[Drake] preselector

[amfone at sbcglobal.net]

I once again stand corrected
----- Original Message ----- 
From: "Paul Blumhardt" <k5rt at verizon.net>
To: "Drake Listserver ( QTH.NET )" <drake at mailman.qth.net>
Sent: Wednesday, May 03, 2006 9:48 PM
Subject: RE: [Drake] preselector


> Most (all?) of the tube receivers made prior to the 2A had a very simple
> impedance matching circuit. Typically these were an L network that had a
> very low Q. Ergo no real band pass/band reject characteristics at all. In
> fact many of those circuits were called "Antenna" or "Peak" or something 
> of
> that ilk.
>
> Its important to note that many receivers of the pre-2A era were not "Ham
> Band only". Most had one stage of RF amplification and while the plate 
> side
> of the RF amp was tuned, but the grid side was basically untuned except 
> for
> the impedance matching network.
>
> The advent of "ham band only" receivers ushered in high-Q front ends
> (Pre-Selectors).
>
> A Preselector by definition is a band pass filter with a fairly high Q 
> that
> is located before the RF amp in the receiver. Its main purpose is to
> minimize the effects of strong out of band signals on the RF amp which 
> would
> manifest as unwanted IP3 products. A good example of how a preselector is
> used in this sense is the Galaxy R-530/1530. That preselector has (as I
> recall) 4 tuned circuits in the filter and employs ganged variable
> inductors - very sophisticated!
>
> If you fast forward to the late 1970's, we even went as far as using 
> crystal
> bandpass filters in front of our R4C's. These were very limited bandwidth.
> As I recall, the ones we used passed 7000 to 7050 kHz and cut off VERY
> sharply. (you read that right, those were crystal lattice filters) 
> However,
> this was the hot set up for contesting on 40 CW from the East Coast 
> pointing
> at Europe with stacked 3 element fullsized yagis. Them Foreign Broadcast
> signals (and some of the Italian hams) got really strong!
>
> One last thing about "Preselectors". If you look at how that function is
> typically implemented, its not preselector in the true sense. Take the R4C
> for example. When you peak the Preselector, you are peaking the front end
> and mixer stages as well. Finally, the Preselector function can be done
> either with ganged capacitors or ganged inductors. Ganged inductors were
> found in your S-Lines, C-Lines and such. This method provides tuned 
> circuits
> with higher Q and more linear tracking.
>
> Just one more example of how the science of engineering receivers evolved
> over time.
>
> VY Sennytree
>
> Paul
>
>> -----Original Message-----
>> From: drake-bounces at mailman.qth.net
>> [mailto:drake-bounces at mailman.qth.net]On Behalf Of Ed Berbari
>> Sent: Wednesday, May 03, 2006 7:34 PM
>> To: Dan Cotsirilos K9DTC; Drake Listserver ( QTH.NET )
>> Subject: Re: [Drake] preselector
>>
>>
>> Dan,
>>
>> I am not sure how the preselector is defined.  Many older receivers (I am
>> thinking of my Hammarlunds - back to the HQ-120)  They had antenna tuning
>> circuits at the very front end.  I believe they were quite common
>> in other
>> brands as well.
>>
>> Ed, W9EJB
>>
>>
>> ----- Original Message -----
>> From: "Dan Cotsirilos K9DTC" <dcsfree at comcast.net>
>> To: "Drake Listserver ( QTH.NET )" <drake at mailman.qth.net>
>> Sent: Tuesday, May 02, 2006 11:27 PM
>> Subject: [Drake] preselector
>>
>>
>> Did Drake with the 2A invent the preselector. Was that the first receiver
>> that had one? Does anyone know?  thanks
>>
>>
>> Dan K9DTC
>>
>>
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>
>
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