Micrel MICRF505BML/YML
August 2006 15
M9999-092904
+1 408-944-0800
Frequency Synthesizer
The MICRF505 frequency synthesizer consists of a
voltage-controlled oscillator (VCO), a crystal
oscillator, dual modulus prescaler, programmable
frequency dividers and a phase-detector. The loop-
filter is external for flexibility and can be a simple
passive circuit. The phase detector compares
frequencies of two signals and produces an error
signal which is proportional to the difference
between the input frequencies. The error signal is
used to control a voltage-controlled oscillator (VCO)
which creates an output frequency. The output
frequency is fed through a frequency divider back to
the input of the phase detector, producing a
feedback loop. If the output frequency drifts, the
error signal will increase, driving the frequency in the
opposite direction so as to reduce the error. Thus
the output is locked to the frequency at the other
input. This input is called the reference and is
derived from a crystal oscillator, which is very stable
in frequency. The block diagram below shows the
basic elements and arrangement of a PLL based
frequency synthesizer. The MICRF505 has a dual
modulus prescaler for increased frequency
resolution. In a dual modulus prescaler the main
divider is split into two parts, the main part N and an
additional divider A, where A < N. Both dividers are
clocked from the output of the dual-modulus
prescaler, but only the output of the N divider is fed
into the phase detector. The prescaler will first divide
by 16. Both N and A count down until A reaches
zero, at which point the prescaler is switched to a
division ratio 16+1. At this point, the divider N has
completed A counts. Counting continues until N
reaches zero, which is an additional N-A counts. At
this point the cycle repeats.
A6…A0 D7 D6 D5 D4 D3 D2 D1 D0
0001010 - - A0_5 A0_4 A0_3 A0_2 A0_1 A0_0
0001011 - - - - N0_11 N0_10 N0_9 N0_8
0001100 N0_7 N0_6 N0_5 N0_4 N0_3 N0_2 N0_1 N0_0
0001101 - - - - M0_11 M0_10 M0_9 M0_8
0001110 M0_7 M0_6 M0_5 M0_4 M0_3 M0_2 M0_1 M0_0
0001111 - - A1_5 A1_4 A1_3 A1_2 A1_1 A1_0
0010000 - - - - N1_11 N1_10 N1_9 N1_8
0010001 N1_7 N1_6 N1_5 N1_4 N1_3 N1_2 N1_1 N1_0
0010010 - - - - M1_11 M1_10 M1_9 M1_8
0010011 M1_7 M1_6 M1_5 M1_4 M1_3 M1_2 M1_1 M1_0