2.5-GHz Signal Source

 

More and more communications systems are operating in the 2.4-GHz ISM (Industrial, Scientific and Medical) band, including Blue

tooth, various WLAN (Wireless Local Area Network) and Home-RF systems. A simple test oscillator for the frequency band between 2.4 GHz and 2.5 GHz can prove useful in testing receivers. Such an oscillator is available from Maxim (www.maxim-ic.com) as a single IC. The MAX2750 covers the frequency range between 2,4 GHz and 2.5 GHz using in internal LC network that can be tuned using a varactor diode that is also built into the IC. An output buffer delivers a level of –3 dBm into 50 Ω.

This component is housed in an 8-pin µMAX package. The circuit is powered from a 9-V battery. The BC238C transistor stabilises the battery voltage at around 4 V. Although the MAX2750 can work with supply voltages

between +2.7 V and +5.5 V, the frequency stability of the free-running oscillator is better with a stabilised supply voltage. All connections to the IC are decoupled using 220pF capacitors, which must be located as close as possible to the IC pins. The tuning voltage at pin 2, TUNE, may lie between +0.4 V and +2.4 V, which provides a tuning range between 2.4 GHz and 2.5 GHz. If it is desired to switch off the oscillator, this can be done by connecting the Shutdown input (SHDN) to earth potential. When the IC is shut down, its current consumption drops to around 1 µA. Here the shutdown input is connected to the Vcc potential by a pull-up resistor, so that the oscillator runs. The –3 dB output level can be reduced using the indicated pi attenuator. A number of resistance values for this attenuator are shown in the table.

source: Elektor Circuit Collections 

Step-up Switching Regulator with Integrated Current Limit

 

In the form of the LT1618, Linear Technology (www.lineartech.com) has made available a step-up switching regulator with a current limit mechanism. This makes it easy to protect an otherwise not short-circuit-proof switching regulator: the input voltage is always connected to the output via an inductor and a diode. We can limit the current at the input (Figure 1), which limits the current drawn by the entire circuit; alternatively, with the circuit of Figure 2, the output current can be limited. This enables the design of constant current sources at voltages higher than the input voltage. In the circuit shown the nominal output voltage of the step-up switching regulator will be around 22 V. The output voltage can be calculated using the formula

Vout = 1.263 V (1+R1/R2)

The output current can be set via R3 as follows:

Imax = Vsense / R3

where Vsense = 50 mV

The IADJ input can be set to a voltage between 0 V and +1.58 V resulting in a linear reduction of the limit current.

The sense voltage of 50 mV across R3 for maximum current is reduced as follows:

Vsense = 0.04 (1.263 V – 0.8 VIADJ )

Hence, for a fixed value of R3, the VIADJ input allows the current limit to be adjusted. Note that in the first circuit the sense resistor R3 is fitted between the input electrolytic capacitor and the inductor. If R3 is fitted before the capacitor, the inductor current can not be properly controlled.

The LT1618 operates on input voltages between +1.6 V and +18 V. Its output voltage must lie between Vin and +35 V. With a switching current of 1 A through pin SW to ground, an output current of around 100 mA can be expected. The switching frequency of the IC is about 1.4 MHz, and the device is available in a 10-pin compact MSOP package.

Li-Ion Protection Circuit

 

When a lithium-ion battery is discharged below the minimum recommended cell voltage its life expectancy is dramatically reduced. The circuit described here can avoid this by disconnecting the load from the battery when the cell voltage reaches a set level.The voltage at junction A may be set to 3 V, for example, by selecting the correct ratio of R1 and R2. When the battery voltage drops below the minimum value, the voltage at junction A will be smaller than that at junction B. The latter voltage is equal to: 

VB = 1.25 V + I R4 = 1.37 V

where:

I = (Vmin. – 1.25 V) / (R3 + R4) = 800 nA

(Vmin. = minimum value)

