Accuprobe Assembly and Use




    The Accuprobe is a descendant of the common diode RF detector.  It uses a special operational amplifier with a compensation diode to cancel out non-linear diode characteristics at low RF input levels.  This allows accurate measurements down to about 50 mV rms as compared to several volts for ordinary detectors.  This translates to a very low QRpp level of about 50 microwatts.  The upper end of the range is 5 V rms or 1/2 watt.  A second uncompensated range is provided to allow measurements up to 35 V rms or 24-1/2 watts.  Compensation is not needed for the higher voltage range since the detector is linear for high input levels.


    The Accuprobe is used as an accessory to a reasonable quality digital multimeter.  It provides a DC output voltage that gives a DMM reading calibrated to the RMS value of the Accuprobe RF input.  On the LOW range the DC is read directly while the DMM reading is 1/10 the RF input on the HIGH range.


    The Accuprobe is AC coupled so it can be connected directly to biased circuits with voltages up to 50V without upsetting circuit bias or inaccurate readings due to presence of DC.  Its high input impedance and low capacitance allow it to be used with minimum loading on high impedance low-level circuits.


    A unique housing method will be described to allow the builder to construct a self-contained shielded detector with a built-in probe and convenient ground lead.  Hints for improving performance and tailoring the detector for other uses will be given later for the advanced homebrewer.





Printed circuit board dimensions: 1.8X1.9 inches

Power:  9 VDC at approx 1 ma using a standard 9-volt battery

Input levels:       LOW range - 50 mV rms to 5V rms (usable to 20 mV)

                        HIGH range – 5 V rms to 35 V rms

Frequency range:  100 kHz to 30 MHz (Upper end not tested but should extend to VHF)

Outputs : DC-compensated to rms input

            LOW range – 50 mV DC to 5 VDC (Uncalibrated readings to 20 mV)

            HIGH range – 5 VDC to 35 VDC (1/10 RMS input)

Output accuracy: approx. 10% of reading decreasing to 25% at 50mv



Printed Circuit Board Assembly


    Before you begin assembly please check the parts supplied versus the attached parts list.  This will ensure that all parts are supplied and will help identify the precision resistors to eliminate confusion as assembly continues.  The parts list shows identifying marking for those components that require it.  In addition it is recommended that you verify resistors using an ohmmeter or DMM.  Also check the Addendum sheet/s for last minute corrections and clarifications.


    Component placement is silk screened onto the top side of the pc board as reproduced in Figure 1. These instructions will provide guidance for several components where physical orientation is important.  Installing components on the printed circuit board is best done in stages.


·         Install all of the resistors. Crimp and solder all resistor leads.  Save two of the longer clipped-off leads for later use.

·         Install diodes D1 and D2.  Both need to have their  leads bent to accommodate the extra wide hole spacing provided.  D1’s banded end should be at the end marked “K”.  D2 should be installed with its banded end the same as the symbol screened on the pc board.  Crimp and solder the leads.

·         Install C1, C2 and C3.  C1’s leads need to be bent with extra wide spacing to fit the pc board holes.  Crimp and solder the leads.

·         Install the 8-pin IC socket for U1.  Align the notched end of the socket with the corresponding notch in the socket outline marked on the board.  If needed hold the socket in place with cellophane tape and solder each pin.

·         Install the pin headers at J1, J2 and J3.  Insert the short-pin end of the headers trough the pc board and hold in place with cellophane tape and solder the leads.  Plug shunts onto the headers to prevent losing them!

·         Strip two-inch bare leads and solder in place on the IN and GND pads on the left-hand side of the pc board.  These leads will be used to connect to chassis ground and a probe feedthru for RF inputs.

·         Solder the 9V battery clip in place.  The red lead connects to the +9V hole and the black lead to the GND lead above it.

·         Strip off about ½ inch of the twinlead cable at both ends. Connect the striped or silver colored lead to the remaining GND pad on the board and the non-striped or copper colored lead to the OUT pad.  Do not put the banana plugs on these leads yet.

