Theory of Operation

The local oscillator (U4)is a programmable oscillator, whose programmatic parameters are set by the microcontroller, U3. U3 is programmed with 16 different values corresponding to the settings of dip switch SW1. Without U3 in the circuit, U4 would default to an output frequency of 56.320 mHz. The IC U8 is needed if the version of U4 is the "LVDS" version. If U4 is a CMOS version, U8 (and R25) is not required and, instead a jumper wire is installed to bypass them.
This LO stage must produce an output RF signal that is four times the desired center frequency for the radio. This is then fed to the dividers section to produce the two center-frequency signals that are in quadrature.

Summary Build Steps

  • Install U8 (ignore if Si570 is CMOS)
  • Install U4
  • Install the 4 SMT Caps
  • Install SW1
  • Install the resistors (ignore R25 if Si570 is CMOS)
  • Install Sockets J1 and J2
  • Install C10
  • Install U3 socket, then plug in U3
  • Testing

Bill of Materials

Designation Component Type Qty Notes
C08 0.01 µF SMT 1206 1  
C09 0.01 µF SMT 1206 1  
C10 0.01 µF ceramic 1 code 103 
C11 0.01 µF SMT 1206 1  
C24 0.01 µF SMT 1206 1  
J1 2-pin socket socket for BPF board P1 1 Install before C10
J2 3 pin socket socket for BPF board P2 1 Install before C10
R01 1k Ohm Resistor 1% 1 hairpin (north-south)
R02 1k Ohm Resistor 1% 1 Hairpin (south - North)
R03 10k Ohm Resistor 1% 1 Hairpin (south - North)
R04 10k Ohm Resistor 1% 1 hairpin (north-south)
R05 10k Ohm Resistor 1% 1 hairpin (north-south)
R06 10k Ohm Resistor 1% 1 Hairpin (south - North)
R07 100 Ohm Resistor 1% 1 hairpin (east- west)
R08 100 Ohm Resistor 1% 1 hairpin (east- west)
R09 100 Ohm Resistor 1% 1 hairpin (east- west)
R10 100 Ohm Resistor 1% 1 hairpin (east- west)
R25 100 Ohm Resistor 1% 1 flat - ignore for CMOS SI570
SW1 4-pos dip switch 8 pin Dip Switch 1  
U3 12F683 CPU and socket 1 (top)
U4 Si570 LVDS Oscillator 1 (bottom)
U8 FIN1002 Diff LVDS Rcvr 1 (bottom)

Installation Notes

Bottomside Components

LO Board Bottom

(You may want to refer to the board bottom view)

Mount U8

. Note: if the kit is a CMOS SI570, then do NOT install U8. Take ESD precautions.  See the guide for mounting SMT ICs
  • (For the LVDS kit version only) mount U8 (see table below for orientation) Take great care mounting U8.  It is very tiny and mounts just below pins 4 and 5 of U4.  This is an area that is very ripe for solder bridges!
  • Carefully check the soldering of U8, especially around pin 2 (the ground connection) and pin 5 (VCC). You do not want to short 5Vdc to ground at that point!
  • A good post-soldering test on U8 is to check the resistance between U8 pin 1 and ground.  If it is low, you have a soldering problem.

Mount U4

  • Mount U4 (see table below for orientation) - note: author's kit was an LVDS version
  • Note: please note that U4 has 8 pins (7 and 8 are at either end of the chip).
  • Also note that you will be installing the IC on the bottom of the board such that the writing on the chip is upside down with respect to the top edge of the board.
Check Designation Component Orientation
U8* FIN1002  (code="FN02X") U8
  U4 Si570 LVDS U4

Mount the 4 SMT Capacitors

  • Mount .01 µF capson the underside of the board (see guide for mounting SMT Capacitors). )
  • Note that all are from the set of ten 0.01 µF caps (no 0.1 µF caps).
Check Designation Component
  C08 0.01 µF
  C09 0.01 µF
  C11 0.01 µF
  C24 0.01 µF

Topside Components

SI570 - CMOS vs. LVDS

You can identify the CMOS version of the Si570 by the presence of a "C" as the first part of the number following the string "Si570" on the top of the metal case. The LVDS version will not have the "C".
  • If the kit is a CMOS version:
    • Do NOT install R25 and U8
    • Instead, install a jumper wire between the hole of R25 and the hole connected to U8-Pin5.  You should use a very fine wire for the jumper, as the holes for it are quite small in diameter.  In addition, you should install the jumper wire as a topside component (i.e., do not try to install the wire to the bottomside - thanks to Pete N4ZR for this suggestion).  Reference the illustration below:

For a discussion of SI570's versions and the need for the FIN1002, see see the analysis by J. K. DE Marco PY2WM.

