[Hammarlund] Fw: Why 2nd RF stage?

[Richard Knoppow]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: "Brian Burns" <brian at lessonsinlutherie.com>
Sent: Sunday, November 30, 2014 11:56 AM
Subject: Re: [Hammarlund] Why 2nd RF stage?


>    The second stage is to increase rejection of images. 
> 455 Khz is a marginal frequency for the IF of a receiver 
> working above perhaps 5 mhz. It was a compromise between 
> selectivity and image response. That is, for a given Q the 
> bandwidth of a tuned circuit is proportional to its 
> resonant frequency. This makes both the IF and RF stages 
> broader as the frequency increases. So, at 455 khz the 
> bandwidth of a single RF transformer is just about enough 
> to give a reasonable reduction of the image response up to 
> a couple of mhz. To go higher one needs more tuned 
> circuits or a higher IF.  For a broadcast receiver there 
> is enough rejection from just the antenna coil so that an 
> RF stage is not necessary but to cover much beyond the 
> broadcast band greater RF selectivity is needed. For 
> instance, a receiver like the Hallicrafters S-38, which 
> feeds the antenna directly into the mixer, has almost no 
> image rejection above about 20 mhz while a receiver will 
> high quality RF coils, like the RCA AR-88 can have nearly 
> 46db rejection at 30 mhz.  To get more one needs a higher 
> IF but the compromise there is the IF selectivity. So many 
> receivers used dual-conversion where the first IF is moved 
> to anywhere from about 1500 khz to 5 mhz and a second 
> conversion stage is used to get the IF down to a frequency 
> where good selectivity can be had such as 455 or even 50 
> Khz.  Since it is possible with modern technology to get 
> good IF selectivity at high IF frequencies many modern 
> receivers have gone back to a single conversion but UP 
> convert the incoming signal to some frequency well above 
> the range of interest.  Since each conversion brings with 
> it a set of spurious responses a single conversion 
> receiver is always cleaner than a multiple conversion one 
> provided one can accomplish the other requirements, such 
> as reasonable IF selectivity and good signal to noise 
> ratio with it.
>    In old receivers, especially those using multiple grid 
> converter tubes, some again ahead of the mixer was desired 
> because the mixers were very noisy. Newer converter stages 
> can be made much quieter so even direct first conversion 
> can result in very good noise factors.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
> ----- Original Message ----- 
> From: "Brian Burns" <brian at lessonsinlutherie.com>
> To: <hammarlund at mailman.qth.net>
> Sent: Sunday, November 30, 2014 10:14 AM
> Subject: [Hammarlund] Why 2nd RF stage?
>
>
>> Hello All,
>>
>>
>>
>> May I bother you with an idle curiosity question?
>>
>>
>>
>> What's the idea behind having two stages of RF 
>> amplification in the front
>> end of a receiver? Often, top of the line models have a 
>> second one. My SWAG
>> (scientific wild-ass guess) is that it's for better 
>> adjacent signal
>> handling. Surely it's not for additional gain, or is it?
>>
>>
>>
>> Cheers,
>>
>>
>>
>> Brian
>>
>