Theory of Operation

This stage lets the SDR filter out the RF spectrum arriving at the antenna into a "chunk" of the RF spectrum corresponding to the desired band(s). This is filtering "in the large", and is designed to minimize interference/harmonics from very strong, out-of-band signals.

There are four separate, pluggable boards which can be built to provide BPF functionality over the range from 160m to 10m.

Mike KF4BQ has conducted tests on the BPF boards to determine the frequency boundaries of these "chunks" (the passbands) of RF spectrum. You can view the results here.

Note: the pluggable bandpass filters may be replaced by the new switchable HF BPF board kit, which implements 4 switchable BPFs on a single board, which can be manually switched or switched via USB control.


This is a subset of the overall schematic. 07 Band Pass Filters (BPF) Stage Built BPF vs a Dime

Bill of Materials

Band Specufuc Values for BPFs
BPF-1160m board
P100-12 pin
P101-13 pin
C100-1390 pF391ceramic
C101-15600 pF562ceramic
T100-11.4 uHT30-2 (red)18T/9T bifilar #30 (10"/5")
L100-118.7 uHT30-2 (red)66T #30 (32")
BPF-280/40m board
P100-22 pin
P101-23 pin
C100-2560 pF561ceramic
C101-2680 pF681ceramic
T100-21.2 uHT25-2 (red)18T/9T bifilar #30 (10"/5")
L100-21.6 uHT25-2 (red)22T #30 (11")
BPF-330/20/17m board
P100-32 pin
P101-33 pin
C100-3180 pF181ceramic
C101-3220 pF221ceramic
T100-30.6 uHT25-6 (yellow)14T/7T bifilar #30 (8"/5")
L100-30.78 uHT25-6 (yellow)17T #30 (9")
BPF-415/12/10m board
P100-42 pin
P101-43 pin
C100-482 pF82ceramic
C101-4330 pF331ceramic
T100-40.13 uHT25-6 (yellow)7T/4T bifilar #30 (5"/4")
L100-40.53 uHT25-6 (yellow)14T #30 (8")

Summary Build Notes

  • Cut the provided board into 4 BPF boards.
  • For each desired band pass filter board (BPF-#):
    • Install pins to board bottom
    • Install ceramic caps to topside
    • Wind and install inductors
    • Test the stage

Detailed Build Notes

There are four bandpass filters (BPFs) you can build, each on its own board with 2 caps, a coil, a transformer, and two sockets for plugging it into the main board. The Bill of Materials above provides you with the parts list for each board. You only need to build one BPF to test out your receiver capability. It is recommended - especially if you are inexperienced in winding coils and toroids - to begin with a BPF for the band you are least interested in (just to get the practice in a non-threatening fashion).

Saw The Boards

Cutting BPF boards

The BPF filter boards are in a strip of four boards and will require the kit builder to hacksaw between the boards to separate the individual BPF boads. It is suggested to use a small plastic miter box and a fine-toothed blade (24 tpi or better) to help cut perpendicularly across the 0.65 inch wide strip. This seems to work well. However, please note the safety warnings on the Softrock reflector (message 23126) concerning the danger in inhaling the dust resulting from sawing.

Winding Inductors

To learn how to wind coils and transformers, please read the Common Component Mounting Instructions - Toroidal Inductors to instruct you in the techniques required for this task.

Concernimg the number of turns in the windings, David WW2R has reported that he had to adjust the number of windings on L100-1 (the 66 turn coil on the 160m band) because of the fact that the toroid was not able to accept 66 turns as a single layer, without winding back over some of the existing winding. Overlapping turns caused him to need 69 turns to reach the required inductance of 18.7 uH.

Pete N4ZR chimed in on this subject, too, adding: "The 160-meter L100 requires 66 turns, but only about 40-45 turns will fit on the core in a single layer. You need to keep winding in the same direction in a second layer until you complete the 66-69 turns. I wound 69 originally, but on checking with my MFJ-259, which may not be very accurate the inductance appeared to be a little high.

When winding bifilar windings, it is a lot easier to wind the bifilar winding if you fold the wire in half but don't cut, and use the folded (closed) end (with or without a sewing needle) to feed through the toroid or binocular core.

Wire Lengths

Refer to the BOM above to see the recommended length of wire (in inches) for each inductor.  These lengths include generous SWAGS to accomodate lead lengths, etc..  These were determined using  DL5WWB's calculator (adding an inch or so to the resultant length, just for good measure.

When the BOM states BPF-80/40: 18T/9T bifilar #30 (10"/5")  this means:

  • Primary: 18 turns of #30, using 10" for the single winding.
  • Secondaries: 9 turns of #30, using a 10" length of wire and fold it over at the 5" point, twisting it together into a bifilar strand, winding it evenly distributed over the primary winding for 9 turns.  The bifilar strand should be about two-three twists per inch.

