High frequency wattmeter and noise generator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed design of a high-frequency wattmeter was developed on the basis of two devices described in [1, 2], where the possibility of using miniature incandescent lamps in measuring equipment was considered.

In addition to the simplicity of design and the availability of the used sensor elements, the author was attracted by the fact that the tuning of such a broadband device does not require high-frequency measurements. All you need is a digital three- or four-digit multimeter. All measurements are carried out at direct current.

The main difference of the proposed design of the wattmeter is that the measuring bridge, to which the sensor-converter on incandescent lamps is connected, is automatically balanced during operation.

The wattmeter, the circuit of which is discussed below, can also be used as a stable noise generator with a matched output impedance of 50 ohms. Since the device has an automatic resistance stabilization (ACC) sensor assembly, the temperature of the filament is also stabilized with high accuracy. The noise level can indirectly judge the operating frequency band of the device. Lamp noise extends up to 1 GHz. and the level drop begins at frequencies of 600...700 MHz, which corresponds to the data given in [1, 2]. You can read about noise generators and measurements with their help in [3, 4].

During the experiments, it turned out that incandescent lamps were very sensitive to mechanical stress. In practice, this means that the device must be protected from shock, otherwise the parameters of the converter may change abruptly. This happens, apparently, due to the displacement of the filament and a change in the heat transfer mode. The most stable level, as tests have shown, is the one that the sensor reaches after turning on the power. Since the ACC node works very stably, the transition to another RL level is easily determined by the dial indicator as a "zero" shift. If an accurate measurement is required, the supply voltage must be switched off and on again. The stability of the sensor, not related to mechanical influences, is quite high: during the day, the device did not detect a zero shift and a limit (by a dial indicator), which does not happen, for example, with an industrial VZ-48 millivoltmeter.

The fundamentals of the applied RF power measurement method are described in [1, 2]. The designations in the text correspond to those adopted in the original articles. The total power that heats the filaments of the lamps,

Рl \u1d Rvch + Pzam. ( one)

where RHF - high-frequency power. Рzam - DC replacement power [2].

Let's transform expression (1):

Rvch \u2d Rl - Rzam \u2d (Ul2 - Uzam2) / R \u2d (XNUMXUl ΔU-ΔUXNUMX) / R. (XNUMX)

where ΔU = Ul - Uzam; Рl = Ul2/R; Рzam = Uzam2/R: R = 200 Ohm (or 50 Ohm for a sensor with lamps connected in parallel, see below).

From expression (2) it follows that the value of the RF power at the sensor input is a function of the voltage difference ΔU = Ul- Uzam. It is this voltage difference (assuming the balance of the bridge) that the wattmeter measures. Formula (2) can be represented in a normalized form:

Rvch/Rl = 2ΔU/Ul - (ΔU/Ul)2 (3)

The form of function (3) is shown in fig. 1. Using the graph shown on it or the analytical expression (3). for a microammeter it is possible to draw a non-linear scale of RHF/Rl values. which is the same for any sensor. The calculation of the measured RF power is made by multiplying the readings of the device by the value of Рl of a particular sensor (the manufactured sample had a value of Рl = 120 mW). If on such a scale the pointer device shows the value "0.75". the measured input power is:

RF = 0.75RL = 0.75-120 = 90 mW.

It can be seen from the graph that if only the initial section of the Рl range is used for measurements, the scale non-linearity will be less. Therefore, in the manufactured sample of the wattmeter, two linear scales of the microammeter are used. corresponding to two limits - 40 and 100 mW. For a specific sensor with Рl = 120 mW, the position of the upper limits of these ranges is shown in Fig. 1. Nonlinear and linear scales are conjugated at two points (zero and maximum). At other points, the device underestimates the readings of the measured power.

Since most RF measurements are reduced to setting the maximum (minimum) voltage or power value, analog indication is the most convenient, and the indicated scale error is not a significant drawback. In addition, the device retains the ability to measure the exact power value with an external digital voltmeter [2].

The schematic diagram of the device is shown in fig. 2. Voltage stabilizers DA1, DA3 are included according to the standard scheme. Capacitors C4, C6 reduce the level of output voltage ripple. The integrated regulator DA2 creates a negative bias of -2.5 V, which is used to power the op-amp. Stabilizer DA4 performs the function of a source of exemplary voltage of 2,5 V (ION).

(click to enlarge)

The ACC node is made on the op-amp DA7 and the transistor VT1. The principle of operation of this node is similar to the operation of a conventional compensation voltage stabilizer, but instead of a zener diode, another non-linear element is installed - an incandescent lamp. The balance of the bridge is maintained with high accuracy (up to 10 ... 20 μV) by changing its supply voltage (R7 - R10 and sensor lamps). The resistances of the bridge resistors are selected with an error of ±0,1%.

