The R2 Single Sideband Direct Conversion Receiver

Posted on Feb 6, 2014

The old common-base audio preamp, having a noise figure of 5dB is an obvious target to improve the noise figure. A common-emitter configuration, with shunt feedback gives 50 ohms input resistance (preferred by the mixer) and a 2dB noise figure. Two paralleled 2N4401 transistors results in better noise performance at such a low input impedance. A g

The R2 Single Sideband Direct Conversion Receiver
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ain of 60 masks most of the noise in the op-amp phase shifter that follows. Common collector ouput stages provide a low output impedance needed to drive the audio phase shifter. A simpler diplexer between mixer and preamp shaves a little off the noise figure too. Rick`s R2 diplexer has steeper stopband slope, but about 2dB loss. The TOKO 10RB inductors simply have too much series resistance: I wound my own on small potcores, 14x8 mm. The potcore bobbins are very easy to wind, but its tricky to get the inductance exact - too much pressure from a mounting screw can change inductance dramatically. Potcore mounting hardware from the manufacturer is recommended. The 1. 2mH inductor should have low internal capacitance - 35 turns #32 wire in a single layer on a FT37-77 ferrite toroid. Diplexer low-pass frequency at 6700 Hz. , and high-pass frequency at 170 Hz are outside the audio passband (350 - 3500 Hz. ). This means that unwanted phase shifts from mismatched diplexer parts are less troublesome, relaxing the need for extremely accurate component values. With the tuning method described below, some of the bad effects of mismatched components can be tuned out. Nevertheless, diplexer components were matched with the aid of a commercial Maxwell impedance bridge. The phasing R`s and C`s were all scaled so that resistor values were much smaller. Otherwise, the Johnson noise from those warm, large value resistors adds so much noise...

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