Ask a Question

sanchayita

"Double-spotting" is a term that means that the wanted station is tuned in at two spots on the dial. These spots would be just 60kHz apart if an IF of 30kHz is used. So how does this occur? In a superhet receiver, the local oscillator frequency is offset from the wanted station by the frequency of the IF amplifier. For example, let's say that the wanted station is on 800kHz and the IF is 30kHz. This means that the local oscillator (which is usually higher in frequency than the tuned station) will be on 800 + 30 = 830kHz. However, if the selectivity of the RF stage is quite poor, a station on 860kHz will also give a 30kHz IF output when mixed with the local oscillator (on 830kHz). As a result, two stations - one on 800kHz and one on 860kHz - will be received at the same time. If the receiver is now tuned to 740kHz the oscillator will be on 770kHz. However, this will also give a 30kHz IF output from the 800kHz station. This means that the 800kHz station is heard at both the 800kHz and 740kHz positions on the dial. Click for larger image This large IF transformer includes a top-cap grid connection lead. Indeed, it was virtually impossible to stop double-spotting on these early sets with very low IFs, as the selectivity of the aerial and RF tuned circuits was quite poor. But even today, with much higher quality materials, double-spotting would still be a major problem using such a low IF. Double-spotting was a real annoyance and so designers set about solving this problem. As a result, triode valves were used for only a short time in superhets, being quickly replaced by the tetrodes and pentodes that were being developed during this time. The latter valve types had greater gain at RF compared to triodes and so generally didn't require neutralisation. And that in turn made it possible to select a higher IF to help overcome the image problem. The next frequency selected was around 175kHz. This meant that the image frequency was now 350kHz (ie, 2 x 175kHz) away from the desired frequency (instead of being just 60kHz away). This meant that the image was rarely observed on those receivers that featured an RF stage - at least on the broadcast band. However, if the receiver had no RF stage, it only had the selectivity of the aerial coil to rely on. Unfortunately, this was insufficient to provide image rejection and so the image was still quite evident - although further away. To overcome this problem, some sets used a bandpass double-tuned aerial coil network. However, this still involved using a 3-gang tuning capacitor, despite the absence of an RF amplifier stage.