R5
R6
C1
C2
C3
R4
C4
CP_OUT
VTUNE_IN
CP_REF
VTUNE_REF
R15
R13
R20
R45
R46
TP12
VCC2
Configuration Options
Figure 1. Loop Filter Reference Designators
VTune may be monitored on TP12 without disrupting circuit operation, because R20 is a high-value
resistance. TP12 can also be used to drive VTune for open-loop VCO measurements when the TRF3765
charge pump is in 3-state logic. A 1-µF capacitor on C4 is also recommended for open-loop
measurements to help stabilize the applied voltage.
By default, reference is tied to ground through shorts on R13, R15, and R45, with R46 open.
2.4
SPI Communication
SPI communication from the TRF3765 GUI passes through mini-USB connector J7. The USB interface is
decoded and encoded by U2 into SPI lines DATA, STROBE, CLK, and RDBK. U2 is powered through the
USB connection instead of the board supply, and LED D1 indicates USB power is applied. Test points
TP6–TP10 can be used to monitor SPI communication with laboratory equipment. The laboratory
equipment must be set to high impedance so that it does not load the communication lines.
When the USB cable is disconnected, U2 I/Os are high impedance. In this case, TP6–TP10 can be used
to directly drive the SPI lines.
The power-on reset default register settings in the TRF3765 do not correspond to a valid operational state.
SPI initialization is required to operate the device. Once the registers have been initialized, the mini-USB
cable may be disconnected without disrupting device operation. However, once the mini-USB has been
reconnected, the link must be reestablished through the GUI Connect button on the Start Up tab. The link
exists between the GUI computer and U2, so loss of power to the TRF3765 device does not require
reestablishing the link. Loss of device power, however, does require re-initialization of the registers.
2.5
Reference Clock
An oscillator is installed on the TRF3765EVM to provide a reference clock to the device. JP2 installed
provides supply voltage to the oscillator, whereas JP3 installed connects the oscillator output to the
TRF3765. The oscillator frequency drifts over temperature and is not rated for the full TRF3765
temperature operating range, so temperature testing must use an external reference clock.
An external reference clock can be supplied through the SMA connector J8. When using an external
reference clock, remove jumpers on JP2 and JP3. The external reference is ac-coupled to the TRF3765
input pin. An external reference can also be used to frequency-lock the device to laboratory equipment.
Verify that any supplied reference clock has low phase noise.
3
Configuration Options
The TRF3765 evaluation module ships configured for either integer mode or fractional mode. Each
configuration is designed to use different reference and PFD frequencies and also has the corresponding
loop-filter components. Differences in integer mode and fractional mode boards are listed in
4
TRF3765 Integer/Fractional-N PLL With Integrated VCO Evaluation Module
SLWU076A – November 2011 – Revised July 2012
Copyright © 2011–2012, Texas Instruments Incorporated