First Mixer, Post-mixer Amplifier, Crystal Filters


To learn about the First Mixer, read the
Circuit Details - First Mixer & Amplifier
before building this section.

To learn about the Crystal Filter, read the
Circuit Details - Crystal Filter
before building this section.


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Approximate time to complete 1:30 - Picture | Picture | Picture

SWL Modifications

Only one crystal and 4 100pf capacitors are used in each of the 3.547MHz and the 4.000MHz crystal filters. A jumper wire is used to bypass the area that is left out of the filters.

The IR LED that normally points to the VFO amplifiers is installed pointed straight up as the Crystal Filter to VFO IR path is not used in the SWL receiver.

If a good outside SWL antenna is going to be used, the 100K resistors at Gate 1 of the Mixer MOSFETS should be replaced by 470 ohm resistors. This modification increases the dynamic range and stability of the first mixer. See image at top of page.

Preparation

This section uses the remaining FT50A-75 ferrite cores (Black colored, Bag 4). Click on the link above for pictures and text on winding the coils for the mixer.

The center transformer of the mixer is only a bifilar coil. The coils at each end of the mixer are bifilar with a 5 turn secondary.

Insert all the components that have their values inside the footprint. They are the following:

____4 - RF Chokes (Bag 3), Footprint on PCB is rectangle with square edges ,"RF" inside the rectangle. Choke is about the size of a 1 watt resistor with rounded edges. Two are located next to the First Mixer and Amplifier and the other two are at the input to the crystal filters.

Please Note!

Correction: DO NOT install the RF Choke next to the "12 Volt" box above the VN0106N3 switching circuitry on this board. Install a jumper wire in place of the RF Choke. See the picture below:

Installation of this choke will cause the First Mixer to oscillate.

Solder

____25 - .01 capacitors (Bag 3)

Solder

____6 - 100 ohm resistors (brown, black, brown) (Bag 3)
____8 - 100K resistors (brown, black, yellow) (Bag 3)

Note: If a good antenna is going to be used with this receiver, use 470 ohm resistors at Gate 1 of the Mixer MOSFET instead of the 100K shown. See image at top of page.

Solder

Resistors:

____3 - 470 ohm (Bag 4), One is located at the top left of the first mixer, and the other two are located to the top and right of the IR LEDs.
____2 - 1K (Bag 3), Located at the top and bottom of the input to the crystal filters.
____1 - 2.2K (Bag 4), Located to the left of the middle bifilar Ferrite coil in the mixer.

Capacitors (Going left to right on the board):

____4 - 100pf NPO caps (Bag 3), Orange color, short leads flared out, labeled "101". All located inside the crystal filters. Only the input and output capacitors are installed. Two are installed right before the first crystal footprint and two are installed just after the third crystal and the output T4-6T transformer in each filter. The four capacitors inside the filter are not installed. Picture

Solder

Other Parts:

____6 - LEDs (Bag 3), Red colored, match the flat on the LED to the flat on the footprint. The short lead is on the same side as the flat.

Note: If you ordered the Super Bright LEDs, you can place two of them at the input of the crystal filter, next to the IR LEDs. Use different colors to help crystal filter identification. If you ordered the Bright Red LEDs, they can replace the ones in the mixer. See Modifying the LEDs.

Solder

Picture

____2 - IR LED (Bag 4), Smoky colored, on a cardboard strip, do not cut the leads off the strip, tear the IR LEDs off the strip and clean the bottom of the leads. Located at the input to the crystal filters, right next to 100pf caps. The short lead is on the same side as the flat. Mount as high as possible using the full lead length.

The one on the left side is installed but not used, just aim this IR LED straight up.

The IR LED near the outside of the board (right side) points to the phototransistor at the Crystal Oscillator on Board 2. Wait till you mount the boards to aim this IR LED.

Solder

____2 - VN0106N3 (Bag 2), static sensitive part, touch a ground lead before removing the part from the bag. Match the flat on the part to the flat on the footprint. Spread the outside leads slightly to fit the footprint. Marked "SI N, 0106, 8302".

Solder

____1 - 3.457MHz crystal (Bag 4), Has a gray plastic cover over the crystal with the markings "NYMPH C O 17915-03". Only one crystal is used in the Crystal Filter Section. Solder the one crystal in the first Crystal Footprint at the bottom of the filter at the input. Picture
____1 - 4.000MHz crystal (Bag 4), The markings are "4000 KSS 2FT". Only one crystal is used in the Crystal Filter Section. Solder the one crystal in the first Crystal Footprint at the bottom of the filter at the input. Picture
____1 - 1 1/2" of Hookup Wire, Solder a piece of hookup wire between the hole in the 100pf capacitor footprint to the hole in the third crystal footprint that is closest to the output in the 3.547 MHz crystal filter. Picture
____1 - 1 1/2" of Hookup Wire, Solder a piece of hookup wire between the hole in the 100pf capacitor footprint to the hole in the third crystal footprint that is closest to the output in the 4.000 MHz crystal filter. Picture

Solder

____1 - T4-6T Minicircuits 1:4 transformer (Bag 4), Note the dot on the part and match it to the dot on the footprint. A dot is outside the footprint so that you can double check your work after it is mounted. Located at the output of the crystal filter.

