GP2010 GP2105 AB5640 GP2015 MLF2012DR56KT MLF1608DR56KT GP2010/GP2015 DS4056

Reduction of VCO-related EMC radiation Application Note AB5640 This Application Brief has been prepared to advise users of the

GP2010 GP2010 and GP2105 GP2105 Reduction of VCO-related EMC radiation Application Note AB5640 AB5640 This Application Brief has been prepared to advise users of the Zarlink GP2010 GP2010 or GP2015 GP2015 GPS RF Front-end how to reduce the radiated emissions of VCO harmonics. It has been discovered that the on-chip 1.4GHz VCO in the GP2010 GP2010 and GP2015 GP2015 IC can radiate significant levels of fundamental and second-harmonic (2.8GHz) EMC energy, via the external PLL Loopfilter components (2 capacitors and 1 resistor). The PLL Loop-filter is predominantly a low-frequency network, designed to set the on-chip PLL's loopbandwidth to approx. 15kHz. This same network also appears to radiate significant amounts of 1.4GHz and 2.8GHz. A diagram showing the existing PLL Loop-filter arrangement for both the GP2010 GP2010 and GP2015 GP2015 GPS RF Front-ends is shown in Figure 1. A simple solution to this problem is to add a series inductor (L1) between Pin 3 (PLLFILT2) and external capacitor C1, of approx. 1µH (any value between 500nH and 3µH should be adequate, as used with the SAW filter matching network). L1 should be a ISSUE 1.0 November 2001 screened inductor (e.g. TDK MLF2012DR56KT MLF2012DR56KT (0805 size), TDK MLF1608DR56KT MLF1608DR56KT (0603 size), or an equivalent). L1 should be mounted as close as practically possible to pin 3 of the GP2015 GP2015, in order to maximise the rejection of EMC energy from this pin. The revised circuit for this network is shown in Figure 2. The new inductor L1 has virtually zero impedance at the PLL-loop-bandwidth frequency of 15kHz, but has high impedance of kohms at >1GHz. Tests performed with this modified PLL loop-filter suggest that PLL Loop stability is unaffected by the use of this inductor. Additionally, and most significantly, the addition of inductor L1 creates approx. 20dB of added rejection of radiated EMC energy at 2.8GHz. This change is strongly recommended in all GPS receiver designs, which employ the GP2010 GP2010 or GP2015 GP2015 RF Front-ends. C2 C1 For 15kHz PLL Loop Bandwidth: C1 = 470nF C2 = 10nF R2 = 270ohms R2 PIN 3 PLLFILT2 PIN 2 PLLFILT1 VCO TUNING VOLTAGE R3