XR2211 quirks (by Gerard Larooy)
Category: Mixed signal devices
This chip is for FSK and/or FM data reception. It would allow simple, low component count modems. The Bipolar internals would give less spreads than CMOS.
- the measured phase noise which is directly bad on FM and FSK because it relates to SNR & BER was significantly worse than the HC4045 which wasn't specifically designed for that purpose. When used in a telephone subcarrier application the margin between actual SNR and Telco spec was wafer thin. Tight audio bandwidth gave a couple of dB but made group 3 Fax operation borderline.
- the PLL would sometimes give a sub harmonic oscillation which would be in the audio passband, these FM birdies were more subjectively annoying than gaussian noise.
- the lock defect outputs (/Q & Q) sometimes give multiple glitches during transitions from lock to unlock and vice versa. Using these bipolar outputs to drive a telephone ringing transformer via pushpull small signal transistors ended in 'mysterious' product failure.
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