HMC703LP4E Datasheet, PDF (22/58 Pages) Hittite Microwave Corporation – 8 GHZ FRACTIONAL SYNTHESIZER

v02.0813
HMC703LP4E
8 GHz fractional synthesizer
As an example, suppose we want to tune a VCO to 7910 MHz. Since the input frequency is > 4 GHz, the
RF divide-by-2 must be engaged, so k=2:
f_vco
7,910 MHz
k
2
f_ps
3,955 MHz
f_xtal
= 50 MHz
R
=1
f_pd
N_int
N_frac
Reg 04h
= 50 MHz
= 79
= 0.1
= round(0.1 x 2²⁴) = round(1677721.6) = 1677722
50e6
1677722
f_ps =
(79 +
1
2²⁴
) = 3955 MHz + 1.2 Hz error
(EQ 7)
f_vco = 2 (3955 +1.2 Hz) = 7910 MHz + 2.4 Hz error
(EQ 8)
In this example the output frequency of 7910 MHz is achieved by programming the 16-bit binary value of 79d = 4Fh =
0000 0000 0100 1111 into intg_reg in Reg 03h, and the 24-bit binary value of 1677722d = 19999Ah = 0001 1001 1001
1001 1001 1010 into frac_reg in Reg 04h. The 2.4 Hz quantization error can be eliminated using the exact frequency
mode if required.
Exact Frequency Mode
The absolute frequency precision of a fractional PLL is normally limited by the number of bits in the fractional modulator.
For example a 24 bit fractional modulator has frequency resolution set by the phase detector (PD ) comparison rate
divided by 2²⁴. In the case of a 50 MHz PD rate, this would be approximately 2.98 Hz, or 0.0596 ppm.
In some applications it is necessary to have exact frequency steps, and even an error of 3 Hz cannot be tol­erated. In
some fractional synthesizers it is necessary to shorten the length of the accumulator (the denominator or the modulus)
to accommodate the exact period of the step size. The shortened accumula­tor often leads to very high spurious levels
at multiples of the channel spacing, f_step = f_PD/Modulus. For example 200 kHz channel steps with a 10 MHz PD rate
requires a modulus of just 50. The Hittite method achieves the exact frequency step size while using the full 24 bit
modulus, thus achiev­ing exact frequency steps with very low spurious and a high comparison rate, which maintains
excellent phase noise.
Fractional PLLs are able to generate exact frequencies (with zero frequency error) if N can be exactly represented in
binary (eg. N = 50.0,50.5,50.25,50.75 etc.). Unfortunately, some common frequencies cannot be exactly represented.
For example, N_frac = 0.1 = 1/10 must be approximated as round((0.1 x 2²⁴)/ 2²⁴ ) ≈ 0.100000024. At f_PD = 50 MHz this
translates to 1.2 Hz error. HMC703LP4E exact frequency mode addresses this issue, and can eliminate quantization
error by programming the N_channels (Reg 0Dh) to 10 (in this example). More generally, this feature can be used whenever
the prescaler frequency, f_ps, can be exactly represented on a step plan where there are an integer number (N_channels)
of frequency steps across integer-N boundaries. Assuming the RF divide by 2 is disabled so that f_ps=f_vco, this holds
when the VCO frequency, f_vco satisfies (EQ 9), shown graphically in Figure 30.
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