LP Filters and Pre-Amp Introduction


This stage adds in the low pass filter bank and the infrastructure for switching in or out the 18 dB attenuator and/or the 15 dB RF preamplifier.

The two switches control the VRX to the two relays K2 and K3. K2 switches the attenuator pad (pi-network of one 220 ohm and two 68 ohm resistors) into or out of the RF chain; K3 switches the 2N3866 pre amplifier into or out of the RF chain.

(go directly to build notes)

LP Filters and Pre-Amp Schematic

LP Filters and Pre-Ampschematic

(go directly to build notes)

LP Filters and Pre-Amp Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

11K5 (1.5k) 1/4W 1%brn-grn-blk-brn-brn brn-grn-blk-brn-brn1/4W
2220 1/4W 1%red-red-blk-blk-brn red-red-blk-blk-brn1/4W
233 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W
1330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W
1390 1/4W 1%ora-whi-blk-blk-brn ora-whi-blk-blk-brn1/4W
1560 1/4W 1%grn-blu-blk-blk-brn grn-blu-blk-blk-brn1/4W
268.0 1/4W 1%blu-gry-blk-gld-brn blu-gry-blk-gld-brn1/4W
16R8 1/4W 1% (6.8)blu-gry-blk-silver-brn blu-gry-blk-silver-brn1/4W
3JRC27F/012 relayJRC27F/012 JRC27F/01212V relay
31N41481N4148 1N4148Axial
9100 nF104 104Ceramic
2220 pF 5%221 221Ceramic
256 pF 5%56JCeramic
2560 pF561Ceramic
147 uH molded inductor 10%yel-vio-blk-slv yel-vio-blk-slvChoke
1470 nH molded inductor (0.47 uH)yel-vio-slv_gld yel-vio-slv_gldChoke
2820 nH molder inductorgray-red-slvr-gld gray-red-slvr-gldChoke
2shunt wire (cut-off lead)Cutoff
4180 pF disc ceramic181 181Disc
122 pF disc ceramic22J 22JDisc
147 pF disc ceramic47J 47JDisc
1misc hookup wireHookup
1TO-39 heatsink Misc
12N3866 NPN Transistor TO-39
2T50-6yellow yellowToroid

LP Filters and Pre-Amp Summary Build Notes

LP Filters and Pre-Amp Detailed Build Notes

Top of the Board

LP Filters and Pre-Amp Top View

Remove the temporary Wire

Remove the wire that was used in the preceding phase to temporarily bridge the antenna signal into the mixers.

Install 2N3866 and heat sink

NOTE: the heat sink is factory supplied so tight that it would not be possible to place it over the 2N3866 after the transistor is soldered to the PCB. Therefore it is important to insert the 2N3866 into the heat sink first.

Prepare the heat sink by gently spreading its collar.

Place the transistor over the heat sink with the marking lip in the middle of the gap, push it down with the flat side of metal tweezers. The grip should be firm and tight (if it is too tight or loose, remove the transistor and adjust the collar)

The transistor and its heatsink will resemble this when ready for mounting

Place the 2N3866 into PCB (marking lip alignment as per photo). Solder it approx 4-5 mm (about 3/16") above PCB, just enough to clear any surrounding components.

It is important to solder this transistor sufficiently raised so that its metal case does not touch any of the plated through holes (won't work correctly if it does so touch).

TIP: Initially, solder one lead only. Then adjust for height and level, as needed, and then solder the other two leads.

misc6-1TO-39 heatsink Misc 
Q6-12N3866 NPN Transistor TO-39 

Wind and install L1 and L2

This transceiver requires exactly two wound toroids! L1 and L2, each wound on a T50-6 core with 0.8 mm (#20 AWG) wire, exactly as per photo.

Make sure to remove enamel coating and tin the leads before mounting. The length of wire required for each coil is about 13-14" (about 35 cm).

  • L1: 11 turns - 0.56 uH (approximately 10" of #20 wire)
  • L2: 12 turns - 0.64uH (approximately 11" of #20 wire)

Rule: Each pass through the center of the core (including the very first pass) counts as 1 turn. See the wb5rvz coil winding hints and tips for guidance on winding coils.

