[ARC5] 80 KHZ IF

[Bob Groh]

Wayne, the 85 kHz in the BC-453 was due to both it's input frequency range
and to make it fit into the mixing and frequency tracking schemes for the
other command sets. In general, the choice of the IF frequency in a
receiver is dictated by (at least in the 'old' days) by the somewhat
conflicting needs for narrow IF bandwidths and image rejection - all cases
with the choices being bounded by practical considerations of the component
Q's (e.g. it is practically impossible to build a 10 MHz IF with normal LC
components which would have an operating Q of 200 (i.e. a bandwidth of 5
kHz) with component Q's of 100).  On the other hand, for an input frequency
of say 10 MHz, it would be very difficult to use an IF of 85 kHz (i.e. the
image frequency would be 2*IF or 170 kHz above or below (depending on LO)
the operating frequency) because you would need a high Q in the front end
tuning filter along with the contradictory choices of low loss and easy
tracking.

So what the heck is a designer to do (again in the old days)? Well, for
input frequencies up to 5 MHz, a 455 kHz IF is a good choice - image is 910
kHz away so input LC is OK, reasonable IF bandwidths are obtainable and the
world is good. For input RF lower than 550 kHz, need a lower IF and 80 kHz
or so is good (e.g. BC-453).  Also good for ham radio because IF bandwidth
is narrower!  Huzzah. Above 40M or so, 455 kHz IF is becoming a bit
constraining (image rejection tough) and an IF bandwidth too wide for good
CW or SSB use - you can use a crystal filter (that helps) but expensive and
image still a problem. So double conversion - use higher IF (e.g. 2215 kHz)
for good image and then a 2nd IF (e.g. 80 kHz) for narrow bandwidth.

It get's a bit more complicated than that but I have nattered on for too
long already. Some good books out there on the subject.

73,
Bob Groh, WA2CKY

On Tue, Feb 2, 2021 at 9:16 AM <spr at earthlink.net> wrote:

> Well, I have a mid-'30s Latvian wood table radio that uses a 135 kHz
> single conversion IF. Image rejection, not so much...
>
> /scott
>
> -----Original Message-----
> From: Robert Eleazer
> Sent: Feb 2, 2021 5:53 AM
> To: arc5 at mailman.qth.net
> Subject: [ARC5] 80 KHZ IF
>
> I recently augmented my collection of National receivers with an NC-300.
> I previously have done quite a lot with the Cosmic Blue series of
> receivers, specifically the NC-155, NC-270, and NC-190, and was quite
> surprised to see that while the NC-300 had the same first IF as those other
> sets, 2215 KHZ, it has an 80 KHZ 2nd IF.
>
> Of course I at once thought of the BC-453 and similar command sets, with
> its 85 KHZ IF.  I had no idea that any HF receiver used that low an IF.
> How common is this?  Was the BC-453 use as Q5er the inspiration for this?
>
> Wayne
> WB5WSV
>
>
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> Virus-free.
> www.avg.com
> <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
> <#m_4502200215178083678_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: https://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20210202/cfe252bd/attachment-0001.html>