Why 455KC?

[AB3FL]

I should know this, but I don't and can't find any real info on it.  Why was 455KC chosen to be the standard IF for AM?


Tom - AB3FL

[flintstone mop]

You are stirring up grey matter!!
Auto radios used 262kc for some reason.
We'll see what comes up. Cuz GOOGLE doesn't know either.....hi

Fred

[WBear2GCR]

For one thing, it is below the broadcast band.

My R-388/51-J uses a 500kc IF.

I think 455 is not "standard" as much as it is popular.

Other than this, I don't know the historical development(s) that made 455kc popular.

                      _-_-bear

PS. a look at Terman (Radio Engineer's Handbook) for an early edition and some of the professional journal references might shed more light on it...

[w4bfs]

$$$$$ .... the higher the if freq, the less expensive the transformer (less physical winding size) ....the higher the if freq, the less the selectivity (all other things being equal).... yup, the bean counters strike again

[AB2EZ]

I would think that the following came into play... in the era in which 455kHz became a popular choice:


You don't want the i.f. output to include the "image" of strong AM station that is on the other side of the local oscillator frequency. So you want the sum of the local oscillator frequency + the i.f. frequency to be outside of the AM band, for any value of the local oscillator frequency.  

With an i.f. frequency of X kHz; the local oscillator (to tune 520 kHz -1610 kHz) will be between (X + 520 kHz) and (X + 1610 kHz).

The image frequency is between (2X + 520 kHz) and  (2X + 1610 kHz).

If you want 2X+ 520 kHz to be outside of the AM broadcast band, then you want 2X + 520 kHz > 1610 kHz.

Solving for X, we get: X > 1090/2 = 545 kHz

I would assume that the industry recognized that having the i.f. frequency above the bottom end of the AM broadcast band would create other problems (e.g. causing interference to neighbors' radios)


So, a compromise was reached... by moving the i.f. frequency somewhat lower.

[K1JJ]

I figure 455 kc was chosen because 455/10 = 45.5.   45.5/45.5 = ONE.  So, 455 was the perfect choice, because ONE is the loneliest number.

(gagging, muffled laugh is heard)

T

[flintstone mop]

DAM I was going to guess images.

Fred

[k4kyv]

Early broadcast superhets used a variety of if's, usually something lower than 455. I believe 165 kc was another popular one.  The problem with the lower if's is that it takes more front-end selectivity for satisfactory image rejection, and that means an additional rf stage with preselector= more $$$.  The vicinity of 455 turned out to be the best compromise between image rejection and selectivity for the BC band range.  Not all receiver if's are exactly 455.  Some are 456, and 465 is fairly common.

The BC-348 uses 910 kc/s for the if, and many of us know how poor the selectivity is. Excessively low if's not only result in image problems; often the selectivity is too high for good broadcast fidelity.

I don't know why Collins chose 500 for the 51J series.

The National NC81X used something like 1600 kc/s for the if, depending on a phasing type xtal filter for selectivity.  It had no rf stage, but depended on the high if for rejection.  I have one  here, but the high if doesn't help with image rejection and the xtal filter doesn't help the selectivity.  The thing is built well mechanically but it still is a POS.

[AB2EZ]

The facts: according to the "Two Thousand Year Old Man"

Interviewer: Carl Reiner
Two Thousand Year Old Man: Mel Brooks

Interviewer: "So tell me... since your were alive long before the invention of modern forms of entertainment... what did you do to entertain yourself back then"

Two Thousand Year Old Man: "Well, we kept ourselves entertained in many ways. We sacrificed virgins, we burned people at the stake, and... in more recent times... we threw Coke bottles off of the roofs of buildings"

Interviewer: "The advent of modern forms of electronic entertainment must have had a drastic impact on that"

Two Thousand Year Old Man: "Yes... with the advent of television, we could sit around and watch people sacrifice virgins, burn people at the stake, etc. It was much more relaxing to watch all of that in the comfort of an air conditioned room and a bowl of microwave popcorn"

Interviewer: "Lets' talk about the introduction of radio"

Two Thousand Year Old Man: "Yes, radio... great stuff... the Shadow, Amos and Andy..."

Interviewer: "I understand that when radio was first introduced, the radio receivers weren't very good. You would hear two or three stations coming through at the same time"

Two Thousand Year Old Man: "Actually, some of the radio programs were kind of boring... so it was good to hear more than one."

Interviewer: "Yes... but then came superheterodyne reception. I am wondering if you remember who came up with the choice of a 455 kHz i.f. frequency?"

Two Thousand Year Old Man: "That was Bernie. 455 was the combination to his gym locker. Everyone thought he was a genius. You know: image band rejection, selectivity... all of that. Bernie couldn't balance his checkbook... no less figure out how to pick a good number for the i.f. frequency. But, as I always say: luck beats brains any day".