Wind and install L1 and L2 photo

Install Diodes, Resistors, and Chokes

Install Diodes, Resistors, and Chokes photo
D6-11N41481N4148 1N4148Axial 
D6-21N41481N4148 1N4148Axial 
D6-31N41481N4148 1N4148Axial 
L6-1470 nH molded inductor (0.47 uH)yel-vio-slv_gld yel-vio-slv_gldChoke 
L6-2820 nH molder inductorgray-red-slvr-gld gray-red-slvr-gldChoke 
L6-3820 nH molder inductorgray-red-slvr-gld gray-red-slvr-gldChoke 
L6-547 uH molded inductor 10%yel-vio-blk-slv yel-vio-blk-slvChoke 
R6-16R8 1/4W 1% (6.8)blu-gry-blk-silver-brn blu-gry-blk-silver-brn1/4W 
R6-233 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R6-333 1/4W 1%ora-ora-blk-gld-brn ora-ora-blk-gld-brn1/4W 
R6-468.0 1/4W 1%blu-gry-blk-gld-brn blu-gry-blk-gld-brn1/4W 
R6-568.0 1/4W 1%blu-gry-blk-gld-brn blu-gry-blk-gld-brn1/4W 
R6-6220 1/4W 1%red-red-blk-blk-brn red-red-blk-blk-brn1/4W 
R6-7220 1/4W 1%red-red-blk-blk-brn red-red-blk-blk-brn1/4W 
R6-8330 1/4W 1%ora-ora-blk-blk-brn ora-ora-blk-blk-brn1/4W 
R6-9390 1/4W 1%ora-whi-blk-blk-brn ora-whi-blk-blk-brn1/4W 
R6-10560 1/4W 1%grn-blu-blk-blk-brn grn-blu-blk-blk-brn1/4W 
R6-111K5 (1.5k) 1/4W 1%brn-grn-blk-brn-brn brn-grn-blk-brn-brn1/4W 

Install Capacitors

Install Capacitors photo
C6-1622 pF disc ceramic22J 22JDisc 
C6-1747 pF disc ceramic47J 47JDisc 
C6-156 pF 5%56JCeramic 
C6-256 pF 5%56JCeramic 
C6-18180 pF disc ceramic181 181Disc 
C6-19180 pF disc ceramic181 181Disc 
C6-20180 pF disc ceramic181 181Disc 
C6-21180 pF disc ceramic181 181Disc 
C6-3220 pF 5%221 221Ceramic 
C6-4220 pF 5%221 221Ceramic 
C6-5560 pF561Ceramic 
C6-6560 pF561Ceramic 
C6-10100 nF104 104Ceramic 
C6-11100 nF104 104Ceramic 
C6-12100 nF104 104Ceramic 
C6-13100 nF104 104Ceramic 
C6-14100 nF104 104Ceramic 
C6-15100 nF104 104Ceramic 
C6-7100 nF104 104Ceramic 
C6-8100 nF104 104Ceramic 
C6-9100 nF104 104Ceramic 

Install Relays and Jumpers

Install Relays and Jumpers photo
misc6-1shunt wire (cut-off lead)Cutoff 
misc6-2shunt wire (cut-off lead)Cutoff 
K1JRC27F/012 relayJRC27F/012 JRC27F/01212V relay 
K2JRC27F/012 relayJRC27F/012 JRC27F/01212V relay 
K3JRC27F/012 relayJRC27F/012 JRC27F/01212V relay 

Install Wires

Install the (permanent) W3 wire. Preferrably on the underside of the board, this wire connects the control of relay K2 to the mount point for the Attenuator Switch, allowing the Attenuator switch to switch VRX in or out of the relay.

You can insert left-over component lead snippings or other wire snippets into the holes at either end of the wire (as markers, from the topside), then turn the board over and connect the permanent wire to each hole through which you inserted the "markers".

Install Wires photo
W3misc hookup wireHookup 

LP Filters and Pre-Amp Testing

Coil Continuity

Test Setup

Test the continuity of the coils' windings by probing the matching (A-to-A, B-to-B) test points with your ohmmeter.

Coil Continuity

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
L1: A-to-Aohms0tbd_______
L2: B-to-Bohms0tbd_______

Voltage Test

Test Setup

Tack-solder wires W1 and W2 (including S1 and S2) temporarily for purposes of this test.

Refer to Final Board Layout Diagram for final wiring configuration.

Test for 2.2 Vdc with S2 (Pre-Amp) on and then Off

S1(Attenuator) Should turn on relay K2

Voltage Test

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Junction of 33 and 6.8 ohm resistors: S2 ONVdc2.2tbd_______
Junction of 33 and 6.8 ohm resistors: S2 OFFVdc0tbd_______

Attenuator Test

Test Setup


To test attenuator preamplifier:

  • Turn S1 (attenuator) switch OFF
  • Turn S2 (pre-amp) switch OFF
  • Power on the board and connect I/Q out to PC's stereo Line-In (or "mic") jack
  • Tune the SDR PC program to a 40m signal (or connect a signal generator)
  • Observe signal strength
  • Turn S1 (attenuator) switch ON
  • Turn S2 (pre-amp) switch OFF
  • Observe signal strength - signal will decrease for approximately 17dB

Pre-Amp Test

Test Setup

With power still on and signal present: in SDR Software

  • Turn S1 (attenuator) switch OFF
  • Turn S2 (pre-amp) switch OFF
  • Observe signal strength
  • Turn S2 (pre-amp) switch ON
  • Observe signal strength - you should see an improvement of 15dB