[Premium-Rx] real basic question on stability etc.

[Karl-Arne Markström]

This is a technique that went away with the advent of the frequency synthesizer.

Very early SSB/ISB receivers often had a "pilot carrier" detector that locked on a weak 
residual carrier which was transmitted along with the information.

The "pilot channel" was very narrow-band in order to get a good S/N even with a 
fading signal, and the information usually was used for both frequency corrections and
amplitude corrections (the AGC also had several inputs, one from the pilot channel and one
from each information sideband channel). The correction range of the AFC was usually quite narrow,
in the order of a few 10's Hz.

In the 50's the frequency synthesizer was introduced, first in transmitters, and later in receivers.
This started to make AFC obsolete for most HF fixed circuits, and it survived longest in ISB telephony
circuits, where the AGC function also could be of advantage.

For mobile HF/SSB use, the pilot carrier system was almost never used,
one exception may have been the SSB systems used on large passenger ships, 
where voice privacy equipment was required. 
Their emission often was R3E (USB with -16 dB relative to PEP pilot carrier).

Usually pilot carrier AFC in HF receivers was handled in an outboard "SSB converter",
which worked on the receiver IF.

Exceptions, providing internal AFC provisions, that I know of are i.a. the Collins HF-8054 
and the RFT EKD300 and EKD500.

73/

Karl-Arne
SM0AOM



----- Original Message ----- 
From: "John P Ceresole" <veloce at tcp.co.uk>
To: "Eugene Hertz" <ehertz at tcaf.org>
Cc: <premium-rx at ml.skirrow.org>
Sent: Thursday, September 01, 2005 5:29 PM
Subject: Re: [Premium-Rx] real basic question on stability etc.


> Hi Eugene
> 
> Tracking receiver is ok with input signal derived AFC whilst this works 
> well with a good signal to noise ratio AND a continuous signal as you 
> correctly point out FM, PM and AM signals but useless on SSB, CW (Morse) 
> owing to their intermittent presence.  WJ got around that problem (I've 
> only just twigged why they did this when thinking about it) by introducing 
> their DAFC (Digital AFC) which holds the LO steady during the "quiet" 
> periods of the received signal.
> 
> 30 MHz to 1 GHz is actually a fairly interference free frequency range but 
> in the presence of interference in the 1 to 30 MHz range (eg PLC and ADSL 
> over aerial telephone lines) I am not sure that standard analogue AFC would 
> cope with a low wanted signal to noise ratio DAFC certainly would provided 
> that the digital loop was adjusted to be low sensitivity to interference.
> 
> Interesting ideas.  I have never played with either AFC or DAFC rxs but I 
> have the luxury of an Rx with a long term LO accuracy better than 1 in 
> 10E10 and any transmitter I used would have a similar synthesiser 
> performance (settable to better than 1 Hz in 100 MHz)
> 
> Best regards
> 
> John G8 BSD
> 
> 
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