Kurt Poulsen, OZ7OU originally gave cause for developing the millivoltmeter. At his home page (Http://www.hamcom.dk/) you can find all documents concerning this measuring box, the purpose of which is the precise measurement of an SMA 50 Ohm terminating resistor for the calibration of the VNWA. The millivoltmeter replaces a high-priced DMM here.
Meanwhile, there is a constant current source available that can be used for these measurements. This "Constant-I/U Source" can be found here.
Norbert, DG1KPN, then had the idea to build this millivoltmeter. Basically everything needed for his task had been available. The level meter_MK served as hardware. Only the input has been slightly changed and a CMOS operations-amplifier with an input resistance of 1012 Ω has been inserted. At this point Norbert has invested much time and some tests in order to find a suitable operational amplifier.
The software also has to be changed accordingly. A "waste product" was a measuring function for the measurement of power levels with an external measuring head with AD8307.
The finished millivoltmeter in mode "level measurement" with external measuring head
Circuit diagram of the millivoltmeter / levelmeter
The AD converter MCP3421 has a resolution of 18 bits and thus a resolution of 15,625 µV at a reference voltage of 2.048V. The voltage measurement can also still be calibrated. Even without this calibration the measurements are quite accurate, because of the precise internal reference voltage source of the converter. The object to be measured is not loaded, due to the very high input impedance. Because of the high input resistance, a protective circuit is provided in the circuit. It can be equipped when required and if the high input resistance is not needed. The offset voltage of the operation amplifier in the input is compensated by software. This Offset voltage can be determined at any time by pressing a button to further increase the accuracy of the measurement.
As display options there are three Measuring modes available.
1. Display of the input voltage and the current offset voltage of the OPA.
2. Display of the input voltage and the resistance value at 10mA measuring current.
3. Display of RF level in dBm via external logarithmic measuring head.
For the measurement of RF-levels an external measuring head was developed by Norbert, DG1KPN. A measuring head with the AD8307, which is operated beyond the linear range, is intended for this purpose. The usable measuring range is approx. -95 dBm to 16 dBm. The calibration is not as usual made by a two-point calibration, but - like a diode measuring head - in steps of 1 dB. Values between the steps of 1 dB are calculated via Interpolation. Thus accurate measurement can also be taken in the non-linear range of the logarithmic detector. This is certainly worth the extra effort while calibrating.
In order to cover the usable frequency range up to 1.5 GHz, frequency correction values have to be determined. But keep in mind, above 500 MHz the dynamic range of the detector degrades due to the increasing mismatch at the detector input. The measured and entered correction values into the EEPROM can be recalled by pressing a button. The frequency correction value makes it possible to take accurate measurements even in the 23cm band. A calibration at a frequency of 10 MHz is intended. Correction values are in this case determined at 30MHz, 50MHz, 145MHz, 435MHz, 1.2GHZ and 1.5Ghz. Unfortunately more correction values are not possible due to the limited EEPROM memory space. If necessary the selected frequencies in the software can be changed. It would be possible to apply an ATMEGA32 to a reconstruction to provide more memory capacity.
Here you can find a documentation of the device as a PDF file (English version).
A through hole PCB can be ordered directly by PCB-Pool (Order History: BS562ce84258763, Millivoltmeter). The HEX file with the Software is available on request. Anyone who sends me a controller will receive it back programed.
To measure RF-power levels in dBm, Norbert, DG1KPN, developed a detector probe using the logarithmic detector IC AD8307. The objective was, to measure RF-power levels with very high precision even outside the specifications of the data sheet, compensating the deviations of the output slope on the software side by an appropriate calibration.
Diagram of the logarithmic detector (image DG1KPN)
In order to improve measuring of extremely low RF-power levels with the AD8307, a low noise voltage regulator was selected. The low dropout feature of the voltage regulator helps to maintain battery operation down to 5.1V. Furthermore a low pass filter at the output of the AD8307 improves the low RF-level handling of the device. To improve EMI, ferrite beads are slipped over the supply wires. Special care was taken to design the input circuitry of the detector. To obtain a 50Ω input match a 52.3 Ω resistor is required in parallel with the input of the AD8307. This requirement is met by using a 100 and a 110Ω resistor in parallel. To improve the high frequency response an inductor of 6.8 nH in serial with the input is used. These simple measures improved significantly the broad band 50Ω match of the device in respect to just terminate the input with a 51Ω resistor.
The millivoltmeter can measure up to a maximum of 2.048V. Resistor values R3 and R5 will determine the slope and the maximum output voltage of the AD8307. Apply a RF-power level of 16dBm at a frequency of 10MHz to the AD8307 input and select R3 and R5 for an output voltage of 2.040V.This detector probe is also used by the RF-power meter created by Georg, DL6GL.
To achieve the above goals, Norbert has designed a circuit board for the RF-detector probe that fits in a commercially available tin plated housing 55 x 20 x 20 mm.
Board (image DG1KPN).
Layout of the board (not to scale, image DG1KPN).
A full-scale template can be downloaded here.
The assembled board in the open tin plate housing (image DG1KPN)
Finished design of the detector
The detector is powered by the millivoltmeter via an additional DC power output socket. The input voltage of the millivoltmeter is wired to this socket.
You can find a detailed description of this Log. Det. on the DL6GL website.
Logarithmic detector from Makis, SV1AFN
An other option is the logarithmic detector from Makis, SV1AFN, with the ADL5513 from Analog Devices. This part has a frequency range from 1 MHz to 4 GHz. The dynamic range amounts to 80 dB (according to the datasheet) and the sensitvity is lower than -70 dBm. The true sensitivity is lower than -80 dBm. Due to the calibration in stepps of 1 dB the nonlinear part of the characteristic curve can be used. Unfortunately for precise measurement above about 1.5 GHz it must be recalibrated. Measurement with the correction factor is not possible due to the change in the gradient of characteristic curve.
The ADL5513 board in the open tin plate housing
Finished design of the detector
The detector is offered by Makis, SV1AFN, at an attractive price. Therefore it is not worth to build it as do-it-yourself project. The PCB is delivered with soldered parts. Only the SMA connector must be soldered by the customer.