[RTTY] Some basic RTTY radio questions

[Kok Chen]

On Mar 6, 2016, at 7:26 PM, Michael Rapp wrote:

> My Yaesu FT-950 has two pre-amp settings.  On phone I typically only
> engage these on the quieter bands.  I'd use preamp 1 occasionally on
> 20 meters, and nearly always on 15 and preamp 2 (which amplified more)
> always on 10.  On RTTY is this useful or is there something
> detrimental to a RTTY decode from using a preamp?

RF preamps improve noise floor but reduces the large signal capability of a receiver (a.k.a. overloading).  

On the lower bands, 30m and lower, the sky noise is usually way over the noise floor of any RF preamp, so there is no reason to engage two of them.  Unless you are using a really low gain receiving antenna, there is really no need to engage any RF preamp on 80m.

On 10m, the improved SNR from the RF preamp(s) can help dig out the weak ones when there are no large signals that can overload the receiver stages that are before selective filters.

Just look at it as a trade off between better SNR (and thus bit error rate) and larger signal handling.

> I must confess that I really don't understand what a roofing filter
> does.  My radio defaults to 3 kHz for RTTY.  Is this okay?

Roofing filters in superhets help to prevent the 2nd mixer from being overloaded by nearby large signals.  With RTTY, you can probably get by with roofing filters as narrow as 600 Hz.  I would not go any narrower if the 1st IF of the receiver is above a few MHz, since the group delay of a narrow filter can be more detrimental than helpful.  With aural CW copy, you can go to 250 Hz or even narrower roofing filters since the ear is more forgiving with intersymbol interference (ringing).

73
Chen, W7AY