Micrel
MICRF506BML/YML
Frequency Synthesizer
The MICRF506 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 MICRF506 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.
Phase
detector
Prescaler
N-Divider
M-Divider
Div/4
1800MHz
Loop filter
XCO
VCO
Charge pump
A-Divider
PA
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
July 2006
15
M9999-092904
+1 408-944-0800
