On Oct 14, 2013, at 8:21 AM, Jim W7RY wrote:
> Has anyone built this circuit?
>
> http://www.w1hkj.com/FldigiHelp-3.21/PseudoFSK.html
pFSK (what cocoaModem calls OOK -- "on-off keying" in textbooks) has been
around for quite a while now, but never really caught on. It has been brought
up often on this reflector.
There are commercial units (MicroHAM USB Interface III, MicroHAM DigiKeyer II)
that will take the keyed tones and convert them to FSK keying signal for you.
You can be lazy and not build a thing.
Think of it as K4DSP's FSKit, but you only send it one tone instead of the two
AFSK tones. In other words, it is equivalent a Mark-only (or Space-only) AFSK
signal. Because of that, you just need one tone detector, and it need not be
frequency selective, so it is much cheaper to implement in pure hardware. Once
you have a microprocessor such as an AVR chip in FSKit, you might as well
implement a full AFSK detector.
The FSKit scheme is somewhat superior because of the "DC restoration" problem
(the two tones ensures equal Mark and Space pulse durations). With an OOK
signal, a keyed Mark pulse can potentially be slightly longer (or shorter) than
the Space pulse depending on the detector threshold (usually just Vbe of a
transistor that is offset by a diode rectifier drop).
On the other hand, with pFSK/OOK, you can choose a higher frequency audio tone
to reduce the uncertainty of the leading and trailing edges of the FSK keying
pulses. I think fldigi uses a tone north of 3 kHz. cocoaModem uses 2500 Hz.
If memory serves, when I tested it, the DigiKeyer II will take very high
frequency tones.
cocoaModem's OOK tone does not use any waveshaping (I am pretty sure it is the
same with fldigi) since the audio tone is not intended to go out on the air.
That also helps reduce the uncertainty at the rising and trailing edges of
pulses.
An active tone detector (for example, a diode in the feedback of an op-amp:
like what the ST-8000 uses for its detectors) with ensure better Mark/Space
balance also.
Since it is not frequency selective, the usual pFSK circuit cannot be used with
regular two-toned AFSK signals that are generated by the computer. I don't
know of programs other than fldigi and cocoaModem that generates a single hard
edged on-off keyed tone. Joe W4TV probably knows of others, if they exist.
Instead of building actual diode rectifiers, you can of course use an AVR chip
to do single tone detector too. You just need to modify Doug's FSKit program
slightly. Heck, if you have an FSKit in hand, all the wiring it already there
and all you need is to burn a different program into it.
If you build your own, just look at the FSK keying signal on the scope to make
sure that it is symmetrical with respect to Mark and Space states, and you are
good to go.
73
Chen, W7AY
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