·         Plug integrated circuit U1 into its socket with the notch in the end of the chip oriented toward the notch in the end of its socket.



Mint Tin Enclosure Details


    The pc board was laid out for convenient mounting in one of the ubiquitous Altoids (TM) mint tins.


    Figure 2 is a photo of a prototype board installed in a tin this way.  RF input connection is made via a homebrew probe detailed in Figure 3.  A nylon machine screw is drilled coaxially with a clearance hole for a brass 6-32 bolt.  The nylon screw passes through a ¼ inch hole in the case with its head outside the box and is held in place by a ¼ inch nylon nut on the inside.  The 6-32 bolt has its head inside the case securing a solder lug. It is held in place by a brass nut tightened on the screw outside the case.  The far end of the brass screw is filed to a conical point for easy circuit probing.


 The input probe is connected to the printed circuit board by the wire soldered to the IN pad. It is also soldered to the probe solder lug.


    The Accuprobe’s RF ground connection is made through a 3-inch alligator clip lead (wire not supplied with kit) mounted to a bolt through a clearance hole in the metal case.  The hardware stackup is illustrated in Figure 4.  A 6-32 bolt protrudes through a hole in the case next to the input probe.  The screw head is inside the case and holds a solder lug in place.  The wire soldered onto this lug goes is one connected to the GND pad on the pc board.  Outside the case a second solder lug is used to hold the flexible ground lead.  A nut secures the lug to the case.  Do not use a longer ground lead since this can result in inaccurate readings.


    The pc board can be mounted in the case several ways.  For very secure mounting, drill four holes in the tin lining them up with the four corner holes in the pc board.  Be sure, however, to leave room for the 9-volt battery.  Use insulated standoffs or rubber grommets to hold the board above the metal case so that the bottom side does not short to the case.


    A less formal but more expedient mounting method is to use double-sided adhesive foam tape commonly used to mount photographs.  It is available at most full-service chain drug stores, hardware megastores and the ubiquitous Radio Shacks ™.


    The battery can be mounted using the same adhesive foam tape.  It should last for months so long as the Accuprobe is turned off when not in use so it can be mounted more or less permanently in this way.  The more adventurous homebrewer can use adhesive-backed Velcro™ strips to allow easy battery swap out.


    DMM output leads pass through a ¼ inch hole in the tin that has a grommet mounted in it to protect them from abrasion from the metal.  The far ends of the leads are connected to banana plugs to plug into a DMM.  This wire is not supplied with the kit so be sure to use leads with different colors so that you can put the black (-) banana plug on the GND lead and the red (-) plug on the OUT lead.





    Before connecting a battery for the first time, ensure that jumpers JU1 and JU2 are in their left-hand position (i.e. the copyright message is at the bottom of the board) and the power jumper JU3 is plugged onto only one pin of the terminal block. Connect the output leads to a DMM, observing correct polarity.  The red plug is positive and the back one is negative.


    Set the DMM to read DC voltage on a range of 10 volts or so.  Now set power jumper JU3 to connect to both leads on the power terminal block.  The DMM should read about 10 mV or less.  If it reads much higher than this check connections and ensure that IC U1 is oriented correctly and that all its pins are making contact with the socket.


    To check operation, you will need a low-level RF source.  The best way to check operation is with a calibrated RF signal generator.  Simply load the output of the generator with its rated load resistance and connect the probe across the resistor.  DC readings on the DMM should be the RMS value of the signal generator output.


    Lacking a calibrated signal generator you can use an oscillator in a receiver.  Clip the input ground lead to a ground point in the receiver close to the oscillator components and touch the input probe to the output of the oscillator.  You should get a reading in the 10’s of millivolts to several volts depending on the oscillator output level.  If you have a circuit where the RF levels are specified you can verify that the readings agree.  Otherwise all you will get is a relative indication.


