[CQ-Contest] Receiver specs

Michael Tope W4EF at pacbell.net
Sat Mar 11 11:29:27 EST 2000

Hi Ford,

If the "splatter" in question is generated in the offending 
stations transmitter, then the only thing you can do is turn
your antenna away from him since the interference actually
falls onto your operating frequency. 

If the offending station transmit signal is clean, then the 
interference is probably being generated in your receiver. 
A quick way to check this is to put some attenuation in front
your the receiver. If the distortion drops more quickly than 
the signal levels, then distortion is being generated inside
the receiver. This is because the kind of interference that 
is generated inside of receivers (intermodulation distortion) 
is very amplitude dependent and falls off rapidly when input 
signal levels are reduced (the so-called third order 
intermodulation distortion products fall off 3dB for every 
1dB drop in signal level). 

In terms of your question about sideband suppression, this 
is seldom a problem with modern transceivers. The main problem 
is intermodulation distortion which has nothing to do with
sideband suppression. The reason for this is simple, unwanted
sideband suppression in an HF transmitter is done at an
intermediate frequency at a fairly low power level where 
amplitude distortion (intermodulation distortion) doesn't really
come into play too much. The sideband suppression is set by
the selectivity of the crystal transmit IF filters which 
have very sharp selectivity (some of the newer rigs may use
DSP techniques to do this). 

Once the unwanted sideband is removed, the transmit signal needs 
to amplified  from a low level (on the order of milliwatts) up 
to a high level - 100 to 150 watts. Since the amplifiers used in
this process only have finite linearity, the various components 
of the voice signal will invariably cross modulate each other to
some degree producing intermodulation distortion products, i.e. 
splatter. This splatter is very drive dependent, so in the same 
way that the intermodulation distortion products generated in a 
receiver drop off quickly as the signal level is reduced, the 
intermodulation products generated by a transmitter increase 
rapidly as the drive power level increased. Automatic level 
control (ALC) is supposed to mitigate this to some extent, but 
some have commented recently that ALC circuits used in some of 
the modern transceivers are of inferior design. 

Hope this answers, your question!

Mike, W4EF.................

P.S. Does this mean I can now ask you questions about my taxes -
grin grin........ 

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