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Teach-In 2026 by Mike Tooley World of Wireless – An Introduction to Radio and Wireless Technology Series 12, part 6: RF testing and measurement I n the last instalment, our focus shifted to antennas, including how several popular types are built, how they work and how well they perform. We investigated several basic antennas and explained the importance of matching them properly. Our Hands-On project featured a 60-300MHz dipole reference antenna. This month, we will be looking at the principles and techniques that underpin a range of useful RF measurements, including voltage, power and frequency. Our HandsOn project is a handy RF probe for use with an ordinary DC meter for RF measurements over a range extending from 100kHz to 100MHz. Measuring RF voltages If a signal is within its measurement range, the easiest method of measuring RF signals is with the aid of an oscilloscope and compensated probe. Reliable measurements of peak-to-peak voltage can usually be made at frequencies up to and beyond 30MHz. Even the budget hand-held oscilloscope shown in Fig.6.1 can produce accurate indications up to 10MHz. If you are lucky enough to possess a high-specification bench ‘scope, you may be able to make measurements up to 100MHz or higher. Where frequencies are beyond the measurement range of a ‘scope, a simple diode detector (like that shown in Fig.6.2) can be used in conjunction with a conventional analog or digital meter display. The detector responds to the peak value of the RF input, but the meter is often scaled in root-mean-square (RMS) units (see later). If you look carefully at Fig.6.2 you should be able to correlate each of the surface-mounted components with those shown in the circuit diagram, Fig.6.3 (there are two diodes in one package). The low-cost diode detector shown in Fig.6.2 is specified for Fig.6.1: using a 10MHz hand-held ‘scope and ×10 probe to measure the RF output of a low-power 7MHz transmitter. 4 operation from 100kHz to 3.2GHz in a 50Ω system via the SMA input connector. To minimise any stray reactance (that would otherwise degrade a detector’s high-frequency performance), the RF voltmeter’s probe tip and ground connection need to be kept very short. Commercial wideband RF voltmeters overcome this problem by using probe mounts to extend the frequency range well into the UHF/SHF range. At signal levels below 1V, it’s important to be aware that the DC output voltage from a simple diode detector responds to the peak value of the applied RF voltage less the diode’s forward voltage drop. This results in nonlinearity at low input levels, as illustrated in Fig.6.4. Where a digital readout is required, a low-cost microcontroller with integral analog-to-digital converter (ADC) can be employed along the lines shown in Fig.6.5. Based on an Arduino Uno and a 16×2 I2C LCD screen, this delightfully simple arrangement only needs a connection to one of the Uno’s analog input ports from the diode detector shown in Fig.6.6. The code fragment shown in Listing 6.1 (later in the article) shows how to cope with the RMS scaling and conversion to dbV (more on that later). Fig.6.2: a low-cost diode-based RF detector module. Practical Electronics | April | 2026 C1 D2 + RF input D1 C2 R1 1.396 V 2.90 dBV 16×2 LCD I2C display Vout Fig.6.3: the circuit of the low-cost diodebased detector module. 10.0 SDA RF input Vout 9.0 A4 A0 GND A5 Diode detector 8.0 SCL GND Vcc +5V +5V +5V GND 0V Arduino Uno GND GND Indicated voltage (VDC) 7.0 Fig.6.5 an RMS-reading RF voltmeter based on an Arduino and 16×2 LCD screen. 