[TowerTalk] Adjacent BC AM sigs DE K0FF

[Tom Rauch]

> In a message dated 08.05.00 08:20:48 Pacific Daylight Time,
> w4ef@pacbell.net writes:
> 
> << 
>  I thought a Q-multiplier was an active stage - wouldn't it be susceptible
>  to IMD just like the front-end you are trying to cure? 
> 
>  As I recall the Drake R4C had a pretty narrow passive preselector - but
>  it was still something like 20 to 30 KHz wide (-6dB?? points) on 160
>  meters - not enough for BC adjacent channel rejection. 
> 
>  Mike, W4EF. >>

K7GCO replies:
 
> I believe intermod is created more in stages with a a mixer?  The Q 
> multiplier as K3BU suggested doesn't have one if I remember right.  A
> preamp only, as frequently used is no different from the 1st RF stage used
> in tube receivers--without a mixer.  Buy cutting it's gain so it doesn't
> overload the front end of a receiver, is capable of giving greater
> selectivity before the receiver and is a very good design as I suggested
> in a previous post..    k7gco 

If the addition of a regenerative RF amplifier, or multi-tuned 
preselector, improves adjacent channel interference the receiver 
would have to be horrible!

The mixer stages or amplifiers ahead of the bandpass filter would 
have to be severely overloaded, rendering the bandpass filter 
useless.

If you ever did get that much selectivity in a regenerative amplifier 
stage, the stability would be horrible. You'd constantly have to ride 
the regeneration control, as well as the frequency control. Not only 
that, the shape factor would be poor. Instead of the flat nose with 
sharply dropping off skirts from a ceramic or crystal filter, which 
allows the sidebands of the desired signal through without 
frequency-vs-level distortion you'd have a steep sharp peak with 
sides smoothly sloping down from that sharp center frequency 
peak.

I suspect people are actually only adding attenuation and bringing 
the overloaded stages out of overload, more than actually improving 
things from additional selectivity.





  
73, Tom W8JI
w8ji@contesting.com

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