HMC703LP4E Datasheet, PDF (23/58 Pages) Hittite Microwave Corporation – 8 GHZ FRACTIONAL SYNTHESIZER

v02.0813
HMC703LP4E
8 GHz fractional synthesizer
Where:
f_vco mod^{���}f_gcd ^{�÷�}= 0, where f_gcd = gcd(f_vco,f_PD)
N_channels = f_PD / f_gcd, and N_channels < 2²⁴
f_PD = frequency of the Phase Detector
f_VCO is the desired output frequency
f_N , f_N+1 are integer multiples of the Phase Detector
f_gcd stands for Greatest Common Divisor

eg.
f_gcd (4000.200MHz, 50MHz)
= 200kHz

therefore N_channels = 50 MHz/200 kHz = 250
f_VCOn are other VCO frequencies we can exactly tune to, given this f_gcd spacing
(EQ 9)
6 - 23
Figure 30. Exact Frequency Tuning
In the previous paragraph, it was assumed that a single frequency was to be achieved with zero error. Exact frequency
mode also applies to cases where many exact frequencies are required, all of which fit on a particular channel spacing.
Example: To achieve exactly 50 kHz channel steps with a 61.44 MHz reference, calculate f_gcd and N_channels:
f_PD = 61.44 MHz
f_step = 50 kHz
f_gcd (61.44 MHz, 50 kHz)

Using the Euclidean algorithm to find the greatest common denominator:

61.440 MHz = 50 kHz x 1228 + 50 kHz

50 kHz = 40 kHz x 1 + 10 kHz

40 kHz = 10 kHz x 4 + 0 (0 remainder, algorithm complete)
f_gcd (61.44 MHz, 50 kHz) = 10 kHz
N_channels = 61.44 MHz / 10 kHz = 6144
For improved spectral performance (to keep spurs low and further out of band), it is best to keep f_gcd as high as pos-
sible (N_channels as low possible) for a given application.
Using Hittite Exact Frequency Mode
To use Exact Frequency Mode, we recommend the following procedure:
1. Calculate the required f_gcd as either gcd(f_VCO, f_PD) or gcd(f_PD, f_step) depending on your application
2. Calculate the number of channels per integer boundary, N_channels= f_PD / f_gcd and program into Reg 0Dh
3. Set the modulator mode to Exact Frequency (SD_MODE in Reg 06h[7:5] = 2)
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