When you are home cooking an IF, it can be whatever you want. 100Khz,
200 Khz, 465Khz, 470Khz..455, 465 and 470 were the three values that became 'standardised' about the time I built my first radio (1965)
You could tweak the coils anyway between 465 and 470...
455 became a standard mainly because that is what the ceramic filter manufacturers decided on.
Well, not been my experience. Valve era lytics are usually fine. All but one in my big 1930s radio are still good. OTOH an older one used oiled paper caps throughout, and all were shot.
NT
IIRC, the original intention was to avoid broadcasting on a frequency around twice the IF, because many listeners will hear a beat note due to distortion in the mixer stage of the receiver (especially if the signal is very strong).
In Europe, manufacturers agreed to standardise on 456kHz, and broadcasters agreed to skip 930kHz. In North America the standard IF is
455kHz and they skip 910.However, that idea has now gone the way of all good intentions, since the vast majority of MW radios are made in the Far East for a global market; but good MW reception is nowhere near as important as it used to be.
(If you want any more, you'll have to find an even older G3 :-)
I think it was just decided on empirically, but yes, I think it was to do with the MF frequency range.
To quote from some very old course notes (I can barely read my handwriting!) :-
"The IF is determined by a compromise between two conflicting factors. A low IF is desirable in order to obtain high amplification with good selectivity and without the risk of instability whilst a high IF will increase the signal-image rejection. For AM receivers an IF between
455 and 470 kc/s is chosen."The LO can be 465 kc/s above or below the wanted RF signal. In practice however the LO frequency is set higher than the signal frequency for the following reason.
Consider a receiver operating over the frequency range 550 - 1500 kc/s. If the LO is 465 kc/s lower than the signal frequency then the oscillator must vary in frequency from 85 - 1035 kc/s - a frequency ratio change of 12·2. If the LO is higher in frequency than the wanted signal then the oscillator must vary in frequency from 1015 - 1965 kc/s - a frequency ratio change of 1·94. Since there are practical limitations to the range of frequencies that can be tuned by a single inductor and variable condenser owing to the min and max values of capacitance then it is far easier to change the frequency over the ratio of 1·94, hence the LO is generally set higher than the RF signal frequency. eg a typical condenser change of 60 - 500 pF - a range of
8·5:1, therefore a frequency ratio of 2·88:1."
This makes sense together with broadcasters avoiding the harmonics.
Also it might have been tricky to listen to the Home Service on Long Wave if the IF had been 465kHz below the RF. :-)
No. Far too young :-)
Dave
.....except in Dribble's alternative universe :-)
I'm not a G3 (whatever that is) but when I were a lad I knew them as tuning condensers too.
Another Dave
A G3 is the prefix to an amateur radio call sign and is quite an old one at that It would go back to the time that tuning condensers where all the rage.
From a relatively young G6
Dave
Condensers? Naww, that's a modern term, we called em jars.
Jars? naww, we called em pith balls.
Variable pith balls? We never ad those, no we had to chew a bit of the pith off to vary them. It wasnt a nice job, hence the phrase 'taking the pith.'
etc
More seriously, in case anyone doesnt know, there were once radios that didnt have tuning.
NT
Just like Radio 1.
Tuning capacitors, or condensers, take your pick, had air spaced vanes that moved. Electrolytic were big round pots and used mainly in power supplies. they have nasty habit of blowing up if they are not used for some time then have power applied. Spent nearly foty years in the ardio trade so seen quiet a few .
Yes I do know that :-)
"there were once radios that didn't have tun(e) in "
"Just like Radio 1"
Still don't get it?
I give up!
ITYM "Light Programme" :-)
The Home Service was never on long wave, being originally a network of regional medium-wave stations. These gradually lost their regionality and became Radio 4 long before the move to 200 kHz (and subsequently 198 kHz).
This is an old myth actually - a saw frequently trotted out in textbooks in the days when valve rectifiers were common. Usually though it was the cathode of the rectifier valve that was said to be at risk from excessive peak current, not the transformer.
In reality, if you analyse the circuit (rectifier with capacitor input filter) properly, with sensible component values, you discover that ratio of the peak current during rectifier conduction to the mean DC load current depends almost entirely on the source impedance of the AC supply to the rectifier - i.e. in practice on the resistance of the transformer windings. For an particular rectifier configuration, the value of the reservoir capacitor has almost no effect on Ipk/Imean, until its value becomes so low that the ripple would be intolerable in any practical situation.
Which could of course be pure coincidence, and nothing at all to do with fitting a larger reservoir capacitor.
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