[Hammarlund] Sweep alignment, crystal filters.

[Kenneth G. Gordon]

On 20 Apr 2006 at 11:23, Julian Bunn wrote:

> What is "ER Mag"? How do I obtain  a copy of that?

Oh. I apologize. I thought EVERYone knew about ER mag! :-)

That is Electric Radio Magazine. Go to:

http://www.ermag.com for information and to subscribe.

For OUR purposes, it is the best magazine out there. I usually drop 
everything to read it from cover to cover when it comes.

> 
> So I think I understand: in fact the crystal IF
> coils are a separate chain from the standard
> coils ... and it's a matter of peaking the crystal
> IF coil circuits to match the frequency of the crystal.

No. That isn't what I meant: what I meant is that normally we align 
an IF chain/strip to its center frequency, say, 455 Khz, because that 
is what we expect. However, in the case where a single-crystal 
crystal filter, with a "Phasing" control, is used, we have to align the 
entire IF chain/strip to the CRYSTAL frequency, rather than to our 
nominal IF frequency.

Now I am not exactly familiar with the SX-62's crystal filter scheme, 
but I am presuming that it is a single-crystal type of filter with a 
tuning capacitor in parallel with it some way. That capacitor used to 
be called the "Phasing" capacitor. What it actually does is to 
balance out the capacity of the crystal HOLDER, and also moves 
the parallel resonant frequency of the crystal, which is a 
REJECTION notch, back and forth across the crystal center-
frequency, and thus the IF passband.

Now, since the crystal center frequency is very seldom exactly 455 
Khz (let's say), in order to get the best effect out of our crystal filter, 
and so that we have minimal loss through it, we must align the 
entire IF chain/strip to the center frequency of that crystal, rather 
than to our desired 455.00000 Khz. 

Those crystals can be off as much as 1.5 Khz: possibly more. So, if 
the actual crystal center frequency is 456.1 Khz, let's say, then we 
must align the entire IF strip to that frequency, and not to 455 Khz.

> 
> But ... when you do your first sweep to look for the
> center of the crystal's frequency peak, what
> prevents the IF coils' responses from masking/swamping
> that peak? I guess I should try it before thinking
> of potential problems :-)

Usually, the crystal filter, when set at its most selective position, 
will be the controlling factor.

I think if you try it, you will see what I mean. It should be most 
obvious.

You're very welcome, Julian. And my thanks to YOU for pursuing 
this technique.

Ken W7EKB