Many descriptions have been written in the relevant amateur radio literature in the past on the assembly of milliwatt meters - and I hope this will continue in the future. Two main measuring methods could be distinguished:
This article will neither be dealing with the theoretical principles of the two options nor contrasting their advantages and disadvantages. Whether a bolometer or a diode detector is used, they have one problem in common - the linearity over a frequency range as large as possible. It is just not good enough if the milliwatt meter merely works well at 23cm - it must give readings just as accurate for shortwave, from 2m to 70cm. Many authors offer interesting analogue solutions to this problem in their publications. But there are others too! Linearisation of characteristics, automatic calibration of the power meter, zero point correction, etc. - you can confidently entrust these jobs to the computer!
What is described below, then, is an mW meter which enjoys the advantages of a PC. A simpler and faster way to measure power is the aforementioned method using high-frequency rectifier diodes. Fig.1 shows the standard circuit. The output voltage, UA, which is dependent on the input power, PIN, and the frequency response are important values for the description of the circuit. The characteristics can be plotted simply, using adjustable output power and a voltmeter as accurate as possible, e.g. a digital gauging instrument. The ratios are shown in graph form in Fig.2 and are listed systematically in the table.
Another simple circuit (Fig.3) operates with simultaneous voltage doubling. Two germanium diodes (type AA119) supply an output voltage proportional to the input voltage.
2.1. The Detector
A measurement detector should fulfil various criteria: The wide dynamic range is provided by a type 1SS99 Schottky diode. Moreover, the wide frequency range goes from the short wave right into the GHz range. Apart from the components selected, the main factor determining the frequency is an assembly appropriate for high frequency! Fig.4 shows the circuit of the measurement detector for frequencies of up to 2.5 GHz.
2.2. The A/D Converter for the mW Meter
The detector supplies an output voltage proportional to the input voltage (Fig.2). The high-Ohm input de-couples the measurement detector from the rest of the circuit. This is brought about via an operational amplifier (¼ LM324) with V = 1. The subsequent comparator compares the reading with a comparison voltage generated by the PC. In accordance with the up-to-date condition, there is 0 V or + 5 V at the PE connection ("paper empty"). The PC controls the entire measurement procedure through the Centronics interface.
The comparison voltage just referred to is generated by a D / A converter (ZN426E-8) from an 8-bit data word. The validity range runs from 0 to 255dez. The procedure uses the successive approximation principle. Rapid D / A conversion is accomplished with the help of this method. An attempt is made to guess the voltage reading. Naturally, this sequential process is not carried out randomly, but is done in accordance with a balancing process, in accordance with which one feels one's way towards the desired value in a few steps. Naturally, this happens very fast if a PC is used.
When the measurement begins, the value is fed into the D / A converter which corresponds to exactly half the maximum voltage. A comparator reports whether the comparison voltage is higher or lower. If the voltage to be measured lies in the bottom half of the range, this is divided again into two halves. If the upper segment is involved, the procedure is exactly the same.
Thus each new experiment narrows the voltage range down more and more. For 8-bit accuracy, the successive approximation procedure requires only eight steps to yield a result.
The comparison voltage (UDAC) of the A / D converter is fed out as a check. Anyone who wants to may connect a simple direct reading instrument to point UMeß to obtain a relative display.
The flow chart opposite (Fig.7) shows the steps of the process very clearly. The screen is set up as a first step. This must be done before the actual measurment routine begins.
The prevailing reading is continuously transmitted in an endless loop, and converted into the proportional power with the help of the detector characteristic line from Fig.2.
Last of all, it only remains for a correction factor to be brought into action, corresponding to the measured frequency. The frequency range can be adjusted for this purpose at any time via input through the keyboard. The measured power is displayed on the screen in mW and dBm. The readings can be displayed as a graph without further ado. There are no limits to what the interested programmer can do here.
Special attention should be paid when assembling the measurement detector. A suitable assembly for high frequency is an absolute must for the desired broadness of band! That means short connecting wires, tidy earth connections and a mechanically stable structure.
The carrier material can be, for example, the hexagon nut from a BNC plug, sawn off in advance. Don't forget to de-burr it!
The small number of structural components is soldered onto or into the residual threaded section and then provided with short wire ends (CuAg, diameter 1mm) for plug and socket. The detector prepared in this way is now screwed into the plug and coupling housing. When functioning has been successfully tested, a drop of rapid-acting adhesive fixes all components of the completed measurement detector.
Fig.8 shows additional details regarding the layout and incorporation of the components.
The board for the A / D converter is double-sided coated epoxy material 1.57mm thick. The board measures 75mm x 75mm.
Matching bores are to be provided for the IC's and the trimming potentiometer. The same applies to the Centronics plug and the UDAC, UDat, UMeß and + 12 V connections.
Right up to the Centronics plug, all the components are mounted on the foil side! This also applies to the IC's and the trimming potentiometer (Fig.9). All earth connections are soldered on both sides. The earth surface may not be milled down in advance here!
Finally, the entire circuit can be incorporated into a housing, as selected. A high-quality plug connection should be taken into account for the measurement detector (Fig.10).
The low current consumption for the circuit - app. 12mA at 9V - means it can also be operated by a simple compact battery.
The complete calibration of the mW meter assembly can be divided into 4 steps:
Characteristic of diode detector
For the diode detector, the output voltage should be measured in relation to the input power. An expedient way to do this is to use a digital voltmeter and a 144 MHz standard signal generator. A transceiver with an adjustable transmission power will also fit the bill, of course. Fig.2 shows the levels and some possible step sizes.
An appropriate measurement graph is usually provided with ready-made measuring heads.
If we increase the number of measurement points, the accuracy of the equipment can be further increased. The program is designed for a minimum step width of 1dB. The corresponding values should be entered in a table. Correction factor df
Particularly at high frequencies, the actual input impedance deviates from the expected value by 50Ohms. A correction factor, df, theoretically balances this out.
The factor should be measured for the frequency ranges 28, 50, 144, 432, 1,296 and 2,320 MHz, with a control power of, for example, 1mW (0dBm):
df = (Rated value at 144 MHz)/(Measured value)
Correction factor dU
Tight tolerances in the components, or even in the D / A converter ZN426E-8, lead to discrepancies in the measured voltage. The factor dU acts as a theoretical correction factor. It has a value of app. 0.01. The reference voltage, URef, can be precisely measured at pin 5/6 of the D / A converter with a digital measuring instrument.
dU = (URef)/(256)
10k potentiometer for relative display of measured power
If no diode detector is connected up, maximum power is displayed. The measuring instrument should now be set to end-scale deflection using the potentiometer.
The detector readings and the correction factors for frequency and voltage are listed in the table MWTABLE.TXT. Any standard text editor makes it possible to correct this table using the updated values for the diode detector used.
This also applies to the Centronics interface desired. LPT1, LPT2 or LPT3 can be used, depending on the configuration of your own PC.