6.0 5.0 4.0 3.0 2.0 Non-linear region 1.0 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Input voltage at 1MHz (VRMS) Fig.6.4: a plot of output voltage vs RMS input voltage for the Fig.6.3 detector. Limitations When interfacing with a microcontroller, it’s important to avoid applying too much voltage to the analog input ports. Fig.6.6 avoids this problem with R1 and D3 protecting the input when more than 5V is applied to an analog input port. Note also that the RF input must be limited so that the maximum reverse voltage of the detector diodes is not exceeded. In Fig.6.6, each of the BAT42 diodes in the voltage doubler arrangement has a maximum reverse voltage rating of 30V, so the maximum input voltage that can be safely applied to the voltage doubler arrangement is about 20V RMS. For greater input voltages, the BAT42 diodes should be replaced with BAT41 diodes (100V maximum reverse voltage). Alternatively, a compensated attenuating probe can be used. Voltmeter calibration & sensitivity Simple diode detectors measure the peak RF voltage and display RMS values assuming the signal is a pure sinewave, making their readings inaccurate for other waveform shapes. True RMS voltmeters instead calculate the effective AC value by determining the square root of the average squared instantaneous values over a full cycle, regardless of waveform shape. Devices like the popular Analog Devices AD8318 offer accurate true Practical Electronics | April | 2026 RMS detection through dedicated circuitry. The sensitivity limitations inherent in a simple diode detector can be addressed by employing a specialised integrated circuit such as the AD8318. It is designed for precise relative received signal strength indication (RSSI) and automatic power regulation. The AD8318 utilises multiple amplifier stages and detectors to compress and sum the input signal, delivering a DC output that reliably represents the RMS value across an extensive frequency range, from 50MHz to over 3.5GHz. The device features nine cascaded amplifiers, each with an individual detector (see Fig.6.7), with outputs that are collectively summed to generate the final DC output voltage. The AD8318 has a dynamic range of 65dB but, because of the high voltage gain, care must be taken to ensure that the input power is kept below 16mW (corresponding to an input voltage of 0.89VRMS in a 50Ω system). An interior view of the C1 RF input 10nF author’s home-built AD8318 RF detector head is shown in Fig.6.8. Using decibels for voltage Decibels provide us with a means of specifying quantities using a logarithmic rather than linear scale. This helps keep numerical values within a reasonable range. Expressed in decibels, the ratio of two voltages, V1 and V2, is calculated as: n=20 log 10 (( ) ) (( )) V n=20 log V dBV=20 log 10 (V )dBV n=20 log 1010 1 V dBrel V ref 2 (V out ( pk− pk )) P out = (in watts) V n=20 log 108 R L dBV=20 log 10 (V )dBV 1V ( ) V 2out ( pk− pk ) 2 12.65 V 2 160 V 2 = =0.4 W P out =(V out ( pk− pk ))= P out = (in watts) 8 RL 8×50 Ω 400Ω 8 RL ( ) P2 2 n=10Vlog V2 12.65 V 2 160 out (10 pk− pk ) dB = an =0.4 W P out = P 1 = Fig.6.8: 8 R L RF detector 8×50 Ω head 400Ω based on an AD8318 module. P n=10 log 10 P 2 dBrel n=10R1 log 10 P refdB 4.7kΩ P1 (( ) ) P Vout log ( P)dBW n=10 log P dBW=10 10 n=10 log 1010 1 W dBrel P ref ( )) ( D3 RV1 10kΩ C2 100nF V2 dB V1 When used to specify a relative V voltage (rather than V 2 gain n=20 log dBrelor loss), log 1010 Vvalue, refdB we use an=20 reference Vref, as V1 the denominator, ie: D2 BAT42 D1 BAT42 ( ) V2 1.3 V 2 P out =5V1RMS =P =0.0025 W or 2.5 mW n=10 logR10L log 10 ( P)dBW 680dBW=10 Ω 1W ( ) GND Fig.6.6: a diode detector circuit incorporating protection for a microcontroller. V2 1.3 V 2 P out = RMS = =0.0025 W or 2.5 mW RL 680 Ω Temperature TEMP Gain bias Slope VPSI sensor I Ʃ DM1 INHI INLO G1 DM2 G2 DM3 G3 DM4 DM9 I V V VSET VOUT CLPF C G9 Nine stage demodulating logarithmic amplifier Fig.6.7: the simplified internal arrangement of an AD8318 IC. 5 +40 +30 +20 dBV +10 0 -10 -20 -30 -40 0.01 0.1 ( ) 1.0 Measured voltage (V) 10 100 V n=20 log 10 2 dB Fig.6.9: V 1 a plot of dBV against RMS voltage. 