Datasheet MAX038 (Analog Devices) - 10

MAX038
High-Frequency Waveform Generator
When the MAX038’s frequency is controlled by a volt- Conversely, if VFADJ is known, the frequency is given age source (VIN) in series with a fixed resistor (RIN), the by: output frequency is a direct function of VIN as shown in Fx = Fo x (1 - [0.2915 x VFADJ]) [8] the above equations. Varying VIN modulates the oscilla- tor frequency. For example, using a 10kΩ resistor for and the period (tx) is: RIN and sweeping VIN from 20mV to 7.5V produces tx = to ÷ (1 - [0.2915 x VFADJ]) [9] large frequency deviations (up to 375:1). Select RIN so that I
Programming FADJ
IN stays within the 2µA to 750µA range. The band- width of the I FADJ has a 250µA constant current sink to V- that must IN control amplifier, which limits the modu- lating signal’s highest frequency, is typically 2MHz. be furnished by the voltage source. The source is usu- ally an op-amp output, and the temperature coefficient IIN can be used as a summing point to add or subtract of the current sink becomes unimportant. For manual currents from several sources. This allows the output adjustment of the deviation, a variable resistor can be frequency to be a function of the sum of several vari- used to set VFADJ, but then the 250µA current sink’s ables. As VIN approaches 0V, the IIN error increases temperature coefficient becomes significant. Since due to the offset voltage of IIN. external resistors cannot match the internal tempera- Output frequency will be offset 1% from its final value ture-coefficient curve, using external resistors to pro- for 10 seconds after power-up. gram VFADJ is intended only for manual operation, FADJ Input The output frequency can be modulated by when the operator can correct for any errors. This FADJ, which is intended principally for fine frequency restriction does not apply when VFADJ is a true voltage control, usually inside phase-locked loops. Once the source. funda-mental, or center frequency (Fo) is set by IIN, it A variable resistor, RF, connected between REF (+2.5V) may be changed further by setting FADJ to a voltage and FADJ provides a convenient means of manually other than 0V. This voltage can vary from -2.4V to setting the frequency deviation. The resistance value +2.4V, causing the output frequency to vary from 1.7 to (RF) is: 0.30 times the value when FADJ is 0V (Fo ±70%). RF = (VREF - VFADJ) ÷ 250µA [10] Voltages beyond ±2.4V can cause instability or cause the frequency change to reverse slope. VREF and VFADJ are signed numbers, so use correct algebraic convention. For example, if VFADJ is -2.0V The voltage on FADJ required to cause the output to (+58.3% deviation), the formula becomes: deviate from Fo by Dx (expressed in %) is given by the formula: RF = (+2.5V - (-2.0V)) ÷ 250µA V = (4.5V) ÷ 250µA FADJ = -0.0343 x Dx [5] where V = 18kΩ FADJ, the voltage on FADJ, is between -2.4V and +2.4V.
Disabling FADJ
Note: While IIN is directly proportional to the fundamen- The FADJ circuit adds a small temperature coefficient tal, or center frequency (Fo), VFADJ is linearly related to to the output frequency. For critical open-loop applica- % deviation from Fo. VFADJ goes to either side of 0V, tions, it can be turned off by connecting FADJ to GND corresponding to plus and minus deviation. (not REF) through a 12kΩ resistor (R1 in Figure 2). The - The voltage on FADJ for any frequency is given by the 250µA current sink at FADJ causes -3V to be devel- formula: oped across this resistor, producing two results. First, the FADJ circuit remains in its linear region, but discon- VFADJ = (Fo - Fx) ÷ (0.2915 x Fo) [6] nects itself from the main oscillator, improving tempera- where: ture stability. Second, the oscillator frequency doubles. F If FADJ is turned off in this manner, be sure to correct x = output frequency equations 1-4 and 6-9 above, and 12 and 14 below by Fo = frequency when VFADJ = 0V. doubling Fo or halving to. Although this method doubles Likewise, for period calculations: the normal output frequency, it does not double the V upper frequency limit. Do not operate FADJ open cir- FADJ = 3.43 x (tx- to) ÷ tx [7] cuit or with voltages more negative than -3.5V. Doing where: so may cause transistor saturation inside the IC, lead- tx = output period ing to unwanted changes in frequency and duty cycle. to = period when VFADJ = 0V. 10 Maxim Integrated Document Outline Blank Page