XR215A Datasheet, PDF (27/32 Pages) Exar Corporation – Monolithic Phase-Locked Loop

XR-215A
The phase-comparator output voltage is a linear measure of the VCO frequency deviation from its free-running value.
The amplifier section, therefore, can be used to provide a filtered and amplified version of the loop error voltage. In this
case, the DC output level at pin 15 can be adjusted to be directly proportional to the difference between the VCO
free-running frequency, fo, and the input signal, fs. The entire system can operate as a “linear discriminator” or analog
“frequency-meter” over a 3:1 change of input frequency. The discriminator gain can be adjusted by proper choice of R₀
or R_F, for the test circuit of Figure 25, the discriminator output is approximately (0.7 R₀R_F) mV per % of frequency
deviation where R₀ and R_F are in kW. Output non-linearity is typically less than 1% for frequency deviations up to +15%.
Figure 28 shows the normalized output characteristics as a function of input frequency, with R₀ = 2kW and R_F = 36kW.
Crystal-Controlled PLL
The XR-215A can be operated as a crystal-controlled phase-locked loop by replacing the timing capacitor with a crystal.
A circuit connection for this application is shown in Figure 28. Normally a small tuning capacitor ( 30pF) is required in
series with the crystal to set the crystal frequency. For this application the crystal should be operated in its fundamental
mode. Typical pull-in range of the circuits is +1kHz at 10MHz. There is some distortion on the demodulated output.
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Normalized Temperature Range (f/f_o)
Figure 26. Tracking Range vs. Input Amplitude
(Pin 10 Open Circuited)
Rev. 1.01
27