[RTTY] Improved Pseudo FSK Keying circuit for 2Tone

[Joe Subich, W4TV]

Remarkably similar to the system used in microHAM DigiKeyer II except
in DK II the monostable function is performed by the processor rather
than a discrete circuit.

73,

    ... Joe, W4TV


On 3/15/2014 10:28 AM, David G3YYD wrote:
> The system of transformers, diodes and smoothing capacitors is far from
> ideal as proposed by the Fldigi team. The system proposed here costs less
> and has virtually no keying bias at all. It has very accurately timed bits
> and stop lengths unlike EXTFSK or keying from many serial port cards.
>
> The rectifier smoothing system causes significant keying bias. Hence 2Tone
> use of 5Khz with smaller capacitors to reduce this bias. Although I have
> never mentioned it before there is a much better way to generate the FSK
> keying signal from the pseudo FSK signals that 2Tone and Fldigi generate.
>
> Take the signal output from the sound card via a blocking capacitor into the
> non-inverting input of an op-amp, which is biased at a fraction less than
> half the supply voltage. (The slightly less is to ensure with no tone the op
> amp does not switch on noise.)
>
> The op amp has its inverting input biased at half its supply voltage. The op
> amp has no feedback resistor and must be able to switch between ground and
> supply rail. A comparator could be used in place of the op-amp.
>
> The op-amp output is fed into the positive going trigger input of a
> retriggerable monostable with its delay setting at 10% longer than one cycle
> of tone - 220uS for 2Tones 5KHz. The Q output from the retriggerable
> monostable goes via a resistor to the base of the keying transistor. Use the
> NOT Q output to invert the keying sense. It is important that the retrigger
> time is greater than one cycle time of tone.
>
> A more sophisticated version would use a full wave op amp rectifier then the
> comparator then the mono stable with delay time set at  110usec. However
> this is not really necessary.
>
> Op-amp and the monostable will need to use the same supply voltage, which if
> CMOS is used could be the filtered shack 13.8v supply. Could even use a TO92
> 9v IC regulator with its decoupling capacitors. Stabilising the supply makes
> the delay timing stable against supply volts.
>
> In CMOS a retriggerable monostable would be a HEF4528B or equivalent
> CD4528BC. The data sheet can be found here:
> <http://www.nxp.com/documents/data_sheet/HEF4528B.pdf>
> http://www.nxp.com/documents/data_sheet/HEF4528B.pdf
>
> This is a dual retriggerable mono so if using SO2R can use the other half
> for the other rig along with a dual op-amp. Then just 2 ICs plus 2
> transistors costing pennies for SO2R FSK keying. Cheaper than a serial port.
>
> The keying bias is very low it will be the monostable's delay time less
> 200uS (one cycle of tone at 5KHz). So with a 220uS delay that is an error of
> 20uS in 22mS or <0.1%. The distortion caused by TX and RX filtering will be
> a lot higher than this. It will also work well at 75 baud.
>
> Personally I always use AFSK so have not developed such a circuit but
> someone else might want to do this and publish it.
>
> 73 David G3YYD
>
>
>
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