Troubleshooting - Linear Technology DC1846A Demo Manual

DEMO MANUAL DC1846A
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Troubleshooting

If the red LED does not illuminate, follow the instructions
below:
1. Verify that you are able to communicate with the
DC1846A. The bottom status line in FracNWizard
should read "LTC6947" and "Comm Enabled". Refer
to FracNWizard's Troubleshoot and Help if not.
DC1846A RECONFIGURATION
The DC1846A is flexible and allows the use of a variety of
VCOs and the choice to employ either an active or pas-
sive loop filter. The following covers the hardware recon-
figuration of the DC1846A. Refer to FracNWizard's Help
and the LTC6947 data sheet to better understand how to
change programmed parameters on the DC1846A.
Installing Different Tunable Devices
The DC1846A permits the use of different tunable oscil-
lators, such as VCOs and VCXOs. There are two different
oscillator footprints on the board: U3, which accommodates
the popular 0.5 × 0.5 in. package, and U4, which accom-
modates another common 14mm × 9 mm package with
four or six pins. An external connectorized VCO can also
Table 1. Oscillator Options
TUNABLE SELECTION
DEVICE COMPONENTS
U3*
C26 ≅ 100pF, depopulate
R2_F3, C25 and C28*
U4 C28 ≅ 100pF, depopulate
R2_F2, C25 and C26
External C25 ≅ 100pF, R15 = 0Ω
Ohms, depopulate R13,
C26 and C28
*These are the default installation options. For the DC1846A-A, U3 is populated with CVCO55CC-2328-2536 from Crystek
4
RESISTOR FOR CAPACITOR FOR
ADDITIONAL POLE ADDITIONAL POLE
R2_F2 = 0Ω for passive C3_F2 = Open for passive
loop filter* loop filter*
R2_F2 = 75Ω for active Pick C3_F2 such that
loop filter 1/(2π × R2_F2 × C3_F2) ≅
15 × the loop bandwidth in
Hz for active loop filter
R2_F3 = 0Ω for passive C3_F3 = Open for passive
loop filter loop filter
R2_F3 ≅ 75Ω for active Pick C3_F3 such that
loop filter 1/(2π × R2_F3 × C3_F3) ≅
15 × the loop bandwidth in
Hz for active loop filter
- -
2. Verify that the 3.3V, 5V and V+VCO have the correct
voltages on them and that the reference frequency is
applied to the REF+ SMA input.
If the red LED is on but you cannot detect an RF output,
make sure the DC1846A jumper JP1 is at the 3.3V position.
Run Help -> Troubleshoot in FracNWizard if the problem
is not resolved.
drive the LTC6947 through J5. Table 1 gives options to
customize the DC1846's VCOs. When using an active loop
filter, an additional pole is required in the loop filter to limit
the op amp's noise contribution to the VCO phase noise.
Selecting Loop Filter Type
The DC1846A can use either an active or a passive loop
filter, depending upon the application. Some VCO tuning
voltage ranges are greater than the LTC6947 charge pump
voltage range (refer to the LTC6947 data sheet). In such
cases, an active loop filter using an op amp can deliver
the required tuning voltage. Table 2 summarizes these
options. FracNWizard helps determine the component
values listed in this table.
POWER
Determined by U3
device power supply
specification, up to 24V
applied to V+VCO turret
Determined by U4
device power supply
specification, up to 24V
applied to V+VCXO turret
-
ENABLE PIN OR
REMARKS
Use EN turret to control
pin 2 of U4 if needed
Selection components
are for an active loop
filter. Connect the
external VCO output to
J5. Use CPGAIN to tune
the external device.
dc1846af