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Skywave · 17th Nov 2006, 11:19 pm

Hi Adi.

In a superhet - such as the type we have here - it is usual to have the local oscillator on the HIGH side of the required receive frequency. This is because it makes the ratio of aerial tuning freq. span to local osc. freq. span smaller than if the local osc. was on the low side.
This, in turn, makes the freq. difference (which is the IF, of course) error smaller, (errors between the sig. freq. tuned cct. and the osc. tuned cct. are known as "tracking errors") and so the output from the mixer stays closer to the nominal IF than would otherwise be the case - which for broadcast sets is usually in the range 450 Kc to 470 kc. [470 kc. was commonly used by Philips valve radios in the '50s. 450 / 455 kc. is very popular with sets of USA & Japanese origin. 465 kc. is the most common for UK radios]. There is no real reason to chose any particular nominal IF over any other in this range for this type of simple superhet.

Padder capacitor.
If the aerial tuned cct. and the oscillator tuned cct. were each simple L/C parallel tuned ccts., it would be possible to achieve a zero tracking error for 2 signal freqs. only. By introducing a padder capacitor in the oscillator circuit, it cuts down the swing of the local osc. tuning capacitor and produces 3 points that result in zero tracking error. A la, this is known as 3-point tracking. It is usual to aim at a zero tracking error towards the LF end of the band - usually set by adjustment of the "L" in the circuits, and another point towards the HF end of the band - adjusted by the "C" of the ccts. - hence the trimmers. The padder is usually not adjustable. Its value is calculated in advance to give a zero tracking point somewhere towards the centre of the band.
The net result is that between the LF point and the mid-band point, there is a positive tracking error, and between the mid-band point and the HF end there is a negative error. The skill of the design is to shuffle all the available variables (L, C and freqs.) to minimise the errors - but with 3-point tracking, they cannot be eliminated.
There is a lot more to superhet design than this - but I'm sure you'll agree with me that that lot will do for now!

Good luck.

/ Al.

17th Nov 2006, 11:19 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: superhet Hi Adi. In a superhet - such as the type we have here - it is usual to have the local oscillator on the HIGH side of the required receive frequency. This is because it makes the ratio of aerial tuning freq. span to local osc. freq. span smaller than if the local osc. was on the low side. This, in turn, makes the freq. difference (which is the IF, of course) error smaller, (errors between the sig. freq. tuned cct. and the osc. tuned cct. are known as "tracking errors") and so the output from the mixer stays closer to the nominal IF than would otherwise be the case - which for broadcast sets is usually in the range 450 Kc to 470 kc. [470 kc. was commonly used by Philips valve radios in the '50s. 450 / 455 kc. is very popular with sets of USA & Japanese origin. 465 kc. is the most common for UK radios]. There is no real reason to chose any particular nominal IF over any other in this range for this type of simple superhet. Padder capacitor. If the aerial tuned cct. and the oscillator tuned cct. were each simple L/C parallel tuned ccts., it would be possible to achieve a zero tracking error for 2 signal freqs. only. By introducing a padder capacitor in the oscillator circuit, it cuts down the swing of the local osc. tuning capacitor and produces 3 points that result in zero tracking error. A la, this is known as 3-point tracking. It is usual to aim at a zero tracking error towards the LF end of the band - usually set by adjustment of the "L" in the circuits, and another point towards the HF end of the band - adjusted by the "C" of the ccts. - hence the trimmers. The padder is usually not adjustable. Its value is calculated in advance to give a zero tracking point somewhere towards the centre of the band. The net result is that between the LF point and the mid-band point, there is a positive tracking error, and between the mid-band point and the HF end there is a negative error. The skill of the design is to shuffle all the available variables (L, C and freqs.) to minimise the errors - but with 3-point tracking, they cannot be eliminated. There is a lot more to superhet design than this - but I'm sure you'll agree with me that that lot will do for now! Good luck. / Al.