[GreenKeys] FSK shifts

[Ralph Mowery]

As many started using SSB transceivers that came with filters about 2.4 khz  and 500 khz.   With audio tones of 2125 and 2975 hz as used on VHF AM and FM the 2975 hz tone would not pass very well on the ssb filter and not at all on the cw filter.   The 170 shift was just 850 divided by 4.  That would allow the tones to pass the 500 hz cw filter and I am thinking the audio filters were designed for a pass band of about 300 hz in the demodulator front end.

 

It is interesting to be on the low bands with the oscilloscope on the tones and watch one tone fade in and out during bad band conditions.  I noticed that even with the 170 hz shift.

 

Ralph ku4pt

 

 

From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Russ Miller
Sent: Sunday, August 22, 2021 12:09 PM
To: N4TTY at arrl.net
Cc: Green Keys
Subject: Re: [GreenKeys] FSK shifts

 

It was about 1968 when 170 Hz narrow shift became popular.  If you go back to RTTY Journal during that period, you will see the slogan "Broad minded?, Try narrow shift!"

 

Why is narrow shift preferred?

 

Here are some of the reasons: - narrow shift maes copy better with QRM.  Period, end of discussion.

 

Why technically? The ability to use a 500 Hz filter greatly improves the signal to noise ratio and helps eliminate adjacent station interference.

 

73

Russ WA3FRP

 

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