9

LT1394

Temperature-Compensated Crystal Oscillator (TXCO)

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.

This open-loop correction technique relies on cancellation

of the temperature characteristics of the oscillator, which

are quite repeatable.

The LT1394 and associated components form the crystal

oscillator, operating similarly to Figure 3’s examples. The

LM134, a temperature-dependent current source, biases

A1. A1 takes gain referred to the LM134’s output and the

negative offset supplied via the 470k

Ω-LT1004 reference

path. Note that the LT1004’s negative voltage bias is

bootstrapped from the oscillator’s output, maintaining

single supply operation. This arrangement delivers tem-

perature-dependent bias to the varactor diode, causing a

scaled variation in the crystal’s resonance versus ambient

temperature. The varactor’s bias-dependent capacitance

shift pulls crystal frequency to complement the circuit’s

temperature drift. The simple first order fit provided by the

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

varactor, allowing accurate output frequency setting. It is

worth noting that better compensation is possible by

including higher order terms in the temperature-to-volt-

age conversion.

18ns, 500

µV Sensitivity Comparator

The ultimate limitation on comparator sensitivity is avail-

able gain. Unfortunately, increasing gain invariably

involves giving up speed. The gain vs. speed trade-off in a

fast comparator is usually a practical compromise

designed to satisfy most applications. Some situations,

however, require more sensitivity (e.g., higher gain) with

minimal impact on speed. Figure 7’s circuit adds a differ-

ential preamplifier ahead of the LT1394, increasing gain.

This permits 500

µV comparisons in 18ns. A parallel path

DC stabilization approach eliminates preamplifier drift as

an error source. A1 is the differential preamplifier, operat-

ing at a gain of 100. Its output is AC-coupled to the LT1394.

Figure 3. Crystal Oscillators for Outputs to 30MHz. Circuit (b)’s

Damper Network Supresses Overtone Crystal’s Harmonic Modes

–

+

LT1394

2k

5V

2k

1MHz TO 10MHz

CRYSTAL (AT-CUT)

0.068

µF

OUTPUT

1394 F03

2k

–

+

LT1394

2k

5V

2k

2k

10MHz TO 25MHz

CRYSTAL (AT-CUT)

200pF

OUTPUT

820pF

22

Ω

(a)

(b)

1394 F04

–

+

LT1394

1k

5V

1k

1k

75pF

D1

OUTPUT

B

A

LOGIC INPUTS

AS MANY STAGES

AS DESIRED

XTAL A

1k

XTAL B

XTAL X

D2

2k

= 1N4148

GROUND XTAL CASES

Figure 4. Switchable Output Crystal Oscillator. Biasing A or B

High Places Associated Crystal in Feedback Path. Additional

Crystal Branches Are Permissible