[R-390] Alignment of the Mechanical Filters and IF with a Spectrum Analyzer

[Charles P. Steinmetz]

Mack wrote:

>>Does anyone have experience with using a spectrum analyzer to:
>>
>>1) *Adjust the mechanical filters? *There is some literature that 
>>suggests just peaking for noise can adversely affect the filter skirts.
>>
>>2) *Stagger tune the IF?* My thought is that you could look at the 
>>overall spectrum of the IF and make optimal adjustments.

David responded:

>1.  *  *  *  The only official adjustment is the trimmer caps on the 
>input and output coils of later-production IF modules, and nobody 
>ever did anything but peak them at the center frequency.  *  *  *
>
>2. The LC portion of the R-390A IF chain is fairly broad and not 
>stagger-tuned; perhaps you're thinking of the R-390.

Except possibly for the 16 kHz bandwidth, the IF adjustments will 
affect the IF bandwidth only way down in the filter stop 
band.  Here's what I wrote in a previous message:

>In the 390A, the mechanical filters provide the actual IF bandwidth
>response (the interstage LC filters do clean up the stop band, where
>the mechanical filters have zeroes and ripple, so they give the IF a
>lower ultimate stop band attenuation than it would have had with the
>mechanical filters alone).  Think of the interstage LC filters in the
>390A as a roofing filter, although in this case they follow rather
>than precede the mechanical filters that actually set the IF bandwidth.
>
>In some cases, the LC "roofing" filter in a 390A may have a narrower
>or more peaked response than the 16 kHz mechanical filter.  If that
>is a worry, the interstage LC filters can be stagger tuned to broaden
>the overall LC response so that even the 16 kHz mechanical filter
>effectively sets the IF response.

Even if the IF response turns out to be narrower than the 16 kHz 
filter itself (these filters often measure more like 20 kHz), I'd 
still just peak them because 20 kHz (or even 16 kHz) is really too 
wide for almost any practical use on today's bands.

Best regards,

Charles