1V is oftenV used as the reference n=20 log dBcase value, in10which the relationrel V ref ship becomes: load, producing a DC output voltage proportional V to signal amn=20 log 10 dBV=20 log 10 (V )dBV 1V plitude To help you understand this rela• applying the 2 (V outFig.6.9 ( pk− pk ) ) shows dBV plotted tionship, attenuated RF Fig.6.10: a wideband AC voltmeter calibrated in RMS volts & dBV. P out = (in watts) 8 R Lvoltage. Note that 1V = against RMS signal from a 0dBV, 10V = 20dBV, 100V = 40dBV matched attenuator to an inte- auto-balancing bridge. The output of 2 2 and soVon. Fig.6.10 a Vtypical grated circuit that senses the true the bridge is a DC voltage that can be 12.65shows V 2 160 out ( pk− pk ) = =0.4 W P out = = meter display separate scales for RMS value of input voltage from displayed using an analog meter or 8 R L with 8×50 Ω 400Ω RMS voltage and dBV. the attenuator converted using an ADC (eg, within The reason • sampling the RF signal as a base- a microcontroller) for digital display. P 2 that the formulae n=10 log dB 10 a scaling above use factor of 20, band AC signal, which can then Due to the intermediate converP1 rather than 10 (as you might exbe directly digitised, processed sion of RF energy to heat, thermal pect from the unit name decibel), digitally and displayed sensors provide a true RMS response P dB isn=10 that log decibels are • calculations of power based on over an exceptional range of frequen10 rel fundamentally P a measure of refpower ratio, not voltvoltage measurements made cies. Unfortunately, while they are age ratio. When comparing voltages with a wideband oscilloscope, excellent for measuring average P impedance, power across the10 same compensated probe and matched power, they can be too slow for some n=10 log dBW=10 log 10 ( P)dBW 1 W to the square of voltis proportional resistive load applications (eg, they will barely age: P =2V 2 ÷ R. Both thermal and detector-type respond to brief high-level bursts of V RMSthe1.3 V2 ofWa squared RF energy due to thermal inertia). PTaking = logarithm =0.0025 or 2.5 mW measurements are often described out = 680 Ω 10(V), and when voltage Rgives 2log as ‘direct sensing’ because the Simple diode detectors provide L this is multiplied by 10 (the ‘deci’ conversion of an RF signal to a DC a fast response, but they can be inpart of decibel), the result is the output is performed by means of a sensitive without additional amplifamiliar 20log10(V) scaling. probe or head mounted close to the fication. Due to the non-linearity of signal source. More complex ‘indi- their characteristic and maximum Measuring RF power rect’ methods involve a multi-stage reverse voltage rating, diode detecSeveral different technologies conversion performed remotely, tors have a limited dynamic range. are used for measuring RF power. where amplitude data is scaled to The modern solution to the They include: provide a power reading. measurement of RF power is with • thermal methods, in which the Thermal power sensors usually the aid of integrated circuits deheating effect of RF power in a employ thermistors attached to a signed specifically for true RMS matched load is sensed using a matched load. The resistance of the voltage and power measurement. thermistor (or ‘bolometer’) thermistor sensor changes with load Supported by a matched attenu• using diode detectors to rectify temperature. This change is detected ating load and software running the RF voltage across a matched using a temperature-compensated on a microcontroller or PC, these devices provide a sensitive and D1 BAT41 highly cost-effective solution to RF power measurement at frequencies SK1 up to several gigahertz. ( ) ( ) ( ) ( ) ( ) RF input 6 R1 50Ω C1 10nF + V - Fig.6.11: the circuit of a basic RF power meter. Using an oscilloscope At lower RF frequencies, it is possible to use an oscilloscope to measure RF voltage and thus, provided Practical Electronics | April | 2026 Power versus RMS voltage (50Ω load) 2.0 +20 +15 +10 1.5 dBW Power (W) +5 1.0 0 -5 n=20 log 10 -10 0.5 -20 0 2 4 6 8 10 RF voltage (VRMS) V2 n=20 dBbetween power and voltage for Fig.6.11. 