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AF7KR FT8 monitoring receiver

1 year ago 2291
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The project is a receiver for FT8 or WSPR monitoring on a single band. It can be used as a base for other applications. Design goals:

  1. Low noise floor
  2. Mono output for cheap soundcards, no SDR software involved, direct feed into the FT8 software (WSJT-X, JTDX)
  3. Low cost. I plan to have 8-10 of these running on all HF bands in parallel. High dynamic range is not one of the goals, but a nice to have. Si5351A used as LO and BFO. I use Arduino Pro Mini to control an Adafruit Si5351A board, but it can be something else. The sketch programs the synthesizer and goes into powerdown mode to reduce the noise. I have 2 prototypes running now on 30 and 40m bands. X1-X4 are 44.545MHz 3rd overtone crystals, 14.848MHz fundamental frequency. This is what I happened to have in quantity allowing me to match the sets. Other frequencies above 10MHz can be used, but C5-C14 need to be changed to get the desired bandwidth. LPF inductors are wound on T37-6 or T37-2 cores. Transformers can be on either FT37-43 or BN43-2402 cores. The first mixer is a very simple DG MOSFET design. Maybe not optimal from the overload point of view, but easy to couple to the BPF. I expect it to have fairly low noise and good gain. Plan to do measurements to confirm this. Impedance matching to the crystal filter is by a 7T:2T transformer. IF amplifier is designed for 200 Ohm input impedance and is termination insensitive. This allows to avoid an extra matching transformer. The product mixer is KISS by Chris N7ZWY, with the switching path DC biased at 0.7V by the AF amplifier input. This voltage is far enough from the rails to yield low switching resistance. 3.3V power is taken from the regulator on the Si5351A board. AF amplifier is my own design, with 50 Ohm input impedance and decent gain, about 50dB. It doesn't have a good PSRR, so the power is stabilized by a 78L05.

Update 07-25-2019 : Got the boards from JLCPCB. Assembled receiver is now working on 17m. Added C27 and C28 around the AF amplifier to reduce the noise from rectified parasitic RF, and a reverse polarity protection diode. Also added some more prototype area to the PCB. It can hold a Si5351A board, or the final AF amplifier for headphones or speaker. Sensitivity is good, getting the same FT8 decodes as from my FT-817. Maybe a bit too much gain, I have to turn the soundcard input gain all the way down. Update 10-01-2019: Assembled 8 boards for all bands except 10 and 12m. 5 are online, sending data to Each has its own Si5351a and Arduino Pro Mini. 20m board has 10.240MHz crystals. 160m board has 11.059MHz low-profile (HC-49S) crystals. Some boards use 6-crystal QER filter topology, which has better bandpass ripple and attenuation.



FT8 RX v2

FT8 RX v3


ID Name Designator Footprint Quantity
1 FSA3157P6X U1 SOT-23-6 1
2 50:2k T1 BN43-2402 1
3 100p C3,C2,C9 1206 3
4 2p2 C1 1206 1
5 0.8u L1 FT37_INDUCTOR 1
6 XFMR T2 BN43-2402 1
7 BF998 VT1 SOT143B 1
8 2k2 R1,R2,R4 RES_1206 3
9 4k7 R3,R13 RES_1206 2
10 10n C4,C26 1206 2
11 LO P1 HDR-1X2/2.54 1
12 ANT P2 HDR-1X2/2.54 1
13 12MHz X1,X2,X3,X4 OSC-49S-1 4
14 47p C5,C13 1206 2
15 DNI C6,C8,C10,C12,C14 1206 5
16 68p C7,C11 1206 2
17 2N3904 Q5,Q1 TO-92(TO-92-3) 2
18 18k R9 RES_1206 1
19 39k R10 RES_1206 1
20 1k R11 RES_1206 1
21 470 R12,R6 RES_1206 2
22 2N3906 Q4,Q2 TO-92(TO-92-3) 2
23 3k3 R5 RES_1206 1
24 680 R7 RES_1206 1
25 BFO P3 HDR-1X2/2.54 1
26 +9V P4 HDR-1X2/2.54 1
27 78L05ML-T92-B U2 TO-92_3PIN-L4.95-W3.94-PITCH2.54-L 1
28 3V3 P5 HDR-1X2/2.54 1
29 56 R8 RES_1206 1
30 100n C16,C17,C18,C19,C21,C22,C27,C28 1206 8
31 39 R14 RES_1206 1
32 22u C20 CAP-AXL-D6.3XF5.0 1
33 4n7 C15 1206 1
34 1u C23,C24,C25 1206 3
35 100 R15 RES_1206 1
36 AF OUT P6 HDR-1X2/2.54 1
38 SMA Connector J1 CON-SMA-EDGE031 1
39 1N4148 D1 DO-35 1




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