“Chinese Pixie” build and LPF LTspice model

I bought a few 40m “Chinese Pixie” (known as such because you can buy them from China and they come with no identified designer or manufacturer) transceiver kits on AliExpress recently and put one of them together this week. To my surprise, it worked! I’ve been having a bit of bad luck on homebrewing lately, so I wanted to step back and make sure I could at least put something like this together. This exercise has given me some new ideas.

Photo of a Pixie transceiver kit that I assembled. Lots of through-hole parts on a small circuit board. I replaced the supplied barrel power connector with a 9V battery connector that you can’t see the end of.

Even managed to send enough of a CQ on the straight key with no sidetone that RBN picked me up. Seems like a win with 9V!

Screenshot of RBN showing a CQ sent by W1CDN, picked up by K3PA in eastern Kansas. About 616 miles on <1W.

A known problem with this design is the lack of ability to block AM broadcast frequencies (530 kHz – 1700 kHz). I plugged in the headphones last night and caught several CW signals as well as a sports announcer! I didn’t stick around to find out what station, but this indicates some better filtering is required. Maybe I’ll build a band-pass filter for 40m to replace the LPF in place.

Here are the LTspice schematic and the frequency response for the filter it came with:

Screenshot of LTspice showing an AC signal on the left, a low-pass filter in the middle, and a 50-ohm resistance on the right.
Screenshot of the LTspice frequency response curve for the above schematic, from 5 MHz to 10 MHz. It shows a pretty flat response up to around 8 MHz, then drops pretty gradually to 10 MHz. The magnitude of the drop is not great (about -6 dB to -7.8 dB).

Because this is a pretty small transceiver with low power, the supplied inductors are all inline parts rather than toroids. I need to dig out my inductor collection and see what I have to work with, before I can swap in something else. I’ll probably start with N6QW’s BPF design and see if I can get away with fewer/smaller parts than what he has.

Here is the LTspice model for the LPF shown above:

Version 4
SHEET 1 880 680
WIRE 16 368 -48 368
WIRE 176 368 80 368
WIRE 240 368 176 368
WIRE 416 368 320 368
WIRE 608 368 416 368
WIRE -48 400 -48 368
WIRE 176 416 176 368
WIRE 176 416 144 416
WIRE 416 416 416 368
WIRE 416 416 400 416
WIRE 608 448 608 368
WIRE -48 544 -48 480
FLAG -48 544 0
FLAG 608 528 0
FLAG 400 480 0
FLAG 144 480 0
SYMBOL voltage -48 384 R0
WINDOW 3 24 140 Left 2
WINDOW 123 24 118 Left 2
WINDOW 39 25 165 Left 2
SYMATTR Value 1
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=50
SYMATTR InstName V1
SYMBOL Res 592 432 R0
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL cap 80 352 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL cap 160 480 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C4
SYMATTR Value 470p
SYMBOL cap 416 480 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C5
SYMATTR Value 470p
SYMBOL ind 336 352 R90
WINDOW 0 5 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName L1
SYMATTR Value 1µ
TEXT 0 624 Left 2 !.ac dec 1000 5meg 10.5meg

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