10 Fig.6.12: the log relationship V1 ( ) 0.01 (( ) ) ( ) ( ) ( ) ( ( )) V V 2 log n=20 10 V10 dBV=20 log dB 0.1 n=20 log 101.0 10010 (V )dBV n=20 log 1 V dBrel 10 V1 V Measured power (W) ref 2 Fig.6.14: dBW plotted against(VRMS power. pk ) ) V = out ( pk− P (in watts) V out n=20 log dB 10n=20 log rel R good-quality 50Ω load. Such instru- with voltage, keeplog nudBV=20 10 8helps 10 (V )dBV V ref this 1 VL that the load resistance is accurately known, to determine power from ments may have digital or analog V n=20 log 10 dBrel of a wavethe peak-to-peak displays and are usually calibrated in V refvalue form observed on an oscilloscope watts (W) or milliwatts (mW). (many oscilloscopes can also perThe circuit of a basic RF power V n=20 log 10RMS measurements). dBV=20 log 10 (V )dBVmeter is shown in Fig.6.11. This form accurate 1V V The following simple arrangement incorporates n=20 log 10 2 dBrelationship is used: V1 a suitably rated 50Ω resistive load 2 (V out ( pk− pk )) across which the applied RF voltP out = (in watts) V 8 RL age is measured. Fig.6.12 shows the n=20 log 10 dB V ref2 intorelcontext, the ‘scope square law relationship between Putting this V out ( pk− pkearlier 160 V 2 12.65in V 2 Fig.6.1 ) and applied voltage in display shown = =0.4power W P out = = 8 RL 8×50 voltage Ω 400Ωof Fig.6.11. The resulting nonlinear indicates aV peak-to-peak n=20 log 10 dBV=20 log 10 (V )dBV 12.65V. This devel- calibration of a typical RF power 1 V voltage is being P 2 load consisting of meter scale is shown in Fig.6.12. oped across a 50Ω n=10 log 102 dB Fig.6.13 shows a typical example two 100Ω 1W resistors connected in P (V out ( pk− pk )) 1 P out = (in watts) of power measurement at RF using parallel. The output power can thus 8 RL a combined power and SWR meter be calculated as follows: P n=10 log 10 dB and an external load. The meter’s P ref V 2 rel160 V 2 V 2out ( pk− pk ) 12.65 digital display indicates an output = =0.4 W P out = = 8 RL 8×50 Ω 400Ω power of exactly 2W. P n=10 log 10 log 10 ( P)dBW If the waveform is dBW=10 not a sinewave 1W P 2 of distortion) (or has a lot and your Using decibels for power n=10 log 10 dB P 1 2has a built in RMS voltoscilloscope Decibels are frequently used to V 1.3 V 2 you can use age measurement specify power in RF systems. As P out = RMS = feature, =0.0025 W or 2.5 mW 680 ΩRMS measureL that to get an PRaccurate n=10 log 10 dBrel ment and then convert the voltage to a P ref power figure using the load resistance. Some high-end oscilloscopes P n=10 log 10 a mathematical dBW=10 log 10 ( P)dBW may have 1W function to perform that conversion forVyou. 2 V 2RMS 1.3 P outIn = that =case, ensure =0.0025 W or 2.5 mW RL 680 Ω you have a sufficient number of signal cycles visible on the screen for an accurate RMS measurement. If you can see, say, oneand-a-half cycles of the signal, the calculation may be inaccurate due to the imbalance that comes from including only half of the second cycle. ( ) V2 dB V1 V n=20 log V dB n=20 log 10 10 V2 refdB rel V1 -15 0.0 ( ) ( ) ( ) merical values within a reasonable range. Expressed decibels,2 the V 2out (in 160 V 2 pk− pk ) 2 12.65 V )= and (V = =0.4 W PVoutpowers, = out ( pk− pk P )log n=20 of log 10 dBV=20 (V )dBV P , is ratio two 10 1 (in P out = watts) 8R 8×50 Ω2 400Ω 1V calculated from: 8 R LL ( ) P out = ( ) 2 P2 (V out ( pk− pk ))V 2 n=10 log dB 12.65 V 2 160 V 2 out(in ( pk− pk ) 10 watts) 8PRoutL = 8 R P 1= 8×50 Ω = 400Ω =0.4 W L ( ) When used to specify a relative 2 2 V (rather 160 V2 12.65 power we out ( pk− pk ) than PV2 P= or n=10 log 10gain dBloss), rel =0.4 W P out = = n=10 log P refdB400Ω 10 8R 8×50 use a reference value, Pref, as the L PΩ 1 denominator, as follows: ( ) P P 2 log n=10 dBW=10 log 10 ( P)dBW n=10 log 10n=10 dB 10 P P 1 log 10 P1 WdBrel ( ) ( ) ( ) ( ) (( ) ) ( ) (( ) ) ref 1W is often the V 2RMS as1.3 V 2reference P =used P = =0.0025 W or 2.5 mW P out n=10 log dB case680 value, in10n=10 which the dBW=10 log 10 ( P)dBW Ω relationP ref logR10relL 1 W ship becomes: ( ) ( ) ( ) P 2RMS 1.3 V 2 n=10 log 10P = VdBW=10 log 10 ( P)dBW W or 2.5 mW out 1 W R = 680 Ω =0.0025 L This 2 relationship is illustrated V 1.3 V 2 graphically P out = RMS = in Fig.6.14, =0.0025 Wwhere or 2.51W mW R L 10W 680 Ω= 10dBW, 100W = = 0dBW, 20dBW and so on. Note that the ( ) ( ) Standalone power meters Fig.6.13: measuring the output power from a hand-held digital transceiver. The power meter indicates an output of exactly 2W delivered to the attached 50Ω load. For regular measurements, a dedicated power meter is a better alternative to an oscilloscope, with the added advantage that many