40m DC receiver (1)

The design for this receiver is based on an earlier design by Steve KD1JV. it is build around two analog switches TS5A3157. These switches have on resistance of ca. 5.5 Ohm 10 times smaller as the often used 74HCT4053 switches.

First Draft of 40m DC receiver – DO NOT BUILD !

The input transformer was wound on a FT37-43 core with 12 turns on the primary side and 2x 12 turns on the secondary side. The switches are powered from a 78L05 regulator. The middle of the secondary winding is connected to 2.5V so the switches are always operating in their lineair region. The astute reader will have noticed that the first audio amplifier after the mixer has been set-up in a different way. In many design the first audio amp is a differential amplifier around an op-amp. Something like the circuit below :

This circuit has a major disadvantage, however. The input impedance for input V1 is R1 + R2 (101K). The input impedance for V2 is less than R3 (1k) ! This is a huge difference. This means that one end of the mixer will be severely loaded and the voltage lowered because of this. The other half will hardly be loaded at all. This will create an asymmetric output of the mixer and you are subtracting apples from lemons so to speak
For a more detailed analysis have a look at the excellent explanation in a Texas Instruments application note.

The common answer to this, is to buffer (and sometimes amplify) the signal before it is offered to the differential amplifier.

If resistor R7 is omitted (infinity), amplifiers A2 and A3 buffer the input signal. Their output has a very low impedance. The difference in loading, caused by the different impedance’s of the inverting and non-inverting input of amplifier A1, is thus eliminated. This whole set-up is a “lot of amplifier” just for subtracting two small signals. Every opamp adds some noise. We are dealing here with low-level signals, so this definitely is not a trivial issue. The resistors need to be well matched (<0.1%) to have good working diff amp. Al this makes it a complex set-up …

After connecting, it all up. It doesn’t work, of course….. Which is very good for this means more joyful time in the lab for troubleshooting…!

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