    The Accuprobe’s HIGH range can be checked using an HF QRP transmitter.  Be sure to set jumpers to the HIGH position with both of them on the right hand set of terminals.  Set the transmitter to output a known level in the QRP range and assure that it is terminated in a 50 ohm load.  Now clip the Accuprobe ground lead to a convenient ground point and touch the input probe to the center conductor of the transmitter output connector.  The DC reading on the DMM will be 1/10th the RMS value of the transmitter output voltage.  At 1 watt this will be 70.7 mV, ranging up to 158 mV for a 5 watt transmitter.


    Be sure to plug power jumper JU3 onto only one lead of its terminal block when you are done using the Accuprobe to turn off the power.



Theory of Operation

    The Accuprobe is an RF detector with compensation circuitry that extends accurate readings well below common RF detector probes.  Reference to the attached schematic diagram shows that it begins with a common AC-coupled half-wave detector formed by input capacitor C1, Schottky diode D1, resistor R1 and filter capacitor C2.  At RF input levels of several volts or more, the DC across C2 is approximately the peak value of the input signals.  However the inherent diode non-linearity causes the DC voltage to be less than the peak voltage at low levels and quite small (only millivolts) when the input RF is below 100 mV.


    For low input levels (the LOW range) jumpers JU1 and JU2 are connected to the left-hand sets of terminals.  Resistor R2 serves as a load for the half-wave detector.  DC from the detector is fed to operational amplifier U1a through R3.  R3 may appear unnecessary since its resistance is so low that it has negligible effect, however it prevents damage to U1 from input voltages higher than the specified range.


    Diode D2 provides feedback to the U1a’s inverting input to compensate for low-signal detector non-linearity.  The amount of feedback applied is adjusted by resistor R4.  This compensation scheme is the brainchild of John Grebenkemper, KI6WX who used it in SWR bridges.  W7EL later applied the same technique in QRP SWR bridges.  Their material has appeared in the ARRL Handbook and QST.


    U1a’s output is the corrected peak value of the input RF voltage.  It is fed through a voltage divider formed by R6 and R7 which convert it to the input’s RMS value.  Again U1b is a unity gain buffer that feeds a DMM.


    For higher voltage levels (the HIGH range) jumpers JU1 and JU2 are set to connected the right-hand sets of terminals.  Resistors R5 and R7 form a voltage divider to convert the peak DC voltage to the corresponding RMS DC value of the RF input and reduces the voltage by 10:1.  Operational amplifier U1b is a unity-gain buffer that feeds the resulting DC level to an external DMM and prevents loading of the detector by the DMM.  The 10:1 voltage reduction is needed so that DC fed to U1b is less than its 9-volt power supply.  This results in DC readings on the HIGH range that are 1/10th the RF RMS input voltage.


    Operating power is supplied by a 9-volt battery.  No on-off switch is provided. Instead Jumper JU3 is used to connect two terminals when plugged onto both leads of the terminal block.  Capacitor C3 serves as a power supply bypass for U1 to prevent instability.





    The Accuprobe is useful for measuring RF signals with predictable results from 100 kHz to at least 30 MHz at levels ranging from 10’s of millivolts up to 35 V rms.  It provides minimal circuit loading so that it can be used for signal tracing in oscillator and mixer circuits as well as multistage QRP transmitters.  Several suggested applications are:


·         Amplifier input and output voltages to determine gain

·         Filter input and output voltages to determine loss

·         Measurement of resonant circuit or filter circuits across a frequency band to check bandwidth or Q

·         RF power levels across a dummy load from microwatts to beyond QRP.  Note that a very accurate 50 ohm dummy load is needed to retain accuracy.

·         Attenuator calibration by accurate loss measurements.

·         Signal generator output calibration

·         The Accuprobe circuit board can also be incorporated into other projects wherever accurate repeatable RF voltage measurements are needed.



Enhancement Ideas


  The Accuprobe can be added to and tailored to improve its utility and accuracy. A short list of enhancements is:


·         Measure its output values with a laboratory grade calibrated signal generator and develop a calibration chart specific to each unit.  Accuracies of 1 or 2 percent are using this method.

·         Panel mount switches can be used for range switching and power on/off.  Use of terminal blocks allows easy connectorization of the off-board leads.

·         Accuprobe low frequency response can be extended below 100 kHz by increasing the capacitance of C1 and C2.

