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Re: RF circuit Alignment in Valve Radios

Geschreven door Henk Roovers op 05. Nov 2004 21:09:00:

In reactie op: RF circuit Alignment in Valve Radios geschreven door PANDU RAJAN op 05. Nov 2004 15:14:41:

This has to do with the total bandwidth of the receiver on SW. The received transmittersignal is mixed with a local oscillator, resulting in two new frequencies. In most cases the transmitter frequency is subtracted, resulting in the intermediate frequency (IF). So if you are receiving a transmitter on 4000 KHZ, in a single superheterodyne the oscillator runs at 4452KHZ. So you get two new frequencies, 452KHz and 8845KHz. The lower frequency is passed by the IF filters and the higher is completely stopped. However, a transmitter working on 4904KHZ (called a mirror), also results in two new frequencies, 4904-4452= 452 KHz and 4904+4452=9365KHz. The first beiing an unwanted signal, because you tuned to 4000KHz. A good preselection in the tuner could reduce this effect, but not eliminate it, due to its bandwdith. The only solution is increasing the IF frequency. The best solution is a dual superheterodyne receiver)First the signal is mixed to an high IF of e.g. 10MHz. Mirror frequencies are at a distance of 20MHz, far enough to be blocked by the antenna filters. To get a good selectivity, the signal is transformed to a lower IF (e.g. 452KHz) giving the possibility to built an IF amplifier with narrow bandwidth giving excellent selectivity.

You'll have to use the lowest frequency on your testgenerator.

Henk Roovers




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