Core Sizes

:  The chart below provides the capacitance values and the winding instructions by band group.   Carefully note that some bands use different size and color cores.  Be sure to use the right core for the board you are building:
  1. 160 m: T30-2 (red)
  2. 80/40m: T25-2 (red)
  3. 30/20/17/15/12/10m : T25-6 (yellow)
Core sizes bandSpecificValues_BPFs

For Each BPF Board

Topside of BPF Board

(referring to the Band Specific Values chart, above):

Horizontal Mounting of T100
Horizontal Mounting of L100

Build Steps for each BPF Board
Check Designation Type Notes
L100-# Coil Wind, prepare, horizontally mount, and solder the coil, L100, using the correct core size and color and turn count..
  • Carefully count the turns1. Each pass thru the center is 1 turn.
  • Leave approximately 1/2 inch for each lead.
  • Use an emery cloth to scrape the insulation off the leads up to the last 1/8 inch.
  • Pull the leads through the holes directly above the circle for L100 on the BPF board (marked in yellow above).
  • Flatten the core horizontally, pull the leads snug, bend them on the bottom side of the board, and solder the leads.
  • Test for continuity (~0 ohms) from the lower hole of C100 through the coil to the lower hole of C101.
  • If there is no continuity, check soldering of the leads and resolder as necessary.
T100-# Transformer
Wind, prepare, horizontally mount, and solder the transformer, T100
  • Transformer T100-# will be mounted horizontally and raised above the board about 1/16 of an inch. In winding T100-#, first wind the primary winding with enameled wire so that the primary winding starts and ends at about the same point on the core and is uniformly spread around the core.
  • Twist two pieces of enameled wire together (bifilar) at about 3 twists per inch and wind the secondary windings with the windings starting and ending where the primary winding starts and ends. When you have wound the transformer, you will have 6 leads, 3 (one primary, one secondary 1, and one secondary 2) on each side of the core.
  • When trimming the wires, recognize that the 3 leads coming from one side of the core may need to be a little longer than those from the other side (to facilitate mounting the transformer horizontally.
  • Insert the leads, following the annotations on the BPF board above:
    • "P" represents the primary leads on each side of the core;
    • "S1" represents the leads for the first secondary winding on each side;
    • "S2" represents the leads for the second secondary winding on each side.
  • Test for continuity on the two primary leads ("P" in the image above) by putting your ohmmeter leads on the two holes for C101. If you do not have continuity, then you likely have a soldering issue on the primary leads.
  • Test for continuity between either of the primary leads and each of the secondary leads. You should see an open circuit.
  • If you do get continuity, look for a short in the transformer or in its solder joints.
  • Test for continuity between pins 2 and 3 of P101. You should get continuity.
  • If you do not get continuity, one or more of your secondary leads has a solder problem.
C100-# ceramic capacitor Mount and solder the capacitor, C100
C101-# ceramic capacitor Mount and solder the capacitor, C101
P100-# 2 pin header Mount and solder the 2-pin header, P100, on the underside of the board, with the shorter pins going through the holes from the bottomside to the topside and the longer pins extending out from the bottom side to mate with the main board (2).
P101-# 3-pin header Mount and solder the 3-pin header, P101, on the underside of the board, with the shorter pins going through the holes from the bottomside to the topside and the longer pins extending out from the bottom side to mate with the main board. (2)

1 The L-100 for the 160m BPF will require overlapping the windings in order to fit all of them on the toroid. The first layer pretty well fills up after 45 or so turns.

2 The BPF board connectors (P100 and P101 headers) are mounted, short ends into the holes for P100-# and P101-#, on the bottom of the board with the other components on top.
Use the main board 9-pin socket (J1) as a "tool" to align the pin headers on each BPF board so that the two will mate properly.

Completed Board (80/40m)

Completed Stage 5 Topside



Test T100 Primary Resistance

  • With the BPF board NOT mounted to the main board and using your ohmmeter, measure the resistance from The C100 hole farthest away from P100 to ANT Return.  It should be ~0 ohms, indicating continuity in the primary windings of T100, through the L100 windings.
  • If you get any appreciable resistance or an open circuit, you should inspect/touch up the solder joints on T100 primary and/or L100.

Test T100 Secondaries Resistance

  • With the BPF board NOT mounted to the main board and using your ohmmeter, measure the resistance between pins 2 and 3 of P101 on the BPF board (which correspond, respectively, to pins 8 and 9 of J1 on the main board) - see the two testpoints circled in the image above..
    It should be ~0 ohms, indicating continuity between the ends of the two secondary windings and through the center tap.
  • If you get any resistance or an open circuit, you should inspect and/or touch up the solder joints.
  • Note: that the two secondaries are center-tapped so both windings are "connected" continuously in the circuit from pin 2 to pin 3.
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