Datasheet LT1394 (Analog Devices) - 9

LT1394
U U W U APPLICATIONS INFORMATION
5V
Temperature-Compensated Crystal Oscillator (TXCO)
1MHz TO 10MHz 2k CRYSTAL (AT-CUT) Figure 5 is a temperature-compensated crystal oscillator (TXCO). This circuit reduces oscillator temperature drift by inserting a temperature-dependent compensatory cor- + rection into the crystal’s frequency trimming network.
(a)
LT1394 OUTPUT 2k This open-loop correction technique relies on cancellation – of the temperature characteristics of the oscillator, which 2k are quite repeatable. 0.068µF The LT1394 and associated components form the crystal oscillator, operating similarly to Figure 3’s examples. The 10MHz TO 25MHz LM134, a temperature-dependent current source, biases 5V CRYSTAL (AT-CUT) A1. A1 takes gain referred to the LM134’s output and the 2k negative offset supplied via the 470kΩ-LT1004 reference 22Ω + path. Note that the LT1004’s negative voltage bias is
(b)
LT1394 OUTPUT bootstrapped from the oscillator’s output, maintaining 820pF 2k – single supply operation. This arrangement delivers tem- 2k perature-dependent bias to the varactor diode, causing a scaled variation in the crystal’s resonance versus ambient 200pF temperature. The varactor’s bias-dependent capacitance 1394 F03 shift pulls crystal frequency to complement the circuit’s temperature drift. The simple first order fit provided by the
Figure 3. Crystal Oscillators for Outputs to 30MHz. Circuit (b)’s Damper Network Supresses Overtone Crystal’s Harmonic Modes
compensation is very effective. Figure 6 shows results. The –70ppm frequency shift over 0°C to 70°C is corrected within a few ppm. The “FREQ SET” trim also biases the XTAL X varactor, allowing accurate output frequency setting. It is RX LOGIC INPUTS AS MANY STAGES worth noting that better compensation is possible by AS DESIRED XTAL B DX including higher order terms in the temperature-to-volt- 1k B age conversion. 5V XTAL A 1k A
18ns, 500
µ
V Sensitivity Comparator
1k D1 D2 + The ultimate limitation on comparator sensitivity is avail- LT1394 OUTPUT able gain. Unfortunately, increasing gain invariably 1k – involves giving up speed. The gain vs. speed trade-off in a 2k fast comparator is usually a practical compromise designed to satisfy most applications. Some situations, 75pF = 1N4148 however, require more sensitivity (e.g., higher gain) with 1394 F04 GROUND XTAL CASES minimal impact on speed. Figure 7’s circuit adds a differ- ential preamplifier ahead of the LT1394, increasing gain.
Figure 4. Switchable Output Crystal Oscillator. Biasing A or B
This permits 500µV comparisons in 18ns. A parallel path
High Places Associated Crystal in Feedback Path. Additional
DC stabilization approach eliminates preamplifier drift as
Crystal Branches Are Permissible
an error source. A1 is the differential preamplifier, operat- ing at a gain of 100. Its output is AC-coupled to the LT1394. 9