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dc1846af

DEMO MANUAL DC1846A

QUICK START PROCEDURE

DC1846A RECONFIGURATION

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.

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,

makesuretheDC1846AjumperJP1isatthe3.3Vposition.

Run Help -> Troubleshoot in FracNWizard if the problem

is not resolved.

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

oscillatorfootprintsontheboard:U3,whichaccommodates

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

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.

Table 1. Oscillator Options

TUNABLE

DEVICE

SELECTION

COMPONENTS

RESISTOR FOR

ADDITIONAL POLE

CAPACITOR FOR

ADDITIONAL POLE

POWER

ENABLE PIN OR

REMARKS

U3*

C26

≅ 100pF, depopulate

R2_F3, C25 and C28*

R2_F2 = 0Ω for passive

loop filter*

R2_F2 = 75Ω for active

loop filter

C3_F2 = Open for passive

loop filter*

Pick C3_F2 such that

1/(2π

× R2_F2 × C3_F2) ≅

15 × the loop bandwidth in

Hz for active loop filter

Determined by U3

device power supply

specification, up to 24V

applied to V+VCO turret

U4

C28

≅ 100pF, depopulate

R2_F2, C25 and C26

R2_F3 = 0Ω for passive

loop filter

R2_F3

≅ 75Ω for active

loop filter

C3_F3 = Open for passive

loop filter

Pick C3_F3 such that

1/(2π

× R2_F3 × C3_F3) ≅

15

× the loop bandwidth in

Hz for active loop filter

Determined by U4

device power supply

specification, up to 24V

applied to V+VCXO turret

Use EN turret to control

pin 2 of U4 if needed

External

C25

≅ 100pF, R15 = 0Ω

Ohms, depopulate R13,

C26 and C28

-

-

-

Selection components

are for an active loop

filter. Connect the

external VCO output to

J5. Use CPGAIN to tune

the external device.

*These are the default installation options. For the DC1846A-A, U3 is populated with CVCO55CC-2328-2536 from Crystek