standalone RF power meters incorporate a Practical Electronics | April | 2026 7 Fig.6.16: a low-cost AD8362 true RMS power-sensing module. True RF power meters A similar arrangement to that shown earlier in Fig.6.11 can be used to indicate RF power, but in this case, the true RMS diode detector is in conjunction Fig.6.15: an RF power meter calibrated in RMS power and dBm. with a matched resistive load same relationship would apply if and the display scaled in milliwe had plotted dBm (dB relative watts or watts. The load needs to to 1mW) against milliwatts (mW). be suitably rated in terms of power Hence, 1mW = 0dBm, 10mW = dissipation and must be purely 10dBm, 100mW = 20dBW and so resistive. True power meters must also on. Fig.6.15 shows a typical analog cope with the waveforms and crest power meter display with separate factors (peak to average ratios) scales for RMS power and dBm. With found in modern modulation sysits companion probe mount, this tems such as quadrature amplitude modulation (QAM) and orthogonal instrument is useful up to 40GHz. Fig.6.17: using the AD8362 with an Arduino Uno and digital display to measure the power output from a hand-held transceiver. 8 frequency division multiplexing (OFDM). This places extra demands on the signal processing, requiring a fast response coupled with a wide measurement range, typically 60dB or more. Typical of the devices currently available is the Analog Devices AD8362. This chip is a true-RMSresponding power detector offering a 65dB measurement range, designed for systems that demand an accurate response to true signal power. The chip operates from 50Hz to over 3.8GHz and accepts inputs from -52dBm to +8dBm. The AD8362 requires a single +5V supply and consumes a mere 1.3mW when in the powered-down standby state. A typical low-cost power sensing module based on an AD8362 is shown in Fig.6.16. The analog output of the module (0-5V) can be applied to a DC meter or to the analog port of a microcontroller like that shown in Fig.6.17, using code to display power in dBW or dBm if required, as shown on the LCD screen in that figure. Note the use of a power attenuator in Fig.6.17 to reduce the signal level into the AD8362. The first line of the display in Fig.6.18 indicates the power supplied to the sensing module (which must be kept below Fig.6.19: checking a dual-band FM transceiver. The DFM on the right indicates a frequency difference of 1kHz. Practical Electronics | April | 2026 Fig.6.20: checking the calibration of an RF signal generator. The 8-digit Fig.6.21: a home-built low-cost DFM being used to check DFM display shows 49467.584kHz with a 1Hz resolution. the output frequency of the VFO described in Teach-In 12.3. 12dBm), while the second line indicates the power dissipated in the attached 50Ω load. When using sensitive RF power measuring devices for indicating high levels of RF power, it’s essential to use a suitably rated combined load and attenuator. Measuring frequencies Frequency is difficult to measure accurately using a conventional oscilloscope. Modern digital storage oscilloscopes (DSO) fare somewhat better in this respect, as they can often display frequency values digitally. However, a dedicated digital frequency meter (DFM) is often a better solution. The desirable characteristics of such an instrument are high sensitivity and high upper frequency limit as well as adequate accuracy and resolution. A sensitivity of 100mVRMS or better is suitable for most applications, and an upper frequency limit of 500MHz will be adequate for most measurements at HF and VHF. Most DFMs offer an accuracy of ten parts per million (0.001%). This is more than adequate for all but the most critical applications. Input Amplifier and pulse shaper Reference TCXO The resolution of an instrument depends on the number of digits in the display. For example, if a resolution of 100Hz is required when measuring a signal at 50MHz, a minimum of six display digits will be required. The block diagram of a basic digital frequency meter is illustrated in Fig.6.18. The signal to be measured is initially sent through an input amplifier, followed by