Schematic RF Probe | Electronic Circuits

Schematic RF Probe

This RF probe can be used at High Frequency (HF) or Ultra High Frequency (UHF) on both 50 and 75 ohm coaxial cables. In additionthe RF voltage can be measured under load or no-load conditions which allows the circuit to double as an RF Watt meter. The RF probecan be used for oscillators and small transistors for powers up to 2 Watts.



Circuit Notes
The circuit is a simple half wave rectifier. In this circuit it works at radio frequencies (RF) and converts any RF signal to a DC voltage, inaddition S1, allows a resistive load to be switched in or out of circuit. S1 is a single pole, double throw switch with a Centre off position.The centre position is no load, and left and right positions are for 50 and 75 ohm measurements. First, a small section on measuring RF voltage, current andpower, then I'll describe how to use this simple test instrument.

Measuring RF Voltage
Digital and analogue multi meters can already measure AC voltages so why can't they be used at radio frequencies? The reason is that they can onlymeasure with accuracy a limited frequency range. My Maplin meter measures frequencies up to 400Hz with 1% accuracy, and up to 20KHz at 4%. This alsorequires that the waveform is a sine wave. At frequencies above 20KHz, accuracy is not reliable.
To measure radio frequencies (RF) a simple diode detector circuit is all that's needed. The detector in this probe is an OA91 germanium diode, butany germanium diode will work. Germanium diodes have a low forward voltage drop (about 0.2V) and are preferred to silicon diodes which have a higher(0.6 - 0.7V) voltage drop. The diode rectifies the RF signal and converts it to a DC voltage, which can be read by a multimeter with good accuracy;the 1nF capacitor is there to smooth the rectified DC signal presented to the meter.

RF Power, Voltage and Current
When measuring any AC or RF signal, the currents and voltages are only in phase if the load is purely resistive. All transmitters are tested witha dummy load which are resistive.
 
Typical RF Voltages
For example, a 1 watt transmitter delivers an average power of 1 watt into a 50-ohm resistive dummy load. Transmitter power is measured in RMS orroot-mean-square. As power, P = V2/R, then re-arranging, V(rms) = sqrt(P x R). Power is also found from P = I2Rand re-arranging in terms of current, I(rms) = √ (P / R)  Peak values are simply 1.414 x the RMS values.
So for a 1 W transmitter V(rms) = √ ( 1 x 50)  = 7.071 Volts.and current, I(rms) = √ ( 1 / 50)  = 0.141 Amps.

Power OutputAC Volts RMSAC Amps RMSAC Volts PeakAC Amps Peak
2 W10 V0.20 A14.4 V0.283 A
1 W7.07 V0.141 A10.0 V0.200A
0.5 W5.0 V0.100 A7.07 V0.141 A
0.2 W3.16 V0.0632 A4.47 V0.0894 A
0.1 W2.24 V0.0447 A3.17 V0.0632 A

RF Probe Functions
S1 allows a 50 or 75 ohm resistive load to be switched in and out of circuit. This allows the probe to read loaded and no-load voltages. Howeveras the load has a fixed resistance (50 or 75 ohm) then power delivered to the load can also be worked out. Finally because the probe has a fixedresistance and can measure loaded and no-load voltages then it is possible to measure output impedance of a transmitter. The RF probe has four functions:

1) Unloaded Transmitter Voltage
In all cases, connect the RF probe between the circuit under test and the meter. The circuit under test could be a transmitter, RF oscillator orother signal source. As the OA91 diode and 10n capacitor are a half wave rectifier, the RF value measured will be a peak value. As V(RMS) = V(peak) / √ 2  then:
V(RMS) = Vpeak = 0.7071 x Vpeak
 2 
To measure unloaded RMS transmitter voltage switch S1 to off and multiply the meter reading by 0.7071.

2) Loaded Transmitter Voltage
To measure a transmitter voltage under load switch S1 to either 50 or 75 ohm position. Normally this will be 50ohm, but for Band II ( 87.5MHz - 108MHz) 75 ohm impedance should be used.

To measure loaded RMS transmitter voltage switch S1 to either 50 or 75 ohm and multiply the meter reading by 0.7071.

3) Measuring Output Impedance
To measure the output impedance of an unknown circuit or transmitter you first need to take two readings, one unloaded and then a reading underload at either 50 or 75 ohms. The output impedance can be found from the following equation:
Z = R ( VNL - VL)
VL
where:
   Z = output impedance of circuit in ohms
   R = resistance of probe ( depending on S1 this is either 50 or 75 ohm)
   VNL voltage RMS reading with S1 in centre position (no-load)
   VL voltage RMS reading under load

4) Measuring Output Power
The output power in Watts can also be calculated. Output power is the loaded (RMS) output voltage squared divided by transmitter impedance:
P = VL2
Z
where:
   Z = output impedance of circuit in ohms
   VL voltage RMS reading under load

Output Power and SWR
The output power as measured by the probe will not be exactly the same as the radiated power by the antenna. This is because there are lossesin the antenna system and the Standing Wave Ratio (SWR). When an antenna and feedline do not have matching impedances, some of the electrical energycannot be transferred from the antenna cable to the antenna. Energy not transferred to the antenna is reflected back towards the transmitter. It is the interaction of these reflected waves with forward waves which causes standing wave patterns. An SWR meter can be used to measure the SWR ratio in order to obtain the best match between antenna and the feedline.

Important Note About Resistors
The components in the circuit are all readily available, however there is one Important consideration. The resistors used Must be carbon type and notwirewound types. The reason is that wirewound resistors contain inductance due to the coiled wire, this is not normally important except at very highfrequencies, as in this circuit.

PCB or Veroboard Layout
A circuit this small with very few components is hardly worth the trouble of producing a PCB. However because of its small size it took me about14 minutes, to draw the schematic and produce the PCB in Kicad. The 3D rendered components are all created by Renie S Marquet, more in the simulationsection.

PCB 3D view

Enlarged Component Side

Actual Size copper track view.


If you are thinking of using this PCB layout first printout the actual size copper track view on paper, then you can match up thecomponents to see if they fit the pads. This is the same for any PCB program. It does not matter if its open source or the program costseveral thousand pounds, the components that you use must fit the footprints on the PCB board. As sizes of components vary wildly thenthis is a problem for all PCB layouts.

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