On 08/23/2011 07:05 PM, Kok Chen wrote:
>
> On Aug 23, 2011, at 3:06 PM, Ed wrote:
>
>> I may be missing something here, but how are the newer rigs generating
>> FSK ? Are they shifting xtals or just sideband tones ?
>
> Various methods.
>
> Back in the FT-990 and FT-1000D days, Yaesu had used keyed-AFSK. The FSK
> keying signal at the rear panel was used to key an internal AFSK generator,
> which fed the SSB exciter. So, even though you "think" you are doing FSK
> with those rigs, technically they are AFSK (which led to people calling them
> "keyed-AFSK").
>
> Funny that you didn't get any complaints about AFSK sidebands from the
> FT-1000D and FT-990. Albeit, Yaesu forces the RF voice processor off when
> you select "RTTY" and the AFSK generator has a fixed amplitude. The AFSK
> generator is crystal controlled, so the shift at least stayed put like some
> of the FSK rigs of that generation.
>
> The Omni V and Omni VI were "direct FSK" rigs and the keying signal shifts an
> RF oscillator with a trim cap. This frequency keyed RF is done at an
> intermediate frequency and mixed to the operating frequency. Yes, there is
> also an image, but presumably very low level and no inside a ham band.
> Because of the fragile trim cap, you can often identify Omnis back then by
> how far off their shift frequency is when the trim cap value changes over
> time, or over temperature :-).
>
> By the time of the FT-1000MP generation, Yaseu had changed to a more direct
> version of FSK, by keying the direct digital synthesizer chip, which is how
> many rigs from that generation do it. Many DDS chips come with a FSK and PSK
> pin, and you can find them in RF synthesized generators such as the Novatech
> 409.
>
> The rig designer cannot modify the FSK modulation from the DDS chip of
> course, so the amount of keyclicks from an RTTY signal in the hands of the
> DDS designer. The DDS typically has to allow at least 300 baud and perhaps
> even higher baud rate -- thus, the keyclicks from most FSK rigs are wider
> than needed for a 45.45 baud transmitter.
>
> Modern rigs such as the K3 generates the FSK in its DSP stage.
> Mathematically, it is not that different from "AFSK" but done at 15 kHz
> instead of at one or two kHz with the more traditional AFSK. The image is
> thus further away than an audio FSK signal. The DSP is probably (I don't
> know for sure, but there is good technical reasons) also helping to suppress
> the image ("the other sideband") further by the use of I-Q (in-phase and
> quadrature) techniques.
>
> As such, the DSP rigs could waveshape the "afsk" signal, but I do not know if
> any of them that does it. The only way to minimize RTTY keyclicks today is
> to use AFSK with a software modem that implements RTTY wave shaping. The
> RTTY wave shaping process is very similar to CW wave shaping that VE3NEA
> talks about in his May/June 2006 article in QEX.
>
> Software modems like cocoaModem actually does I-Q processing internally.
> However, it has to reduce the I-Q signal into a real waveform to feed the
> audio line input of the SSB transmitter. If the time ever comes when newer
> rigs will accept I-Q inputs, you can generate really clean "AFSK" signals
> from software.
>
> 73
> Chen, W7AY
As always, your input is very informative.
Ed W3NR
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