a Schmitt trigger buffer, which converts and shapes nearly all input signals into a logic-compatible pulse train. While the signal’s waveform changes, the pulses still match the original input frequency. Having a logic-compatible signal is important for connecting with the signal gate and the circuits that handle gating, counting, and latching. The reference oscillator provides a very stable and accurate source for the time base circuit, typically using a temperature-controlled crystal oscillator (TCXO). TCXOs offer stability better than 1ppm, compared with standard quartz crystal oscillators, which usually have an error of about 30ppm, taking into account both initial accu- Gate Pulse counter racy, ageing, temperature drift etc. The control pulses from the timebase have an accurate time duration and are applied to the signal gate which, in effect, opens and closes to allow a train of signal frequency pulses to pass through into the counter over a very precisely controlled interval. These pulses are then electronically counted, and the result is passed to the display latches, which retain the count during the subsequent counting process. To clarify this concept, consider a scenario in which a 25MHz signal is applied to the input of a five-digit DFM with its timebase set to 1ms. During this 1ms interval—when the gate remains open—25,000 pulses are counted, and the latch subsequently updates the display to indicate “25000kHz”. Conversely, if the time base is set to 1µs rather than 1ms, only 25 pulses are registered, resulting in a display of “00025MHz”. Figs.6.19 through 6.22 show examples of DFMs being used. In Fig.6.21, the five-digit display indicates a frequency of 744.64kHz with a resolution of 10Hz. Note how in Fig.6.22, an inline 30dB Display Display latch and drivers Time base dividers Range Fig.6.18: the simplified block schematic of a digital frequency meter (DFM). Practical Electronics | April | 2026 9 power attenuator is connected between the transmitter output and DFM. This acts both as a matched load for the transmitter and as an attenuator to prevent overdriving the input of the DFM. Hands-On: Handy RF voltmeter probe This month’s Hands-On project is a voltmeter probe that’s ideal for signal tracing in RF circuits. The probe will allow you to quickly and easily make in-circuit measurements of RF voltages using only a low-cost digital meter for display. The probe can be calibrated for RMS (sinewave) voltages up to 20V over a frequency range from 25kHz to 95MHz. The circuit of the RF voltmeter probe is shown in Fig.6.23. The probe uses two BAT42 Schottky diodes, D1 and D2, connected in a voltage-doubler arrangement like that shown previously in Fig.6.6. The DC output voltage (approximately twice the peak value of RF voltage applied to the probe tip) is developed across C2. To overcome the non-linear diode characteristics, a small amount of bias voltage is applied to the detectors. This voltage can be adjusted by RV1 so that the two diodes are just on the point of conduction. The bias supply is derived from a single 3V coin cell, which is mounted in a compact printed circuit board holder. Lithium coin cells are a severe hazard to young children if they swallow them, so this board with the onboard cell holder must be secured in a plastic case held together with screws. Luckily, there is a neat case available that’s the perfect size for the job (see the parts list for details). As with our previous Hands-On projects, this one is constructed on a small piece of perforated copper stripboard measuring 25 × 64mm, arranged as nine strips each of 24 rows. The component layout (viewed from the top) is shown in Fig.6.24, while the corresponding track layout (viewed from below) is given in Fig.6.25. The required 16 track breaks can be made using a spot face cutter or small drill bit, and the six links on the upper side of the board can be made using short lengths of tinned copper wire. Take care to observe the correct polarity of the two BAT42 diodes (the stripe on the package is the cathode connection). 