed as in Figure S2.  Either connect 5V or let the circuit where the remote ATMega328P-PU chip is running provide 5V.  Then simply program sketches into the ATMega328P-PU chip as if it were still plugged into the Arduino Uno.  This does not work unless the crystal is 16.0 MHz, as on the Arduino Uno.  Remove the RESET line if you remove Arduino power to allow the remote ATMega328P-PU chip to run. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


SIDEBAR 3 – Convert Arduino Uno to Pseuduino

I first built this instrument around an Arduino.  In order to operate efficiently from battery power for portable use, I built a copy with minimal current drain.  I eliminated all functions other than the ATmega328P-PU and the external reference supply.  Folks call such an Arduino copy a Pseuduino. 

To simplify the process of making a Pseuduino, I attempted to turn an Arduino into a Pseuduino. 

I started with the removal of the surface mount ATmega16U2-MU.  This reduced the supply current by about 20 mA. 

Then I removed the yellow LED next to the “L” and the green LED next to the “ON”.  This further reduced current drain by about 8 mA. 

Finally, I removed the NCP1117ST50T3G 5.0 V LDO regulator next to the external power connector, which draws 3.4 mA, even with power supplied from the output side. 

The LP2985-33DBVR 3.3V LDO regulator drew less than 100 µA, so I reinstalled it.  The Sparkfun hot air rework station[20] made removal of these extra parts relatively simple. 

Figure S3 shows these parts removed and sitting to the right of the circuit board. 

Without the ATmega328P-PU CPU, the supply current for this Pseuduino implementation is now less than 550 µA.  I replaced the ATmega328P-PU CPU after taking the photo. 

To program the ATMega328P-PU Chip remotely in a Pseuduino as in the second Sidebar, simply connect the 5 pins on the unmodified Arduino with the ATMega328P-PU Chip removed to those same 5 pins on this Pseuduino which now hosts the remote CPU.    



[1] Wes Hayword W7ZOI & Bob Larkin W7PUA, Simple RF Power Measurement, QST June 2001, pp 38-43

[3] Bill Kaune W7IEQ, A Modern Directional Power/SWR Meter, QST Jan 2011 pp 39-43    

[10] Sam Green WØPCE, A Fully Automated DDS Sweep Generator Measurement System - Take 2, QEX, Sept 2012