BGT24MTR11
User's Guide
VCO Section
Application Note AN305, Rev. 1.0
2012-11-15
6 / 15
3
VCO Section
BGT24MTR11’s signal generation section consists of a free-running VCO with two separate tuning voltage
inputs followed by a buffer amplifer to reduce frequency pulling effects. Two prescalers are available to monitor
the frequency of oscillation. The first prescaler devides the transmitted frequency by 16, the second prescaler
further reduces the output of the first one by a factor of 65536.
3.1
Tuning Voltage Inputs
BGT24MTR11 has two inputs for tuning the VCO’s frequency of oscillation, FINE (pin 4) and COARSE (pin 5).
Both inputs can be used independently of each other to adjust the frequency output. As the pin names imply,
COARSE has a steeper tuning slope compared to FINE.
If there is only one voltage available for tuning the VCO it is possible to connect both pins to this single voltage
source. The resulting tuning sensitivity will be then the sum of the sensitivities of the respective pins.
Both tuning pins are connected internally via a pull-up resistor to Vcc. This means that when a pin is left open, it
will be internally at Vcc. So if both pins are left open the oscillator will be around 26 GHz at room temperature.
Note: It is mandatory for each of the two pins to be at a voltage equal or higher than 0.5 V. If any voltage at the
pins drops below that voltage level the oscillator will fail to work. This might lead to problems when starting
a control loop and the loop’s control output voltage at the start is below 0.5V. In this case some additional
DC voltage needs to be present at the tuning inputs.
It is possible to cover the whole 24 GHz ISM band with tuning voltages between 0.5 V and 3.3 V - both over the
device’s specified temperature range and production related device variations.
Figure 2 shows the temperature behavior of the VCO. The measurement was conducted with both tuning pins,
COARSE and FINE, connected together.
Figure 2
VCO frequency vs. tuning voltage V
FINE
= V
COARSE
and temperature
Below are two graphs that show how the the frequency output of the oscillator behaves for different pairs of
V
COARSE
and V
FINE
.
21
22
23
24
25
26
27
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Fr
e
q
u
e
n
cy
/
G
Hz
Tuning Voltage / V
Frequency vs. Vfine = Vcoarse and Temperture
-40°C
25°C
85°C
105°C
125°C
