Superheterodyne Receivers - Tracking & Alignment of Tuned CircuitsAuthor: R.J.Edwards G4FGQ © 24th October 2002
RF Signal Tuned Circuit Local Oscillator Tuned Circuit
C ¦ ¦ ¦ Cpad C
RF C ¦ ¦ ¦ C osc
coil C === Ctune === Ctune === Ctrim C coil
C ¦ \_____ganged_____¦____\ ¦ C
C ¦ capacitors ¦ ¦ C
The RF and local oscillator tuning capacitors are mechanically coupled on the same shaft and have the same capacitance/rotation law. As the shaft is rotated both circuits increase and decrease together in frequency. When the local oscillator is set such that its difference from the signal frequency is equal to the intermediate frequency then any mis-tuning of the RF tuned circuit results in a loss of signal strength, an increase in adjacent channel interference and possibly between-channel cross-modulation. This program computes component values. The tuning capacitor can be swept over the frequency band for error checks.
For the circuit shown, oscillator frequency is always higher than the signal frequency. The intermediate frequency can be either higher or lower than the signal frequency.
In this program RF mis-tuning is described as an 'error' and is measured in kHz. The error is also measured as a fraction of the wanted signal frequency. If the fractional error exceeds approximately 1/Q of the RF tuned circuit then best use of RF signal circuit selectivity is not being made.
Within a particular waveband there are three frequencies of zero-error. Optimum performance is obtained when the errors at the four frequencies of greatest error all have similar magnitudes. Two of these four frequencies are at the band edges.
Optimum performance over the whole band is obtained when the outer pair of zero-error frequencies are about 5 percent of the bandwidth inboard from the edges. The remaining zero-error frequency is best located half-way between the others.
In practice, both of the ganged capacitors have trimmer capacitors and these are used to align the higher frequency band edge with tuning dial markings. Depending on circuit strays the oscillator trimmer will be set to the larger value.
Small tuning coils can be set to precise value with iron-dust or ferrite slugs.
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