At this point the output of opamp LT1495 will go high, causing SW1 (a P-channel logic level MOSFET) to block and break the connection between the battery and the load. Because the battery voltage will rise when the load is disconnected, a certain amount of hysteresis is created by the addition of R5. This prevents the circuit from oscillating around the switching point. The value of R5 shown here provides 92 mV of hysteresis. So the battery voltage has to

rise to 3.092 V before the load is reconnected to the battery. An increase or decrease of the hysteresis is possible by reducing or increasing the value of R5, respectively. The required hysteresis depends in the internal impedance of the battery and the magnitude of the load current. The switching point defined by the values of R1-R2 is

quite critical with a circuit such as this. If the switching point is too high, then the available capacity of the battery is not fully utilised. Conversely, if the switching point is too low, the battery will be discharged too far with all the harmful consequences that may entail. Using the values shown here and including the tolerances of the parts, the switching point is between 2.988 V and 3.012 V. In practice it may be easier to select slightly lower values for R1 or R2 and connect a multi-turn trimpot in series with it. This makes an accurate adjustment of the switching point possible and has the additional advantage that R1 and R2 may be ordinary 1%-tolerance types.

Finally, before using the protection circuit it is advisable to first connect it to a power supply instead of a battery and carefully verify the operation of all its features!

Mains Powered

 

Many circuits can be powered directly from the mains with the aid of a series capacitor (C1). The disadvantage of this approach is that usually only one half cycle of the mains wave form can be used to produce a DC voltage. An obvious solution is to use a bridge rectifier to perform full-wave rectification, which increases the amount of current that can be supplied and allows the filter capacitor to be smaller. The accompanying circuit in fact does this, but in a clever manner that uses fewer components. Here we take advantage of the fact that a Zener diode is also a normal diode that conducts current in the forward direction. During one half wave, the current flows via D1 through the load and back via D4, while during the other half wave it flows via D3 and D2. Bear in mind that with this circuit (and with the bridge rectifier version), the zero voltage reference of the DC voltage is not directly connected to the neutral line of the 230-V circuit. This means that it is usually not possible to use this sort of supply to drive a triac, which normally needs such a connection. However, circuits that employ relays can benefit from full-wave rectification.

The value of the supply voltage depends on the specifications of the Zener diodes that are used, which can be freely chosen.

C2 must be able to handle at least this voltage. The amount of current that can be delivered depends on the capacitance of

C1. With the given value of 220 nF, the current is approximately 15 mA. A final warning: this sort of circuit is directly connected to

mains voltage, which can be lethal. You must never come in contact with this circuit! It is essential to house this circuit safely in a suitable enclosure.

source : Elektor Circuit Collections

Simple mV Source

 

This design can be used to simulate millivolt (mV) sensor signals for industrial control systems. Most of the new sensors used to day include some form of ‘intelligence’ at the measurement head, that is, the point at which the sensor comes into contact with what it is to measure. At this point, the sensor signal is conditioned/digitized and fed into a microcontroller that transmits a digital representation of the sensor value to the remote control system. However, there are still a number of ‘elderly’ control systems still in the field that have the intelligence remote from the sensor head. These systems rely on field wiring to convey the measured signal back to the control system.

During commissioning of these types of plant, it is useful to simulate the sensor signal to ensure amongst other things, that the sensor signal gets back to the correct terminals on the control system as they invariably pass through various junction boxes on the way. It can also be used to ensure that the control system operates correctly in response to the sensor signal. The design shown here has been used by the author to ‘bench test’ a control system prior to being installed. Please note that the design is only suitable for simple simulation and is not accurate enough for calibration purposes. Power from a ‘plugtop’ PSU (when bench testing) or a battery is fed to three current sources (diodes). Of these, I1 generates a 1.00 mA current signal, which when switched across the 100-Ω pot creates a 100-mV signal. Likewise, I2 generates a 0.25-mA signal which generates 25 mV across the pot. Current source I3 develops 3.0 mA and is used to illuminate the LED to give a power indication. The selected current source is switched via S2 to the 10-turn pot. Switch S1 is used to cleverly swap the polarity of the output signal. If the Type MTA206PA DPDT switch from Knitter is used, you get a centre-off position which actually shorts out the output signals (S1 pins 2 and 5) together, ensuring a zero output signal.

The current sources, despite being pretty expensive, are not very accurate — they have 10% tolerance! (hence the unsuitability for calibration use). If the output is too high, the tolerance can be ‘trimmed’ by fitting a bleed resistor (R1, R2) as shown in the diagram. The current sources are manufactured by Vishay/Siliconix and stocked by Farnell. The circuit draws a current of about 4.25 mA.