A pair of simple 4 khz usb transceivers



These transceivers were made to be as simple as possible in use - just a volume control, tx key and warble tone switches.

Why 4 khz?

An Osram SFH 4715AS 5w 850nm IR LED had been purchased and I wanted it to be used as a receive detector as well as to be the transmit source. The receiver circuit used was the Finningley Mk II, ie, the LED detector feeds the gate of a high impedance  FET buffer.

Feeding the receiver output into a spectrum analyser and taking two plots of a) no signal and b) low level signal allows a comparison to be made that indicates the relative signal to noise performance against frequency, and is shown opposite. Both plots are taken in a dark background environment.

The highest frequency that can be used (so as to be as far away from mains generated light polution as possible) is in the region of 4 - 7 khz, so usb with a carrier frequency of 4 khz was adopted.
receiver
                      noise/sensitivity v frequency

The transceivers

4 khz ssb transceivers
 
There is nothing very original circuit wise - the same is used in each transceiver, with the larger box being made first because I wasn't sure what space would be needed. The second unit just managed to fit into a small die-cast box:

a tight squeeze

A usb signal is first generated at 4.192 MHz using an SA612 mixer IC followed by a 6 crystal ladder filter. There are still some 60 of these crystals left in the bag, so a lot of future filters there. Down-conversion uses up another SA612 mixer and the carrier/conversions local oscillator feeds are reversed to go from tx to rx. To aid optical alignment, a warble tone is generated directly at 5 khz, so the far end should hear this as a warble (identification is much easier with a warble tone rather than a straight 1 khz tone.

Circuitry:

ssb generation and conversion

tx audio:

tx audio processing

This is simple enough. Mic audio is amplified and pre-emphasis added, since this always seem to improve readability with my voice. A switched capacitance low pass filter cuts the audio off above 3 khz before being applied to a simple clipper (letting the op-amp bash against the supply rails). Harmonics appearing above 3 khz are removed by the 4192 khz crystal filter.

One thing not shown on either circuit diagram is the loud speaker audio amplifier, which uses a TDA2030A.

Here is the response of the ssb filter:

6 pole filter response