High-Impedance Millivoltmeter HF-Level Meter

Transcription

1 Instruction Manual High-Impedance Millivoltmeter HF-Level Meter Author Hans-Peter Prast, DL2KHP Version 1.2 page 1 of 11

2 1. Preliminary note The device was developed for high precision measurements of resistance. Values should be immediately readable in Ohms. The device needs an external 10 ma constant current source to allow valid measurements. The measurement is carried out by a voltage measurement across the measurement object. To minimize measurement errors, the instrument has a very high input impedance of Ω. The measuring device can also be used as a high resolution precision millivoltmeter. The maximum measuring voltage is volts; the maximum safe input voltage is 5.0 volts. Another feature of the instrument is to measure RF-power levels in a range from -95 dbm up to 16 dbm. A logarithmic amplifier is used as a RF-detector. The type of detector determines the dynamic- and frequency range. Correction factors can be entered for several frequency ranges. Instrument calibration is performed under software control. The voltage measurement mode can be calibrated by connecting a precise volts reference. In 1 db steps the RF-detector is calibrated. For six frequency bands correction factors can be determined. 2. Hardware The hardware of the level meter fits on a double-sided PCB with the dimensions of 86 x 97 mm. The board should be installed in a tin plate metal housing or preferably in an aluminium housing. The two-line LCD display with backlight is mounted on the circuit board with standoffs and pin connectors. The contrast is adjustable with a potentiometer. Power is supplied by a 9 volt battery. The internal battery is automatically disconnected when external power is supplied. The BNC connector will handle the input voltages from various sources. 3. Circuit The input voltage is routed to an operational amplifier with an input resistance of Ω. The operational amplifier works in the unity-gain-mode to deliver a low source impedance for the following analogue/digital-converter. A 18 bit A/D converter will digitize the analogue input voltage. The A/D-converter is connected to the ATMega-Controller via the TWI-Bus (I2C-Bus). The internal high-precision-reference-voltage of the converter is volts. This determines as well the full scale value of the converter. The board features a protection circuit (D3, D4). This should only be implemented if the extremely high input resistance is not required. After use of the instrument it is good practice to terminate the BNC input connector for ESD protection. A 5.0 volts low drop, low noise, voltage regulator is used to supply power to the internal circuits. The internal or external supply voltage is continuously monitored by the controller. First a warning will be displayed If the voltage drops below 6.5 volts. Next the instrument will be locked to avoid false measurements. Measured values are displayed on a two-line LCD display with standard pin-out and standard Controller (HD). The backlight of the display can be set by the ATMega16, using PWM control with the transistor T1. The display contrast can be set via potentiometer P1. page 2 of 11

3 MFT (multi-function), Up and Down buttons allow selection of the measurement mode and other functions of the device. The central processing and control is performed by the ATMega16. The controller can be programmed via the ISP-interface in the circuit. The corresponding 10pin connector with standard assignment is available. An external crystal is not required because no time-critical processes are present. The Input is connected to a CMOS operational amplifier TLC-271. This Op-Amp operates in the "Low Bias" mode. According to the data sheet a hardware offset correction is not required., An offset correction is performed by the software and can be repeated at any time - prior to the measurement, to preserve the accuracy of the measurement. There is no voltage range extension provided, because this would require a resistor voltage divider, and thus the high input impedance is not feasible any more. The used delta-sigma analogue/digital converter has an integrated reference voltage source of 2.048V with an accuracy of 0.05% and a drift of 15 ppm/ C! The converter is self-calibrating. This self-calibration is performed prior each conversion. The internal programmable preamplifier (PGA) is not used in this application. At 18 bit resolution the LSB value is µV and the sample rate is 3.75/s. page 3 of 11

4 4. Software The software is implemented in the programming language PASCAL. The micro-pascal Pro Compiler of mikroelektronika, Version 6.0, was used. At power on, the instrument shows a Start up screen followed by an initialization. Millivoltmeter * Version 1.2 * This message is displayed for approximately one second and then the last used measuring mode is loaded. 4.1 Calibration For accurate measurements the device must be calibrated, especially for the RF-level measurement. Voltage measurements can be done without calibration, due to the precise internal reference voltage. A calibration is monetary for RF-level measurements. The calibration of voltage measurements and level measurements is performed in one sequence Voltage Measurement calibration The calibration routine is called by pressing and holding the button "MFT" (multi-function button) while switching the device on. Hold the MFT button until the following message appears on the display: Input 0V? Weiter mit MFT continue with MFT The BNC input of the instrument must now be terminated, because before the calibration starts the offset voltage of the operation amplifier must be measured. With the input terminated the "MFT" key is now actuated. Then the following message is displayed: Display Example: Neuer Offset mv = new offset This message is displayed for approximately two seconds, then connect a known accurate reference voltage of 1.0 volts. This is displayed as follows: 1 Volt anlegen Weiter mit MFT apply 1 volt continue with MFT With a push of the button "MFT", this value is transferred, stored and used for the correction of measurements. A touch of the key "Down" deletes an existing reference value and a default value is stored. If the button "Up" is pressed, then nothing is changed and the calibration of the voltage measurement is finished and the calibration of the RFlevel meter will be continued in step page 4 of 11

