The G2NJ Trophy is sometimes awarded for a really good technical article, and sometimes for an outstanding contribution to international QRP. This year’s winner described a Cross-coupled Double Balanced Product Detector in SPRAT 191. The winner is Cor, PA3COR” Steve G0FUW, Sprat 194
Needless to say it was a complete surprise! I feel very honored with this! I looked up who has been awarded this trophy in recent years and then I felt completely hunbled! I can hardly believe it…
Updated : January 25th; Input amplifier Updated : January 26th; Audio filter
A couple of days ago I found the circuit of 20m DC PSK31 receiver at the website of va3iul. The website of va3iul is probably well known in the radio amateur community. It features 100’s of circuit design ideas on all kind of topics related to RF design : oscillators, noise sources, filters, phase shifters, it is all there.
Now my google search came up with an image of a 20m PSK31 receiver designed by DG2XK hosted by va3iul. it didn’t state in which magazine it was originally published and it did not show the component values.
The circuit below shows an active low pass filter that is universally applicable. Only 4 equal (!) resistors needs to be changed to change the filter corner frequency fc. It uses only standard parts, is suitable for a large supply voltage range and needs only a modest 2mA of supply current.
R2, R3 and R4 set the input bias voltage at half the supply voltage. R5-R8 and C3-C6 are the low pass filter network. C3, C4, C5 are bootstrapped to provide a sharper roll-off. Q1 and Q2 are a series-feedback pair and function as a ‘super’ emitter-follower. A standard emitter follower has a gain of ≈ 0.90 … 0.95x. This ‘super’ emitter follower has a gain of ≈ 0.99x. Just like an OpAmp with feedback, in this series feedback amplifier, the feedback increases the input impedance, lowers the output impedance, stabilizes gain and lowers distortion. Not bad for a single €0.10 transistor that is added!
You cannot get it more simple than this, a 1 transistor CW filter. This filter is build according to the (surprise !) Multiple Feedback BandPass Filter (MFBPF) topology. Below, you see the circuit, originally designed by KE3IJ (alas now SK).
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