1 TEST&MEASUREMENT. Digital RF Wattmeter with LC Display for 1 kHz to 1 GHz Design by Thomas Scherrer OZ2 CPU Any radio amateur knows the importance of an accurate RF power meter. A Wattmeter can be used to measure gain in amplifiers, bandwidth in filters, field strength from antennas, transmitter power, SWR, return loss and many other things. 16 Elektor Electronics 10/2002. TEST&MEASUREMENT. engineers are conversant with the dBm unit, Specifications others prefer watts' and still others like to talk about RMS voltage'. This meter displays Frequency coverage: 1 kHz to 500 MHz (calibrated) all three units at the same 1 kHz to 1000 MHz (uncalibrated, for relative power measurements only).
2 Nominal input impedance: 50 About the AD8307. Input power range: 60 dBm to +30 dBm (1 nanowatt to 1 watt). The AD8307 monolithic logarithmic amplifier Input power range using 50-dB attenuator: up to 100 kwatts. from Analog Devices was first described in Dynamic range: 90 dB with good RF shielded case. the article RF Decibel Meter, see Elektor Resolution: dBm (1 dBm on bar-graph) Electronics, January. For reference purposes, Input return loss: @300kHz: 35dB the block diagram of this hugely successful @100 MHz: 27dB IC is given in Figure 1. The AD8307 is a rela- @500 MHz: 25dB tively low cost component at least accord- Input SWR: @ 300kHz: ing to the datasheet in practice, the @100 MHz: author paid about 13 (approx.)
3 For one @500 MHz: off plus postage. Accuracy before calibration: 1 dB from 1 MHz to 450 MHz. The author initially tried the DIL version of After calibration: dB at each calibrated frequency. the AD8307 this is also the type used in DC Voltage Measurement: 0 to 20 volts. the previously mentioned article. Although it DC Voltage Resolution: 20 mV. is easier to solder and use than the SMD chip, DC Voltage accuracy after calibration: 20 mV. its longer connecting pins make it unusable Power supply: 9 to 20 VDC. Current consumption: with no LCD light: 30 mA; for frequencies higher than about 100 MHz. with normal LCD light:120 mA.
4 Several experiments were carried out using this type before it was found that the SMD. type could be used up to about 500 MHz. This RF Wattmeter uses an AD8307 storage, plus many extra features, Again several experiments were set up, to measure the power level. The see the software description further boards where made, input resistors and AD8307 front end circuit is both fre- on in this article. capacitors where changed to optimise the lot. quency compensated and optimised At frequencies above 300 MHz input for return loss to give optimum input power should not exceed +20 dBm (100. SWR over a wide frequency range. The decibel milliwatt mW) to maintain accuracy.
5 This is a docu- A pre-programmed microcon- (dBm) unit mented weakness of the AD8307. Not a big troller type PIC16F876 with built-in In RF technology 0 dBm represents problem, really, if you are aware of it it is 10-bit analogue to Digital converters 1 milliwatt into 50 . Similarly, just a matter of using the right input attenu- is used to convert the analogue volt- 0 dBW represents 1 watt into the ation, to reduce the actual RF power applied age output from the AD8307 into dig- same impedance. to the chip to a level below 20 dBm and so ital values. Next, a set of lookup So, +10 dBm = 10 mW; +20 dBm = guarantee optimum accuracy.
6 Tables are used to convert the dBm 100 mW; +30 dBm = 0 dBW = 1 W The author is grateful to all the radio ama- values into RF voltage and RF power and so on. teurs and skilled engineers who have con- (watts). The readout of all values The term dBm' is used at any tributed with their experience, good ideas, including a bar-graph appears on a professional radio development, help with measurements, and lending out large 20 2 LCD Display with back repair and servicing facility as well expensive equipment so that it was possible light. There is also a DC voltmeter as by radio amateurs, to describe to construct this input circuit.
7 with minimum and maximum peak (relative) RF power levels. Some Circuit description The circuit diagram of the RF Wattmeter is AD8307 given in Figure 2. It consists of four sections, SUPPLY. VPS. BANDGAP REFERENCE ENB. ENABLE. which will be discussed briefly below, leav- AND BIASING. ing a bit more space in the article to ponder SIX 900 MHz INT. AMPLIFIER STAGES INT. ADJ on the second main components yes that's INP. +INPUT the PIC16F876. MIRROR. INPUT The RF voltage converter is draw as a sep- INM 3 2 A OUT. /dB OUTPUT. arate unit around the AD8307. The log'. NINE DETECTOR CELLS 2. SPACED equivalent of the voltage representing the RF.
8 COMMON COM COM power applied to socket K4 appears on K5 in OFS. INPUT-OFFSET. OFS. ADJ. the form of a step level between 0 and V. COMPENSATION LOOP. The input resistor network is dimensioned for 020026 - 11 50 input impedance which is the de facto standard in RF work. The input network can Figure 1. Block diagram of the AD8307 (courtesy Analog Devices). handle power levels of up to 1 watts. C1 and 10/2002 Elektor Electronics 17. TEST&MEASUREMENT. type for ease of software develop- LCD. S1 S2 ment and debugging. Furthermore, LCD Display 2 x 20 Chars 4 Digital inputs, 2 for the pushbut- SELECT MENU. tons and 2 for the encoder, 7 outputs LED2.
9 LED1. VSS. VDD. RW. VO. RS. D7. D6. D5. D4. D3. D2. D1. D0. E. K1. +5V +5V +5V. 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1. to the LC Display in 4-bit mode or 11. outputs in 8-bit mode. The Microchip K6. R9. 16F873 and 16F876 PIC microcon- 47k C10. +5 K3 trollers with 4 and 8 kwords of Flash 0. 100n +5V C14. A program memory proved an excel- 100n DC. R15 R17. C8. 20 P1 lent choice. Their price being almost DC. the same, the author went for the 120k 180k VOLTMETER 1n 1. INPUT MCLR 10k 2. IC2. 11 RS K3 larger 8 k type. RA0/AN0 RC0. R16. C7. 3. RA1/AN1 RC1. 12 D0 2 1 The 16F876 has 5 analogue inputs 10k 4 13 RW RS 4 3. 5. RA2/AN2 RC2.
10 14 E E 6 5 RW. with 10-bit resolution, representing RA3/VREF RC3. 100n 6. RA4/T0 RC4. 15 D5 D1 8 7 D0 a range of 0-1023 in discrete values 7 16 D6 D3 10 9 D2. RA5/AN4 RC5. 17 D5 12 11 D4. when the input voltage goes from 0. 28 RC6/TX. 27. RB7. RC7/RX. 18 D7 D7 14 13 D6 to 5 volts. The DC signal from the RB6 16 15. ENCODER K2. 26. RB5 OSC1. 9 AD8307 covers 0 to volts for the ENCODER D1. D2. 25. 24. RB4. PIC16F876. X1. LED2 LED1 entire operation range. To get full RB3 R14. D3 23. RB2. Digital resolution the ADC inside the 10 . 4 MHz +5V D4 22. RB1 OSC2. 10. PIC could use an external positive 21. RB0/INT. R10. reference for the full-scale voltage.