10 The completed stripboard is mounted, together with S1, inside an ABS probe or small project case. Suitable enclosures are available from several online suppliers at a reasonable cost. However, when purchasing, it’s important to check that the dimensions of the case are sufficient to accommodate the board (25 × 64mm). The case used for the prototype measured 120 × 40 × 25mm. As explained above, the board must be mounted in a case to make the coin cell inaccessible. Don’t skip that step! Make sure to use a case that doesn’t come apart without tools. Fig.6.26 shows the internal wiring of the RF voltmeter probe. The probe tip is connected to P1-2 using a short length of insulated hookup wire, and a 100mm length of insulated stranded wire (terminated with a crocodile clip) is connected to P1-1. This wire should be kept reasonably short to ensure good high-frequency performance. The DPDT slide switch, S1, is connected to P2-2 and P2-3 using Fig.6.23: the circuit of the RF voltmeter probe. Probe tip Ground clip C1 10nF D2 BAT42 D1 BAT42 C2 10nF P1-2 P1-1 C3 10nF Fig.6.22: using a low-cost DFM to measure the output frequency of a 1W 7MHz transmitter. short lengths of insulated hook-up wire. 500mm lengths of black and red insulated multi-core flexible wire are used to link the DC output from the probe to the voltmeter. These leads should be fitted with 4mm banana plugs and the positive (red) and negative (black) connections terminated at P2-1 and P1-1, respectively. P2-2 P2-3 + B1 3V S1 On/off R1 1kΩ RV2 10kΩ P2-1 RV1 500Ω C4 100nF + (red) Output to DC voltmeter - (black) Figs.6.24 & 25: the component & track layouts for the RF voltmeter probe circuit board. Practical Electronics | April | 2026 2.0 Indication (V) 1.5 Probe ON 1.8 1.0 Indicated voltage (VDC) 1.6 1.4 0.5 Probe OFF 1.2 0.0 10k 1.0 100k 1M 10M 100M Frequency (Hz) Fig.6.28: the frequency response of the RF voltmeter probe. 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Applied voltage at 1MHz (VRMS) Fig.6.27: indicated voltage (VDC) plotted against input voltage (VRMS) with the probe switched on and off. Adjustment and calibration The RF voltmeter probe requires initial adjustment and calibration using an RF signal source. If you don’t have a suitable RF signal generator, you can use the VFO (‘variable frequency oscillator’) or reference source module that we described in Part 3 of Teach-In 12 (in the January 2026 issue). The following steps are required: 1. Switch the probe off and set both RV1 and RV2 on the probe to their fully anticlockwise positions. Insert the CR2032 3V coin cell into the holder with the positive (+) side up. 2. Leaving the probe tip and ground lead shorted together, connect the red and black output leads from the RF voltmeter probe to a digital voltmeter. Select the 20V DC range on the meter and keep it on this range for all subsequent steps. 3. Ensure that the probe tip remains unconnected and switch the RF voltmeter probe on. Ignore any reading on the meter at this stage. 4. Advance the setting on RV1 on the probe (turning it slowly clockwise) and adjust it for a reading Practical Electronics | April | 2026 of between 0.02V and 0.03V. 5. Connect the probe tip to Fig.6.29: using the RF voltmeter probe to investigate voltages in SK1 on the an RF power amplifier. VFO (or an alternative signal source of against a known instrument and about 2V peak-to-peak at around RV1 re-adjusted accordingly. 1MHz). Clip the ground connecFig.6.27 shows the response of tion to the 0V common rail. the RF voltmeter probe to voltages 6. Adjust the VFO to produce an below 2V. The result of applying output at about 800kHz and set a bias is evident, as is the error at RV1 on the VFO to mid-position. applied voltages of less than 0.8V. At this point, the meter should For most purposes (and when carbe indicating a reading of around rying out relative adjustments), this 2V. small error can be ignored. Note the 7. Note the reading on the meter difference with the probe switched and then adjust RV2 on the probe on and off. so that the meter reading falls to The frequency response of the exactly 0.4 times (40% of) the RF voltmeter probe is shown in initial reading