[ARC5] 80 KHZ IF

[Christopher Bowne]

I believe that a number of Hallicrafters  receivers did an 85 kHz second IF, the SX-71 (iIRC Halii’s first dual conversion receiver), SX-101, and SX-111,  possibly others.

Chris AJ1G 
Stonington CT

Sent from my iPhone

> On Feb 2, 2021, at 11:20, Bob Groh <bob.groh at gmail.com> wrote:
> 
> 
> 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    
>> 
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