Interviewer: "Thank you very much for enlightening our audience on that very interesting piece of history"

[Jeff W9GY]

The OT that trained me in the design biz. at Delco told me that 262.5 kHz was chosen for auto radios to minimize the number cities that whistles between two close proximity transmitters would occur in.  Apparently an extensive study was done using tx location in major cities and frequency.  In my 31 years I was constantly reminded of the extreme varied environment the car radio had to accommodate.  FWIW

[KA2QFX]

Excellent question, and a very well reasoned reply from Stu.  I wondered just the other day why 3.579...MHz was selected for a color burst sync freq.

[w4bfs]

STOP THAT ....my head is going to explode

[K6IC]

Stu,  that IS well reasoned,  and quite technical.  FUNNY !

Re 3.579545 Mhz,  it is  so the numbers work out,  close ENUF:
http://en.wikipedia.org/wiki/Colorburst

72, Vic

[WA1GFZ]

and my grandparents had a rubber bag with a long hose hanging on the back of the bathroom door

[AB3FL]

Excellent question, and a very well reasoned reply from Stu.  I wondered just the other day why 3.579...MHz was selected for a color burst sync freq.

That one is simple

http://en.wikipedia.org/wiki/Colorburst#Rationale_for_NTSC_Color_burst_frequency

Tom - AB3FL

[Opcom]

and therefore the 14.31818 4Fsc crystals found in much video gear. The memory clock for much digital video gear was chosen to be 4 times the subcarrier. Everything was timed on this. I used to find the Burst signal to be mysterious in a sort of beautiful way until I realized it is a shade of OD-like green, sort of a dark watermelon-green color. The same sort of bubble was burst when I learned CTDM recording method for betacam R-Y and B-Y components. Always, the mysterious beauty of a thing is lessened by the knowing.

[KM1H]

The primary reason for 262kc was selectivity since autos traveled close to many transmitter sites. The study mentioned determined the best frequency to provide selectivity and reasonable image rejection plus all sets, except super cheap aftermarket, had an RF stage. The earlier sets were 455.

20's superhets had almost as many IF frequencies as their manufacturers. Ive heard of them as low as 12-13 kc. A bit later in the 20's 175kc became popular which was used again in the WW2 Navy RAS ( basically a pre octal HRO) to allow full coverage of the 200-500kc range.

Carl
KM1H

[KM1H]

The National NC81X used something like 1600 kc/s for the if, depending on a phasing type xtal filter for selectivity.  It had no rf stage, but depended on the high if for rejection.  I have one  here, but the high if doesn't help with image rejection and the xtal filter doesn't help the selectivity.  The thing is built well mechanically but it still is a POS.

That certainly wasnt one of their better ideas! Didnt sell well even after they cut the price and mighty scarce today. What a waste of a catacomb coil assembly and dial drive.

Then Hallicrafters had to do them one better and come out with the S-53, no RF stage and a 2075kc IF. That didnt last long either. The later SX-140 was another hi IF dog, I changed mine to double conversion with a couple of stages of 262kc IF and a few other things and its a half way acceptable AM radio. It and a Knight T-150A were given to me a few years ago and after getting both changed around a bit they went into the cottage in Maine. The small size of both is nice and before that I used to lug a Viking I, 122 VFO, and HQ-129X back and forth. That was a royal PITA.

Carl

[Jeff W9GY]

Auto radios had much wider selectivity in the 40's and early 50's.  When the FCC really started letting lots of station on the air and "shoehorning" them in with tight antenna patterns all radio manufacturers were forced to tighten selectivity.  We used to make our own 262 IF's and had good control of selectivity. 

[N2DTS]

455 KHz is a nice pick for reasonable selectivity/fidelity in an inexpensive transformer.
I bought a bunch of IF cans from a tv repair shop going out of business many years ago, both the 262 KHz and 455 KHz ones, and used the 455's in the homebrew as I wanted the filter to do the bandwidth, not the IF cans.
The Gonset G76 used 262 KHz, then they used resistors across the primary and seconary windings, and a small cap from primary to secondary to get the response they were looking for.
An easy mod to change the resistors, higher value gave more gain and a narrower response, lower values gave less gain and a wider response.

I don't have the G76 anymore, but it would have been fun to look at the response curves with the sdr-iq or flex radio spectrum displays.

Brett
 

[KM1H]

Collins also used the resistor swamping in the 455kc 75A4 IF.

When I rebuilt the SX-140 a few years ago I used 2 stages of 262kc for the 2nd conversion; its just right for AM. It and the modified T-150A are compact enough for the Maine cottage.

Carl