Circuit Block Descriptions
2.2
Power Supply
The TRF3765 uses three primary power supplies: VCC1, VCC2, and VCC3. In the default configuration,
VCC1 and VCC2 are connected onboard by R41 and VCC3 is unused. The entire board can be supplied
through 3.3 V on TP2 or Revision D boards. Earlier board revisions require multiple power connections.
A clean power supply is critical to optimal phase noise performance of the synthesizer. The impact of the
power supply is discussed in detail in the application report Supply Noise Effect on Oscillator Phase Noise
(
). Linear power supplies are the best sources available. Switching power supplies degrade in-
band phase noise by 10 dB compared to linear laboratory supplies. Onboard regulators U3 and U4 are
ultra-clean TPS74201 linear regulators that also provide excellent performance when they are driven by
most laboratory power supply equipment. These regulators provide performance comparable to a clean
linear supply. To use these regulated 3.3-V supplies, disconnect power from TP2 and remove R41.
Connect 5 V to TP26, using TP27 for ground. Place jumpers on JP4 and JP5 to shunt jumper pins 1 and
2.
VCC3 can be used to drive VCC_TK, a 3.3-V/5-V tolerant supply on the TRF3765. VCC_TK is normally
driven by the 3.3-V VCC2 supply, but some applications perform better with a 5-V supply on VCC_TK. To
use VCC3 to drive VCC_TK at 5 V, move FB2 onto FB11. Populate R12 with a short. Then drive VCC3
through TP5 with a clean linear laboratory supply at 5 V.
VCC3 can also be driven at 5 V by onboard regulator U5. However, this regulator is not as clean as a
linear laboratory supply, and some phase noise performance loss occurs. To use the VCC3 onboard 5-V
regulator, drive TP25 with 6 V using TP28 for ground and place a jumper on JP1 to shunt jumper pins 1
and 2.
The TRF3765EVM includes a power supply filter. This filter can be used to reduce in-band frequency
noise from a switching power supply so that an external supply can drive 5 V on VCC_TK. Phase noise
performance using a high-quality laboratory switching power supply to drive VCC3 through TP5 is similar
to performance measured using a linear supply. The filter is integrated on Rev. E and later boards. Rev. D
and earlier boards include an external filter that is equipped with BNC connectors for a convenient
connection with power supply banana jacks.
Each VCC pin on the TRF3765 connects to an individual test point. The test point may be used for
monitoring. Because each supply pin is isolated through a ferrite bead, by lifting the ferrite bead these test
points may also be used to drive single-device supply pins.
Spurs occurring in the LO signal at 60 kHz and 100 kHz offset from the carrier are usually the result of
ground loops in power supply cabling.
2.3
Loop Filter
Loop-filter components are also critical to optimal phase noise performance. The loop filter must be
matched to the selected phase frequency detector (PFD) frequency. TRF3765 boards are shipped with
components matched to the onboard reference clock and configuration file. However, to use a different
PFD frequency, the loop-filter components must be updated. The Loop Filter Design Tool available at
in the TRF3765EVM product folder is an intuitive software package that identifies proper
component values.
Loop filter reference designators are shown in
. The assembly layout of these components is
shown in a silkscreen blow-up on the EVM.
3
SLWU076A – November 2011 – Revised July 2012
TRF3765 Integer/Fractional-N PLL With Integrated VCO Evaluation Module
Copyright © 2011–2012, Texas Instruments Incorporated