Since the bridge is balanced, when connecting the sensor with a series connection of lamps (Fig. 2), the equality is fulfilled:

Rd \u9d R10 + R200 \uXNUMXd XNUMX Ohm,

where Rd is the resistance of the sensor.

A digital 3.5-digit device does not allow measuring resistance with the indicated accuracy, but it can be calibrated using precision resistors (eg C5-5V) with a tolerance of 0.05 - 0,1%. Since the elements of the bridge heat up during operation, it is not recommended to use MLT resistors due to the large value of TCR ±(500... 1200)-10-6 1/°C [6]. It is important that the resistances of the resistors R7. R8 differed by no more than ±0,1%, and the value can be in the range of 47 ... 75 ohms. It is not recommended to reduce the power of the resistors included in the arms of the measuring bridge indicated in the diagram.

Immediately after turning on the power of the device to start the ACC, the resistor R6 creates a small initial current flowing through the bridge, so the maximum power measured by a specific sensor is somewhat less than Rl.

The high-frequency connector XW1 also removes noise voltage in a wide frequency band.

For normal operation of the ACC assembly, the lamps must operate in a mode where the thread glows weakly or does not glow at all. With a bright glow, the dependence of the voltage on the lamp on the current flowing is close to linear, and in this "linear" section of the ACC is inoperable.

The maximum power of the sensors with which the wattmeter works does not exceed 250 mW. Only sensors with an input impedance of 50 ohms are considered here. but you can also use sensors with a resistance of 75 ohms [2]. The resistance of the bridge resistors in this case: R9 = 225 ohms. R10 = 75 Ohm. The power of the sensors with the same lamps will increase approximately twice, so the bridge supply voltage will have to be increased.

Sensor type "A" is described in detail in [1, 2]. In the on state, its DC resistance is 200 ohms. and from the RF input side - 50 Ohm. Lamps for such a sensor must be selected in pairs so that in the on state the voltage drops on both lamps are approximately equal. By checking several instances of the lamps, it is easy to see that this condition is often not satisfied, even when the resistances of the lamps in the cold state are the same. Assuming that the input resistance should be within 50 ohms ±0.25%. then in this case the voltages on the lamps connected to the wattmeter may differ by no more than 15%. The sensor sample, with which the operation of the device was tested, had the following parameters: Ul = 4,906 V (Pl = 120 mW). Un1= 2.6 V. Un2= 2,306 V (voltage difference across the lamps is about 12%).

On fig. 2 for CI. C2 in the sensor "A" is set to 0,44 μF, which allows you to reduce the lower limit of the frequency range to 1 ... 1,5 MHz. To reduce the inductance of the input circuit, two 0.22 μF CHIP capacitors connected in parallel were used. With the values ​​of capacitors indicated in [1, 2] (0.047 μF), a measurement accuracy of about 1% is achievable only at the frequency range boundary of at least 15 MHz, and not 150 kHz.

In contrast to that described in [2]. The proposed wattmeter allows using two types of sensors, in which the lamps are connected in series (type "A" sensor) or in parallel (type "B" sensor).

The type "B" sensor connected to the device with a jumper on pins 1 and 4 in the sensor connector closes the resistor R9 of the bridge, therefore Rd \u10d R50 \u0.25d 0.5 Ohm. For sensors of this type, the selection of a specific pair of lamps is not necessary. To get the required value of Rl. one to four lamps can be used in the sensor, and they can be of various types. To expand its frequency range down, an increase in the inductance of the inductor should not lead to an increase in its active resistance (preferably no more than 50 Ohm, i.e. 0.3% of 0.4 Ohm). The inductor has to be wound with a wire with a diameter of 50 ... 1 mm in order to obtain a coil inductance of the order of 16 μH with the dimensions of the MLT-1 resistor. With such an inductance, the lower limit of the frequency range of the "B" sensor is XNUMX MHz, in contrast to the inna "A" sensor, which is quite accurate already at a frequency of XNUMX MHz.

On DA6 chips. DA7 and LEDs HL1. HL2 made comparator. Its purpose is to indicate the balance of the measuring bridge. When it is balanced, both LEDs turn off. With the values ​​​​of resistors R29 and R31 indicated in the diagram, the dead zone of the comparator is approximately ± 60 ... 90 μV. If the RF power at the sensor input is equal to the maximum allowable value Рl (actually somewhat less). ACC is unable to balance the bridge, and one of the LEDs HL1. HL2 turns on, indicating that the measurement is not possible.

The inertia of incandescent lamps allows you to visually see the process of regulation (duration 1 ... 2 s). As a result, the indicator has another positive function. It allows you to determine small and fast changes in the amplitude of the RF signal at the input of the device. It is known that such amplitude fluctuations are characteristic of unstable amplifying cascades or generators, which are prone to self-excitation at spurious frequencies as well. For example, when checking the wattmeter from the G4-117 generator, it was found that at frequencies above 8 MHz and an output signal level of more than 2 V (at a load of 50 Ohms), the internal stabilizer of the output signal amplitude practically does not work in the generator.

