Figure 1. - low cost plastic ganged variable capacitor in an AM radio
LAST MODIFIED:
I'm asked every so often to produce a page on antenna pre-selector filters.
An antenna pre-selector filter is nothing more than a narrow band filter, a topic already covered at some length on another page.
Some confusion results because the narrow band filter topic contains no provision for variable tuning. There are several reasons for this.
Contrary to your immediate impressions I find no particular benefit in introducing variable tuning to an antenna pre-selector filter. I can see a number of drawbacks however because your tuning element would of necessity be a variable capacitance. This would perhaps be a standard air variable capacitor or possibly varactor diode tuning.
These comments do not apply to variable inductance tuning which you are highly unlikely to ever achieve. It is of course superior, but not readily achieved cheaply.
If you study quite carefully and at full length my tutorial on narrow band filters you would readily appreciate that as the value of "C" alters then so the circuit impedance varies. Given fixed source and load impedances it follows the loaded bandwidth will similarly vary.
If you feel a tunable antenna pre-selector filter will give you the benefit of picking out one particular frequency e.g. 7.047 Mhz then clearly you haven't been paying attention in class.
I could give you a number of mathematical calculations to back that statement up but on reflection, if you don't already understand my reasoning then it is highly unlikely further maths would be of any help to you.
The most common example is with receivers which cover significant bandwidths which EXCEED twice the IF of the receiver. Your good old AM radio receiver is a very good example.
Your AM radio receiver usually has an IF of around 455 Khz and covers the band of around 540 Khz to 1650 Khz. From our topic on superhetrodyne radio receivers we know if our receiver were tuned to say 630 Khz the local oscillator would be tuned to 1085 Khz. The image frequency would be on 1540 Khz.
Both 630 Khz and 1540 Khz fall within the AM band of 540 Khz - 1650 Khz. It is impractical to construct filters of one octave bandwidth e.g. 540 Khz to 1080 Khz.
Here it is imperative to remove the image frequency and this is achieved by a tunable antenna pre-selector filter, the variable element being a low cost plastic variable capacitor ganged to a lower value variable capacitor which controls the local oscillator.
The RF value is usually around 160 pF max and the Oscillator section about 60 pF. Special coils are manufactured to match these ganged capacitors. AM RF Tuning Unit Coils are also manufactured to suit modern varactor diode tuning. The older plastic ganged variable capacitor is shown in figure 1 below.
Figure 1. - low cost plastic ganged variable capacitor in an AM radio
The ganged variable capacitor in an AM radio is the white square depicted in the middle of the photo with a yellow lead, situated adjacent to the band switch and to the left of the speaker. Check out any cheap AM Radio for a look-see.
These two ganged capacitors more or less track one another in frequencies. Frequency tracking is an art form and can involve considerable calculations. Suffice to say that in a cheap AM radio the tracking is only accurate on two points of the dial, about one third of the way in from both ends of the AM radio band.
In other words, at only two points on the dial will the antenna pre-selector filter and oscillator be precisely 455 Khz apart. This is called "two point tracking".
Another example is the FM band which covers 88 - 108 Mhz with an IF of 10.7 Mhz. Similar story. Again the "sole" purpose is to minimise out-of-band signals, i.e. image frequencies.
In order of my preferences:
earlier tutorial on Active Receiving Antennas
Newer Active Receiving Antennas tutorial
antenna basics
low pass filters
high pass filters
narrow band filters
double balanced mixers
Superhetrodyne Oscillator Tracking
reactance tutorial
earth dangers
baluns
impedance
inductance
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Created 7th March, 2002
Updated 13th March, 2002