Solder

____2 - FT50A-75 Ferrite Cores, enamel wire (Bag 4), Cores were wound with a Bifilar winding and a 5 Turn winding in the "Preparation" section. These are located at the ends of the Mixer/Amplifier. Be careful to get the windings soldered on the board correctly according to the footprint. The lead that has two wires twisted together goes in the middle hole.

Solder

____1 - FT50A-75 Ferrite Core, enamel wire (Bag 4), Core was wound with a Bifilar winding in the "Preparation" section. This core goes in the middle of the Mixer/Amplifier.

Solder

Please Check!

Carefully inspect the wires on the bifilar side of the cores and make sure they are not shorted. There is no ohm meter check for this error, so you must look carefully. Make sure the bare parts of the wires are not touching!

Double check underneath the board to make sure there is not a solder bridge between the leads.

The second and third core (counting from the front) carry 12 Volts. If the bifilar leads are shorted there will be some burning (smoke) of the wires, the LEDs will not light up, and the RF Choke that feeds 12 Volts to that core will need to be replaced.

____4 - MOSFETs (Bag 2), (Picture) static sensitive part, touch a ground wire before taking it out of the bag, notice the dot on MOSFET (may be difficult to see, hold at an angle to a light source and you can see the shadow of the dot), the dot is located to the left of the second line of the text on the MOSFET.

A dot is placed outside the footprint so that you can double check your placement after it has been soldered to the PCB. The leads on each side of the part need to spread apart slightly to fit the footprint.

Solder

____1 - SPDT Miniature PC Mount switch (Bag 5), There is a switch footprint in front of the First Mixer. There are two hole patterns. Mount the switch in the holes that work. Picture

____Crystal Filters (Picture) - Between the VN0106N3's, there is a BOLD circle that is labeled "Gnd for 30/17" and marked "CF". Solder a 7 1/2" length of wire to the circle and to the terminal marked "To CF" on the Crystal Filter switch.

____Miniature Coax Cable, cut a 12 1/2" piece (Bag 6), The coax is used between the VFO OUT in the upper left hand side of the board, to the VFO IN at the lower right hand side of the board.

The coax supplied with the kit is Teflon 75 ohm cable. The best way to strip the insulation is to get a very sharp utility knife and slice a 1" section length wise, then fold over the cable and pull the shielding/center out of the insulation.

Pull back the shielding slightly to loosen, make a small hole at the bottom, and pull through the center wire.

Since it is Teflon, there is no danger of burning up the insulation on the center wire and causing a short while soldering.

Pictures and Instructions for using the Teflon coax.

Best to mount this jumper underneath the board for a cleaner appearance and avoid interfering with the IR switching at the bandpass filters.

When soldering underneath the board, solder to the traces and the ground plane. Saturate the ground braid with solder first, then solder the braid to the ground plane. Next solder the center wire to the output(VFO OUT)/input(VFO IN) pads. Pictures

____Miniature Coax Cable, cut a 10" piece (Bag 6). Strip and prepare as explained above.

____Solder the center wire to "XTAL FILTER OUT", solder the braid to the "Ground" box. Leave the other end loose at this time.

Double Checking Your Work

____Check the dot on the Minicircuits T4-6T transformer.
____Check the primary/secondary on the ferrite cores in the mixer/amplifier.
____Check the dot on the MOSFETs.
____Check the flats on the LEDs and make sure they fit the flats on the footprint.
____Check the flats on the VN0106N3s, they should both be facing to the right.
____Place the PCB in front of a bright light. If you see light shining through any of the soldering holes, you missed a solder connection.
____Look at all the 100 ohm (brown, black, brown) resistors and check the size of the footprint underneath the resistor. It should be shorter than the length of the resistor body.
____Look at all the 100K (brown, black, yellow) resistors and check that the size of the footprint is as long as the resistor body. The yellow band is a dead giveaway to find these resistors.

____1 - 5 section tuning capacitor (Bag 1), Note the double holes inside the footprint of the tuning capacitor: this is where the mounting legs of the capacitor should lie. Tin the mounting legs of the capacitor before soldering to the board to get a good solder flow between the ground plane of the PCB and the capacitor.

Take the capacitor and put the soldering tabs into the holes on the PCB footprint. Then roll the capacitor onto the ground plane and solder in place. This will automatically mount the capacitor straight and leave plenty of room for soldering to the tabs.

Be careful, as the capacitor begins to lie flat, the tabs come out of the holes in the PCB.

After the capacitor has been mounted to the board, solder the 39pf capacitor to the middle tab, and the 47pf to the back tab of the capacitor (where the 10pf normally goes), as shown on the silkscreen.

____1 - Large Knob (Bag 6), Mount on tuning capacitor using small screwdriver.

Please note: When mounting the capacitor on a ground plane, without the holes for the soldering tabs, the tabs will be forcefully grounded on the ground plane because their length is as long as the mounting legs.

Bend the soldering tabs up and against the stator connections to keep from grounding the stator sections, when mounting on PCB ground planes other than the receiver's PCB. Picture


You can test Board 1 now, then build and test Board 2, or build both boards, then test both after you finish.

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Last Update: 7/14/08
Web Author: David White, WN5Y Free Website Translator