We were are about to apply LMX2571 Low Power Synthesizers in our company’s developed RF transmitters.
We have made prototypes of both VHF and UHF transmitters. UHF transmitter output frequency range is from 430MHz up to 510MHz. LMX2571 synthesizer OSCin frequency = 16MHz; PDF frequency = 56MHz; PLL N Pre-scaler = 2; Cp_Ixx = 1250uA; CP_GAIN = 1; 3rd order PLL loop filter components C1 = 15nF, C2 = 100nF, R2 = 220Om (loop bandwidth = 22.3kHz).
+/-125kHz spurs are present at -47dBc/-70dBc level at output frequencies Fout = Foscin x N / CHDIV1/CHDIV2 +/-0.0125MHz. There: Foscin = 16MHz; N = 269, 270, 271, ……, 318; CHDIV1 = 5; CHDIV2 = 2.
Similar spurs are seen in LMX2571EVM Evaluation Module output signal too, at frequencies Fout = Foscin x N / CHDIV1/CHDIV2 +/-0.0125MHz. There: Foscin = 20MHz; N = 215, 216, 217, ……, 265; CHDIV1 = 5; CHDIV2 = 2. PDF frequency = 50MHz.
Our transmitters must to conform to the European Community standard EN300 113 (channel separation 12,5kHz). The transmitter’s output power are up 5W. Spurious emissions shall not exceed -36dBm. Could you please advise how to reduce the level of the spurs at least to –73dBc to the carrier level?
Thank you in advance,
Saulius