5 4.1.2 Calibration level measurement This calibration routine automatically starts after the voltage calibration is finished. To cancel the calibration, press the UP button. For calibration, a RF-power level starting at -95 dbm in 1 db step increments up to 16 dbm must be provided. First a offset measurement is necessary. The display shows the same message as before with the calibration of the voltage measurement. However, now the measuring head must be connected to the device and its input terminated with 50 ohms. set. Det. 50 Ω Weiter mit MFT continue with MFT After a pressing the button "MFT", the offset voltage of the measuring head is measured and saved. This offset voltage is only used for level measurements. The offset of the voltage measurement remains unchanged. Neuer Offset mv = new offset Apply a RF-level of -95 dbm at the input of the measuring head. Then press the "MFT" button. The display shows the message "-94 dbm anlegen". Now you need -94 dbm at the input to the measuring head. Then press the "MFT" button again. Repeat until 16 dbm is reached. -94 dbm anlegen DW= MFT apply -94 dbm Digital value continue with MFT In the second line of the display the digital value of the A/D-converter is displayed, allowing you to check the actual value. After the last calibration value, the message "level meter calibrated" appears briefly, then the level meter operates in the measuring operation. Level Meter Kalibriert calibrated The calibration process can be terminated at any time by pressing the "Up" button. A touch of the key "Down" skips the current position, without saving or modifying the existing data at the current position. page 5 of 11

6 4.1.3 frequency correction values For frequency correction values a separate calibration routine is provided. This is accessed by pressing and holding - during power up the "Down" button. First execute an offset measurement according to Then connect a level of -30 dbm to the input of the measuring head. The required frequency is shown in the display (second row left). -30 dbm anlegen 145 MHz MFT apply -30 dbm continue with MFT Now when a level of -30 dbm and a frequency of e.g. 145MHz are applied, with a push of the button "MFT" the difference to the (at 10MHz) calibrated value is calculated and stored for the actual frequency. After the last value was determined, briefly the following message is displayed and the device operates in the measuring mode. Korrekturwerte gespeichert correction values stored The operation can be cancelled at any time by pressing the "Up" button. A touch of the "Down" button skips the current position without changes. 4.2 Display background light When turning the instrument on while the "Up" button is pressed, the background light can be adjusted via the buttons "Up" and "Down" until the desired brightness level is reached. Then press the "MFT" button. The message "Lighting adjusted" appears, then the level meter operates in the measuring operation. The preselected value is saved and used again at the next power on. If an external voltage (> 9V) is detected, the backlight is switched to approx. 80% of the maximum brightness. After removal of the external power supply, the preset brightness will appear again. 4.3 Measuring functions The measuring function can be selected with the "MFT" button Measurement Function 1, dbm Level measurement In this function the display shows the input level in dbm in the top line and the input level in milliwatt in the bottom line. The power will be displayed in the values pw, nw, µw, mw and W. The frequency for the measured value correction appears in the upper-left corner. If the button "Up" is pushed, the next frequency correction value appears and its value is considered in the measurement. Therefore it must be ensured that the correct correction value is used! With each push of the "Up" button the value is changed to the next correction value. 145MHz µw = dbm page 6 of 11

7 4.3.2 measurement function 2, millivolt measurement with display of offset In this function the input voltage in the ratings µv, mv and V is displayed in the lower line. In the top line the offset value of the operational amplifier is displayed. Off. = mv V = measurement function 3, millivolt measurement with display of resistance In this function if the input voltage is measured and displayed in the reference variables µv, mv and V in the lower line. The value of a resistor is displayed in the top line. The value of this measurement is only correct, if the current value through the resistor is exactly 10 ma!! Ω = mv = With this measurement it is possible to determine resistors very precisely. 5. Offset-correction In this function the offset of the operational amplifier is determined and saved. This can be performed at any time, to preserve the measurement accuracy. The offset voltage is set depending on the measuring function. In case of voltage measurements, the BNC input must be shorted. For level measurements the input of the instrument must be connected to the output of the measuring head while terminating the RF-input with 50 Ω. To start the offset correction routine in normal measurement mode press the "Down" button. Otherwise the procedure is similar to the calibration just as previously described. page 7 of 11

8 attachment Circuit diagamm Layout page 8 of 11

9 Pictures ready-made device, function level measurement PCB with components, (error message I2C-bus) PCB with components (resistor measurement) measurement heads for RF-level (optional) page 9 of 11