The display unit of the device is made on the OS DA4. DA5. microammeter RA1. Variable resistors R19 (zero corrector) and R24. R26 and R25, R27 ("range" corrector) make it easy to set up the wattmeter to work with any sensors with Pl < 220 mW. With wide adjustment ranges, it is best to use multi-turn wirewound resistors. Therefore, to adjust the "zero" in the device, a variable resistor of the SP5-35B type with a high electrical resolution is installed [6]. Additional zero correction when switching to another measuring range, as a rule, is not required. Zero and span adjustments do not affect each other. The presence of the diode bridge is due to the fact that power is a positive value. With this option of turning on the microammeter, its arrow does not cross zero.

Most of the elements of the device are located on the same board, and those that heat up during the operation of the wattmeter (DAI, DA2. VT1. R7-R10). have thermal contact with the rear aluminum panel of the instrument. It is better to set up the device in a closed case. The design must provide access to all adjusting elements.

The designs of sensors and drawings of printed circuit boards are shown in fig. 3, 4. The foil on the reverse side of the printed circuit board is completely preserved. The high-frequency connector and cable braid are soldered on both sides of the board. To minimize the intrinsic inductance of the sensors, they use surface-mount capacitors (0.22 and 0.022 uF, two in parallel). The body of the high-frequency connector is soldered to the foil on both sides of the board.

The wattmeter uses precision wire resistors S5-5V 1 W with a resistance of 100 Ohm with a tolerance of ±0.1% (TCS ±50 10-6 1/°C). As R7, R8, R10, two such parallel-connected resistors are installed, and R9 is formed by the series-parallel connection of three. It is also possible to use other precision resistors, for example, C2-29V, C2-14. Resistors R24 - R26 - tuning. wire SP5-2, SP5-3. XS1 socket for sensor connection - ONTS-VG-4-5/16-R (SG-5). high-frequency connectors XW1 - СР-50-73Ф. Power connector - male, socket DJK-03B (2.4/5.5 mm).

Instead of the KD906A bridge, you can use any diodes, for example, the D9, D220, KD503 series. KD521. Microammeter - M24. M265 with a total deviation current of 50 - 500 μA.

KR142EN12A can be replaced with a low-power imported analogue - LM317LZ, and KR 142EN19 - TL431.

The wattmeter is adjusted in assembled form 10 ... 15 minutes after switching on.

First, any pair of CMH2-3 lamps is connected to pins 1, 9 of the XP60 connector. connected in series, and to sockets "A" and "B" - a digital voltmeter, which is included in the minimum measurement limit (200 mV). By rotating the tuning resistor R15, zero readings of the voltmeter are achieved.

After balancing the measuring bridge, adjust the comparator. Resistor R21 (or R23, depending on the initial bias of the op-amp DA8. DA9) is temporarily replaced (the device case will have to be opened) with a variable resistance of 100 kOhm. By changing the resistance of the resistor, a state is achieved in which both LEDs will be extinguished. Then the variable resistor is replaced by a constant one with a resistance close to the found one. The limits of such offset adjustment are relatively narrow, so it is advisable to check the initial offset value of all op amps before installing it in the board. Chips with a minimum offset should be used as DA8. DA9. For other microcircuits, the value of the initial offset is not so important, since their operating modes can be controlled by the corresponding variable resistors.

After adjusting the comparator, you need to make sure that its dead zone is ±60...90 µV. It is permissible to unbalance the bridge with resistor R15 within a small range, and determine the mismatch voltage at which the LEDs turn on using the connected digital voltmeter. It is desirable that the dead zone of the comparator be symmetrical (with respect to the balance point of the bridge). To expand it, you can increase the resistance of the resistor R29.

Having finished setting up the comparator, the measuring bridge is finally balanced with resistor R15. Using resistor R19, you should check that for arbitrarily selected lamps, zero readings of the PA1 microammeter are set.

Having completed these operations, pairs of lamps for the sensor are selected on the switched-on device according to mechanical stability and voltage difference. The digital voltmeter must be switched to sockets "0", "B". It will show the voltage Un, from which it is easy to calculate Rl. The upper points of the ranges "100 mW" and "40 mW" can be set by calculation, since at a given value of Pp it is known what voltage the digital voltmeter will show at the indicated points (Uzam). A signal can be applied to the sensor input from any generator with a frequency above 2...3 MHz and an output voltage of at least 2,5 V (at a load of 50 ohms). The signal level of the generator is adjusted according to the readings of a digital voltmeter as follows. so that the voltmeter shows the calculated value Uzam, after which, by adjusting the resistor R24 ​​(R25), set the microammeter needle to the last division of the scale.