indicated. For ex- Fig.6.28. The response is substanample, if the initial reading was tially flat from 100kHz to 10MHz, 2.1V, RV2 should be adjusted so with a small resonant peak due to that the meter reads (2.1V × 0.4) stray reactance at about 50MHz. = 0.84V (the RMS equivalent). Thereafter, the probe is useful to 8. The RF probe should now indi- about 80MHz, above which the cate the approximate RMS value output falls rapidly. of the sinewave. If possible, the calibration should be checked Using the RF voltmeter probe Fig.6.29 shows a typical application for the RF voltmeter probe when fault-finding on an HF power amplifier. The probe tip is taken along the signal path to the points under investigation, and the ground clip left attached to a suitable ground point. In this case, the grounded metal enclosure has been used, but a nearby ground point on the PCB Fig.6.26: the internal wiring of the RF would also be suitable. voltmeter probe. 11 n=20 log 10 ( VV )dB rel ref Fig.6.30 Vshows how the RF probe n=20be logused dBV=20 logoperation 10 10 (V )dBV of can to check the 1V the amplitude modulator described 2 in Teach-In 4 (February 2026 (V out ( pk− pkPart )) issue). the ground clip P out = Here again, (in watts) 8 RL is attached to a convenient point on the common 0V rail 2and the2 probe V 2out ( pk− pkto 160 V 12.65 V ) different tip moved = test points. =0.4 W P out = = 8 RL 8×50 Ω 400Ω For comparison, we’ve shown the DC and RF test voltages at various P circuit. points in the n=10 log 10 2 dB The signal P 1 path can be clearly seen. It’s also possible to determine the output Ppower from the modulan=10appearing log 10 dBrel tor, the 680Ω load P ref across resistor, R4. The 1.3VRMS measurement at this point (dropped across P n=10can log 10 be used dBW=10 log 10 ( P)dBW R4) to determine the 1W power dissipated in R4, as follows: ( ) ( ) ( ) Coming up In next month’s instalment, we will be investigating low-cost spectrum and virtual network analysis. We will also have two Hands-On projects. One takes the form of a combined load and power meter that makes adjusting low-power HF transmitters a breeze; the other is a separate 25W load suitable for PE operation up to 450MHz. 9.5V RF 0V R5 560Ω C5 47µF L1 100µH DC 4.4V RF 0.7V R3 100kΩ TR2 2N2222 C1 10nF SK1 RF input 1.3V RF 0V 1.3V e DC 3.3V c RF 0.7V TR1 2N2222 e R1 100kΩ P1-2 Fig.6.30: DC and RF voltage measurements made on the amplitude modulator circuit from Teach-In Part 4. DC 0V RF 1.3V Mod. RF output C4 1nF b Com. Listing 6.1: sample code for RMS scaling and dBV conversion. RF RV1 500Ω P2-3 C6 100nF + SK2 + DC 9.5V +12V c R2 100kΩ AF input P1-1 DC b C2 10µF ( ) V2 1.3 V 2 P out = RMS = =0.0025 W or 2.5 mW RL 680 Ω DC + C3 10µF R4 680Ω DC 0.7V RF 0V P2-1 Com. P2-2 void loop() { int rawValue = analogRead(inputPin); float voltage = (rawValue / ADC_STEPS) * V_REF; float voltRMS = voltage / 1.414; lcd.setCursor(0, 0); lcd.print(voltage/1.414, 3); lcd.print(" V"); double dBV = 20.0 * log10(voltRMS); lcd.setCursor(0, 1); lcd.print(dBV, 2); lcd.print(" dBV"); delay(1000); lcd.clear(); } Parts List – RF voltmeter probe 1 25 × 64mm piece of stripboard (9 × 24 holes) 1 ABS probe case (129 × 40 × 25mm) [eBay 291019910762] 1 panel-mount probe (eg, machine screw sharpened to a point and matching nut) 1 2-pin male 0.1in/2.54mm header (P1) 1 3-pin male 0.1in/2.54mm header (P2) 1 DPDT miniature slide switch (S1) 1 PCB-mounting CR2032 coin cell holder (B1) 1 CR2032 3V lithium coin cell (B1) 4 brass or nylon hexagonal M3 × 10mm stand-off/spacer 4 M3 panhead machine screws 4 M3 countersunk head machined screws 1 red 4mm banana plug 1 black 4mm banana plug 1 insulated crocodile/alligator clip 1 50cm length of red PVC flexible test lead 1 50cm length of black PVC flexible test lead 1 10cm length of black PVC-insulated flexible wire Semiconductors 2 BAT42 Schottky diodes (D1, D2) Capacitors 1 100nF 50V ceramic (C4) 3 10nF 50V ceramic (C1-C3) Resistors 1 1kΩ ¼W axial, 5% or better (R1) 1 500Ω top-adjust miniature preset variable/trimpot (RV1) 1 10kΩ top-adjust miniature preset variable/trimpot (RV2) 12 Practical Electronics | April | 2026