The PY2OHH pages contain a useful link to a TI paper on active filters. In my experience, virtually any audio or RF transistor (e.g. 2N3904 NPN) can replace the op-amps with little effect. Some prefer "perfect" op-amps whereas I find transistors easier to work with and place. The performance difference is very very small.
2N3906 is a ubiquitous PNP equivalent. The 2N3904 and 2N3906 devices are available from a range of suppliers for just a few pence each only. A few 2N3904s in parallel with small emitter resistors make a good HF/LF QRP PA replacement for a 2N3866 too. My recommendation is buy 100 of each (2N3904 and 2N3906) and stock your junk-box. Less than £10 and it will be a long time before you run short! Think of all those projects.
See http://py2ohh.w2c.com.br/trx/filtroativo/filter%20design%20in%20thirty%20seconds.pdf for the active AF filter paper.
Showing posts with label 2n3904. Show all posts
Showing posts with label 2n3904. Show all posts
14 Apr 2014
27 Jul 2012
The Lesser Chirpy 10m CW transceiver
SPRAT 151 had my article on Chirpy, an ultra-simple, very basic crystal controlled CW transceiver for 28MHz. This does work and has even managed a few decent QSOs but, boy, does it chirp! So, today I followed up on an idea I had to reduce the chirp by FSKing the oscillator when on TX. The schematic is attached.
The basic idea is to switch to TX by closing S1 (increases the oscillator output to around 150mW) then key a capacitor in series with the crystal. As the oscillator is on at full power with the key off or on (FSK shift) the chirp goes. There are a couple of disadvantages (a) the rig is no longer full break-in, and (b) when the key is up there is the "opposite" morse about 1kHz up the band, which is wasteful of power and may confuse. Still, this is a "for fun" rig and, with virtually no chirp, a much better radio. As before, a small low pass filter on the output is advisable unless your ATU provides this.
I also adjusted the turns on the main collector inductor finding that a tap 3t from the cold end worked better. You will almost certainly have to experiment with the values of capacitance around the crystal and key to get the right amount of TX-RX offset.
UPDATE 29.7.12: corrected the circuit diagram (crystal going to wrong place!)
The basic idea is to switch to TX by closing S1 (increases the oscillator output to around 150mW) then key a capacitor in series with the crystal. As the oscillator is on at full power with the key off or on (FSK shift) the chirp goes. There are a couple of disadvantages (a) the rig is no longer full break-in, and (b) when the key is up there is the "opposite" morse about 1kHz up the band, which is wasteful of power and may confuse. Still, this is a "for fun" rig and, with virtually no chirp, a much better radio. As before, a small low pass filter on the output is advisable unless your ATU provides this.
I also adjusted the turns on the main collector inductor finding that a tap 3t from the cold end worked better. You will almost certainly have to experiment with the values of capacitance around the crystal and key to get the right amount of TX-RX offset.
UPDATE 29.7.12: corrected the circuit diagram (crystal going to wrong place!)
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