Topside Layout

LO Topside Layout

Mount SW1

  • Mount the 8 pin Dip Switch.  Orient the switch such that the numbers (1, 2, 3, 4) are closest to the right hand edge of the board and read, in their ordinal sequence, from top to bottom.1.
  • This switch provides a means to select one of 16 "center" frequencies, depending upon the settings for SW1.
Check Designation Component Orientation
  SW1 4-pos dip switch SW1 orientation

1 If you accidentally mount SW1 backwards (as the author did on the first go around), all is not lost. The switches will still work; you'll just have to remember that the MSB corresponds to switch #4 and the LSB corresponds to #1 when dialing in the frequency "nibbles".

Mount the 11 resistors

(see the guide to mounting resistors)
  • Mount the 10 hairpin resistors, R1 thru R10 (see table below for orientation)
  • (For the LVDS kit version only), Mount R25 (mounted flat). This step does not apply if the kit is the CMOS version.
Check Designation Component Orientation Notes        .
  R01 1k Ohm hairpin (north-south)
  R02 1k Ohm Hairpin (south - North)
  R03 10k Ohm Hairpin (south - North)
  R04 10k Ohm hairpin (north-south)
  R05 10k Ohm hairpin (north-south)
  R06 10k Ohm Hairpin (south - North)
  R07 100 Ohm hairpin (east- west)
  R08 100 Ohm hairpin (east- west)
  R09 100 Ohm hairpin (east- west)
  R10 100 Ohm hairpin (east- west)
  R25* 100 Ohm flat - ignore for CMOS SI570
*Note: if the kit is an SI570 CMOS kit, do NOT install R25

J1 and J2

Mount BPF Board Sockets, J1 and J2

Mount the 2-pin and 3-pin sockets on the topside (J1 and J2) with the ends going down through the holes in the main board(1) Mounting J1 and J2
  • Use the 9-pin header piece provided in the kit to align sockets J1 and J2 to ensure a good fit with the header pins on the BPF boards (see later instructions on BPF Boards).
  • Note: These need to be mounted before C10, so that this header pin alignment method can be used without C10 interfering with the "bridge" (see photo above).

Mount C10

Check Designation Component
C10 0.01 µF  (code 103)

Mount CMOS Jumper (if Si570 is CMOS version)

Check Designation Component
CMOS jumper wire (use cut-off resistor lead)

Mount the socket for U3

U3 Socket

Plug U3 into its socket

Check Designation Component Orientation
  U3 12F683 (topside)

Completed Stage Board


Completed Stage 2 Topside




Current Limited Power Test

  • Connect a 100 ohm resistor in series with the power line and apply 12 V dc power
  • the current should be relatively low (around 10 mA or less)
  • Measure the voltage WRT ground at the +5 V and at the 3.3 Vdc testpoints.
  • A voltage of around 2 V dc on each testpoint indicates the power rails are not shorted

Current Draw (DMM - 93 mA)


SW1/U3 Tests (DMM - 3.3 Vdc, 33 mVdc)

You can test the SW1 programming of U3 by checking the dc voltages at pins 1 thru 7. Pins 1 thru 3 should always be approximately 3.27 Vdc. Voltages at pins 4 thru 7 will vary, depending upon their corresponding dip switch settings (pins 4 thru 7 correspond to dip switches 1 thru 4):.

Frequency Test

  • Set the SW1 switches for 7.046 MHz, as above
  • Connect a clip lead to the left-hand lead of R11 (to act as an "antenna" for the Si570's signal)
  • Connect a wire to your transceiver's RX ANT jack lay it in proximity to the clip lead to loosely couple the signal
  • Power up the transceiver and tune it to 28.184 MHz
  • Power up the board. You should hear the signal on your transceiver.

LO Output (Scope/Freq Counter)

Testpoint for LO Output

The Local Oscillator should output a signal at the four times the center frequency selected by the switch settings of SW1. 


U3 U4 U8 Dividers Stage Power Supply Stage BandSwitch