·         A BNC or other coaxial input connector can be used instead of the probe provided.  A Tee connector on this input connector would ensure that a good 50 ohm termination could be made for accurate power readings in a coaxial cable hookup.

·         Diodes D1 and D2 can be matched for improved accuracy below inputs of 100 mV RMS.

·         A small adjustable bias circuit could be added to null out the inherent operational amplifier offset voltage for improved accuracy with small input signals.

·         The Accuprobe can be interfaced with a precision analog to digital converter, a microcontroller and numeric display to build a dedicated RF voltmeter or RF wattmeter.  Calibration values could be measured and stored in the microcontroller memory to provide improved measurement accuracy.

·         Analog meter fans can also add an external analog DC meter with appropriate ranges to make an analog RF voltmeter or RF wattmeter.





The Accuprobe project was conceived and promoted by Doug, KI6DS who also provided necessary prodding and encouragement.  AmQRP provided funding for prototype development.  Printed circuit layout was masterfully carried out by Lenny, W2BVH.  Circuit design and checkout was performed by Joe, N2CX.  Technical questions on the Accuprobe can be directed to him at



Figure 1 – Accuprobe Component Layout



Figure 2 – Accuprobe Prototype installed in Altoids ™  mint tin


Figure 4 – Accuprobe Schematic Diagram


Accuprobe  Parts List


Ref Des.            Value                           Marking


C1, C2  .01 uf, 10%, 50v                        103

C3        .1 uf, 10%, 50v                           104      

D1, D2  1N5711                                    1N5711  Cathode end marked with stripe on body

R1         1k, ¼ w  5%,                            brn-blk-red bands

R2        1 Meg, ¼ w  5%,                      brn-blk-grn bands

R3         15k, ¼ w  5%,                          brn-grn-ore bands

R4        150k, ¼ w  5%,                        brn-grn-yel bands

R5        649k, 1/4w, 1%                         blu-yel-wht-ore bands

R6        20.5k, 1/4w, 1%                        red-brn-grn-red bands

R7        49.9k, 1/4w, 1%                        yel-wht-wht-red bands

U1        LMC6482AIN                            LMC6482AIN dot at pin 1 and notch on pin 1/8 end of body

JU1, JU2 3 position,  .025” post hdr strip 

JU3       2 position .025” post header strip

3 – shunt                                              

1 – printed wiring board

1 – 6-32x3/16 pan head screw

1 – 6-32 nut

1 – 6-32x1 brass screw

1 – 6-32 brass nut

1 – 1/4-20 nylon pan-head screw

1 – 1/4-20 nylon nut

1 – nylon hole grommet ¼” hole             

1 – minigator clip                                              

3 – solder lug                                        

1 – 9V battery terminal                          

1 - red banana plug

1 – black banana plug

1 - 8 pin IC socket                                 


Note:  Also required but not supplied is some wire that you likely have around the shack. 


A 3-inch long piece of insulated stranded 22 ga or so hookup wire to use for the probe ground lead. 


A 2-ft or so two conductor lead to connect the Accuprobe to an external digital multimeter.  You can use common two-conductor speaker cable, audio twisted pair cable or twist together two lengths of stranded hookup wire to make your own twisted pair.















Accuprobe Addendum – 6-3-2004


1.       Development of the Accuprobe circuitry led to the need for the pc board to accept several different component configurations.  Because of this, hole locations for diodes D1 and D2 and capacitor C1 are not optimum.  Bending of the diode leads should be obvious.  Since C1 will not drop directly into its mounting holes its leads must be prepared as described below:





















2.  Silk screened marking for D1 and D2 are not consistent.  D1 has a “K” at the top end of the diode outline to show how the negative (banded) end should be oriented.  Diode D2 has the band in the outline.


3.  Some kits may have two styles of 0.01 uf capacitors for ease of installation.  One will have a radial lead arrangement as shown above.  This one should be mounted at the C2 location.  The other will have an axial lead arrangement as shown below.  This should be used for C1.  Both will be marked “103”.