10 Parts list Vom: Blatt: um 18:30:42 Uhr "1: Präzisions-Millivoltmeter" Bat1 = 9V (Anschluß über Batterieclip, Best.Nr. CLIP 9V, Reichelt) BNC0 = BNC-Buchse (BNC Printbuchse 50 Ohm, BestNr. UG 1094W1, Reichelt) C1 C2 C3 C6 C7 C8 C9 C10 C11 C16 C17 C18 C19 = 10µF (Miniaturelko radial, 35V, RM 2,5 mm, BestNr RAD 10/100, Reichelt) = 22µF (Miniaturelko radial, 63V, RM 2,5 mm, BestNr RAD 22/63, Reichelt) = 10µF (Miniaturelko radial, RM 2,5 mm, BestNr RAD 10/100, Reichelt) = 10µF (Miniaturelko radial, 35V, RM 2,5 mm, BestNr RAD 10/100, Reichelt) = 2,2nF (SMD 1206, Best.Nr.X7R-G1206 2,2N, Reichelt) D1 D2 D3 D4 = 1N4001 (Silizium-Diode, Best.Nr. 1N 4001, Reichelt) = 1N4001 (Silizium-Diode, Best.Nr. 1N 4001, Reichelt) = BAT 41 (Schottky-Diode Mini Melf, BestNr. BAT 41 SMD, Reichelt) = ZF 5,1 (Zenerdiode Mini Melf, BestNr. SMD ZF 5,1, Reichelt) IC1 IC2 IC3 = ATmega16 (ATmega16, 16 MHz, DIL 40, BestNr. ATMEGA DIP, Reichelt) = MCP3421 (SMD_SOT-23_A/D-Wandler, Best.Nr. MCP 3421A0T-E/CH, Reichelt) = TLC271 (Operationsverstärker TLC271, BestNr. TLC 271 DIP, Reichelt ) J1 J2 = 3,5mm Klinkenbuchse (Klinkeneinbaukupplung Stereo 3,5mm, Best.Nr , Conrad) = NEB21 Hohlbuchse (Einbaukupplung NEB21, Best.Nr.LUM NEBJ 21R, Reichelt) L1 L2 L3 = 10µH (Induktivität, Best.Nr. L-MICC 10y, Reichelt) = 1µH (Induktivität, BestNr. L-1206AS 1,0µ, Reichelt) = 1µH (Induktivität, BestNr. L-1206AS 1,0µ, Reichelt) LCD1 = 2x16 Zeichen (Dot-Matrix Display, HD44780 komp. Best.Nr. LCD 162C LED, Reichelt) P1 = 10k (Trimmer, liegend10mm, Best.Nr. PT 10-L 10K, Reichelt) PL1 = Version 1.0 (Platine "Millivoltmeter", PCB-Pool) R2 R3 R4 R5 R8 R9 R10 R14 R15 R16 = 22 (Widerstand 1/4W, RM 10, Best.Nr. METALL 22, Reichelt) = 12 (Widerstand 1/4W, RM 10, Best.Nr. METALL 12,0, Reichelt) = 10K (Widerstand 1/4W, RM 10, Best.Nr. METALL 10,0K, Reichelt) = 2,2K (Widerstand 1/4W, RM 10, Best.Nr. METALL 2,20K, Reichelt) = 100 (Widerstand 1/4W, RM 10, Best.Nr. METALL 100, Reichelt) = 8,2K (Widerstand 1/4W, RM 10, Best.Nr. METALL 8,20K, Reichelt) = 2,2K (Widerstand 1/4W, RM 10, Best.Nr. METALL 2,20K, Reichelt) = 4,7K (Widerstand 1/4W, RM 10, Best.Nr. METALL 4,70K, Reichelt) = 4,7K (Widerstand 1/4W, RM 10, Best.Nr. METALL 4,70K, Reichelt) = 10K (Widerstand SMD 1206, Best.Nr. SMD 1/4W 10K, Reichelt) S1 S2 S3 S4 = Druckschalter (Taster 222,5mm, Best-Nr. TASTER 3FTL6, Reichelt + KAPPE 1S ) = Druckschalter (Taster 222,5mm, Best-Nr. TASTER 3FTL6, Reichelt + KAPPE 1S ) = Druckschalter (Taster 222,5mm, Best-Nr. TASTER 3FTL6, Reichelt + KAPPE 1S ) = Schalter (1pol. Ein-Ein, Best.Nr. MS 500A, Reichelt) St1 = 10pol (Wannenstecker 10pol, Best.Nr WSL 10G, Reichelt) T1 = IRLR 110 (IRLU110 HEXFET für Logikansteuerung, Best.Nr. IRLR 110, Reichelt) VR1 = LM2940CT5 (Spannungsregler, Best.Nr. LM 2940 CT5, Reichelt) page 10 of 11

11 Technical Data: Input impedance 1012 Ω (without protection) Measurement range 0 to volt, resolution µv -95 dbm to 16 dbm (depending on the measurement head) Recorder output max V, Impedance 100 Ω Power Block-shaped 9 V battery external 9 12V via hollow connector Necessary for calibration: Reference voltage Volt (calibration optional) Calibration level Calibration frequency -95 dbm to 16 dbm (in 1 db-steps) 10 MHz Values of correction 30, 50, 145, 435 MHz and 1.2 and 1.5 GHz (different values possible via software) page 11 of 11