To power the device, any source with an output voltage of 15 ... 24 V inflow of 150 ... 200 mA is suitable. If a low-power mains "adapter" is used, make sure that the lower limit of the input voltage ripple is at least 2.5 V higher than 12 V.

A direct check of the characteristics of the manufactured device could not be carried out due to the lack of appropriate devices. Therefore, there is no need to talk about checking the frequency properties of the sensor at frequencies of hundreds of megahertz. The author had at his disposal only a digital multimeter DT930F + (accuracy class 0.05 when measuring DC voltage and 0.5 when measuring resistance, rms AC voltage up to 400 Hz [5]), a GZ-117 low-frequency generator (up to 10 MHz), and a VZ-millivoltmeter. 48 (accuracy class 2.5 In the band 45 Hz ... 10 MHz).

Verification of several points of the scale (the control was carried out on a digital voltmeter, and not on a microammeter scale) at a frequency of 5 MHz showed that the wattmeter works more accurately and more stable than the VZ-48! It's good that this millivoltmeter had control sockets on the back wall, to which you can connect an external (digital) voltmeter. Assuming that the VZ-48 does not have a frequency error in the middle part of the operating frequency range, three voltage points were calibrated at a frequency of 400 Hz. according to the available digital voltmeter class 0.5.

After that, the generator was tuned to a frequency of 5 MHz and the previously measured voltage values ​​at the sensor input were restored using a digital voltmeter (and not the VZ-48 analog scale). According to the VZ-48 readings, the input power was calculated from the ratio Pl = U2/50. and the power that the wattmeter showed was calculated by formula (2).

The results of these measurements are shown in the table. It is especially impressive that in the obtained error values, the presence of a systematic error is clearly visible [7, 8], which means that the parameters of the wattmeter can be even better!

Various thermistors can serve as sensors - both with positive and negative TCR. In order for the ACC unit to work with negative TCR thermistors (incandescent lamps have a positive TCR), jumpers are provided in the device circuit (highlighted by a dash-dotted line), which must be rearranged to the position between contacts 1 and 4, 2 and 3.

To test the operability of ACC with a sensor having a negative TCS, a bead-type thermistor MKMT-16 with a nominal resistance of 5,1 kOhm [6] was used when switched on according to the "B" sensor circuit. Despite the large value of the initial resistance, the supply voltage of 10 V was sufficient to heat up the miniature thermistor and balance the bridge. But since the operating temperature for the thermistor is significantly lower than for the filament, and the thermal insulation is worse, this sensor works more like a temperature meter and zero stability is very low. The value of Рl = 102 mW.

For those who want to experiment with different sensors, here are some general tips. The initial resistance of the thermistor (for any sign of TCR) must be chosen such that the resistance of the heated thermistor (or a combination of several thermistors) is 50 ohms. achieved at the highest possible heating temperature. For example, thermistors ST1 -18. CT1-19 bead type are operational up to +300°С [6]. At the same time, measures must be taken in the design of the sensor for passive thermal stabilization and thermal insulation of the thermistor.

NTC thermistors at the moment of switching on may have too much resistance, therefore, a significant increase in the supply voltage may be required to create conditions for self-heating. When using posistors, there will be no problems with power supply.

Except CMH9-60. you can use other types of miniature incandescent lamps, the parameters of which are given in [1, 2]. It is easy to obtain transducers with Rl value from units to hundreds of milliwatts. The measurement of higher RF signal power is carried out through matched attenuators. The calculation of attenuators can be found in [9,10].

Literature

1. Trifonov A. Noise generator. - Radio. 1997. No. 7. p. 31.32
2. Trifonov A. High-frequency wattmeter. - Radio. 1997. No. 8. p. 32.33.
3. Zhutyaev S. G. Amateur VHF radio station. - M.: Radio and communications. 1981.
4. Skrypnik V. A. Instruments for monitoring and adjusting amateur radio equipment. - M.: Patriot. 1990
5. Nefedov S. Influence of voltage form on voltmeter readings. - Radio amateur. 1997. No. 10. p. 10.
6. Aksenov A. I., Nefedov A. V. Elements of household equipment circuits. Capacitors Resistors: A Handbook - M .: Radio and Communication. 1995.
7. Nefedov S. Metrological characteristics of measuring instruments. - Radio amateur. 1997. No. 12. p. 10.
8. Zaidel AN Elementary estimates of measurement errors. - L.: Science. 1968.
9. Red E. Reference manual for high-frequency circuitry. - M.: Mir, 1990.
10. Vinogradov Yu. Antenna attenuator. - Radio, 1997. No. 11. p. 80.

Author: O. Fedorov, Moscow

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