Bravo bravo to Chen.
That is why he has his own folder in my email client. ;-)
73,
Jim AC0E
Sent from my iPad
On Jan 28, 2013, at 3:24 PM, "Ed Muns" <ed@w0yk.com> wrote:
> Thanks, Chen! I love your mini-tutorials. I always learn something and
> sometimes I even understand most of it.
>
> The K3 FSK wave shaping is applied to the Mark-Space slew. The trade-off
> for the resulting narrower FSK bandwidth is a shorter dwell time of the
> transmit energy. So there is less energy in the receive filter. That's all
> I was referring to.
>
> Its a good trade-off for anyone trying to operate nearby, but worse for the
> station with the cleaner signal because he gets more of the clicks from
> broader signals that are now closer to him. That is, until all radios clean
> up their FSK. ;>)
>
> Ed W0YK
>
>
>
> Chen W7AY wrote:
>> On Jan 27, 2013, at 10:53 PM, Ed Muns wrote:
>>
>>> I didn't feel empirically that other stations had more
>> trouble copying
>>> me due to the somewhat lower energy in the signal.
>>
>> Unless Elecraft did something terribly wrong to the first 280
>> Hz of the filter (and unless there is a bug, that is no
>> reason to think they would do that), then any loss of power
>> is (1) tiny, and (2) is discarded by the demodulator anyway
>> (certainly true if you use 2Tone).
>>
>> What is filtered away is unused power by the receiver in the
>> first place, just as CW keyclicks are filtered away when you
>> tune in to the CW signal. Many of you even use 250 Hz filter
>> to receive RTTY. Your modems would not even know Elecraft
>> has filtered the keyclicks away (but the stations that
>> operate near your frequency sure will be able to tell.)
>>
>> Unless the receiver uses a Matched Filter, those keying
>> sidebands are not used when RTTY is demodulated by a Nyquist
>> filter. There are only two software modems I know which uses
>> Matched Filters to gain a few tenths of a dB of SNR when
>> propagation conditions are quiet and when there is no QRM.
>> None of the common software on Windows uses Matched Filters.
>>
>> From what someone else has measured, the Elecraft FSK filter
>> profile is at least similar to the "400 Hz filter" they use
>> as their AFSK transmit filter. So, as long as the tone pair
>> is properly centered, you would expect the filter to
>> introduce no intersymbol interference (ISI). As I indicated
>> in my RTTY Transmit Filter write-up, a properly designed
>> bandpass filter can be as narrow as 280 Hz at the -6 dB
>> points and still incur no extra decoding errors at the receiving end.
>>
>> I have been modeling RTTY filters through transmit IMD
>> (intermodulation distortion). You will notice in Andy
>> K0SM/2's plots that there is only so much you can do to
>> narrow down an RTTY signal before IMD takes over.
>>
>> Stare closely at his plots and what you will see is that the
>> filtered AFSK signals start off being nice and narrow. But
>> when it drops down to about 40 dB or 50 dB below the carrier
>> peaks, you see a sudden broadening of the spectrum.
>>
>> This sudden broadening comes from 3rd order transmit IMD
>> (second order IMD does not cause broadening). The amount it
>> broadens is pretty much the FSK shift (170 Hz) plus perhaps
>> one set of RTTY keying sidebands. I.e., that "pedestal" adds
>> about 216 Hz to the width of the transmitted signal.
>>
>> With a transmitter that has better IMD that the one Andy
>> measured, that "pedestal" will start at a lower level. For a
>> transmitter with worse 3rd order IMD, that pedestal will be
>> higher up and causes more harm.
>>
>> So, even if you start off with good intentions and use a 280
>> Hz filter, the transmitter will broaden it to 500 Hz when you
>> are down to the -40 dB or -50 dB level relative to the carriers.
>>
>> The FSK filter in the K3 will produce a similar "pedestal"
>> since it also goes through the transmitter's IMD.
>>
>> I am currently looking into methods of producing filtered
>> RTTY that are less affected by transmitter IMD.
>>
>> 73
>> Chen, W7AY
>>
>>
>>
>
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