NE602 40m DC Receiver

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.

20m PSK31 DC receiver by DK2SK
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Simple remote numeric keypad with ATmega88

Numeric keypad for radio control
Numeric keypad for radio control
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Easy to Customize Universal Low Pass Filter

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!

PA3COR - Universal RC Low Pass Filter fc = 3.3KHz
PA3COR – Universal RC Low Pass Filter fc = 3.3KHz
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1 transistor CW – filter

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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40m DC Receiver – CW Filter

So after studying some designs of multiple feedback band-pass filters for CW by other designers (see my previous post), I’ve decided to try my hand at designing and building one myself. Actually, I’ve made two different versions. The first one is designed by myself and is a two stage filter with an fc of 750 Hz. The second one is the filter designed by Tom Hall GM3HBT and published in Ham Radio Today 1987. I selected this filter because the gain ( ~1x), the center frequency ( ~750 Hz) and the Q / bandwidth ( ~5x / 100Hz).

DesignStagesR1
[ kΩ ]
R2
[ kΩ ]
R3
[ kΩ ]
C1=C2
[ nF ]
Gain
[ ]
fc
[ Hz ]
Q
[ ]
BW
[ Hz ]
13. GM3HBT2-3390128202.21.17404.20176
PA3COR21102.1220101.07475.17145
Overview of MFBP CW filter found in radio literature – PA3COR.nl

Continue reading 40m DC Receiver – CW Filter