Usually it is a frequency with less images and harmonics and that is not generally used for transmissions that is chosen to used as IF (See Chapter 9). When 9 MHz itself is chosen to be IF, along with these inherent problems, stage gain also will be considerably reduced because of being a HF signal. Here, the 9 MHz signal from the Product Detector is to be converted into 455 KHz., though after an IF stage of 9 MHz. Even though double conversions overcome these issues, a 455 KHz IF is always better. In 9 MHz IF circuits Crystal Oscillators are unavoidable. Coil-Capacitor combination tuned oscillator circuits always are subject to variations which is to be checked accordingly. There are two types of Crystal Oscillators - parallel tuned and Series tuned. This feature decides how the fine tuning trimmer capacitor is added - series or parallel.
If a Crystal in a Crystal Oscillator circuit is to be tuned exactly to the intended frequency, manufacturers instructions are to be observed seriously. This means that the attached circuit and the components used are to be strictly as recommended. In short, Crystals Oscillators require a signal frequency check using a frequency counter.
Subsequent losses due to being High Frequency when being used as IF can be considerably reduced by shielding the IF stages properly and using high efficiency coils and active components in the stages. In Fig. C-13/1 the circuit diagram of a high efficiency IC (TDA 7231) audio amplifier is given. This IC is small like LM 386 but not cheap as it is. This TDA 7231 has only very few external components. Also feed back related issues are comparatively negligible with reference to LM 386. If the speaker leads of IC LA 4510 IC is connected to the positive power supply line, TDA 7231 IC speaker leads are free and at the same time gives pretty good audio output also. Whatever be he IC used, the impedance of the speaker used should be matching with the output impedance of the IC. Even if the signal that passes through the Crystal Filter has the correct bandwidth, there could be significant strength loss. In short if the signal is from a simple general front end, practically nothing could appear in the filter output. The signal fed into the input of the Crystal Filter should be strong enough to withstand the damages done in the filter. This sort of signal strength loss happens not only with Crystal Filters like CFU 455 also, though not that seriously.
Only intended signals come out of the front end of an excellent receiver. Noise, harmonics, images, distortion ... everything will be removed from at the front end. Along with we may want more active stages that compensates the signal losses at filters. That is a front end inevitably requires effective filters of varying kinds and the regular resonant frequency loading circuits are not enough.
There are various types of filters (Ref. Chapter -4). The type and efficiency of a filter is very important here. Generally filters are classified as 3 pole, 5 pole, 7 pole, 9 -pole etc.
The umber of poles show the number of components in the circuit. A three pole filter contains either two capacitors and one inductor or two inductors and one capacitor.
There are various types of filters (Ref. Chapter -4). The type and efficiency of a filter is very important here. Generally filters are classified as 3 pole, 5 pole, 7 pole, 9 -pole etc. 
The umber of poles show the number of components in the circuit. A three pole filter contains either two capacitors and one inductor or two inductors and one capacitor. 
Low pass filters let through signals below a definite frequency and high pass filters let through signals above a definite frequency. Because signals below and above the central frequency of a bandpass filter have higher attenuation filters are not recommended at the output of VFOs. The word Attenuation refers to all sorts of strength reductions in a given signal. Both Lowpass and High pass filters are found in 'T' and 'P' configurations. 
The efficiency of a filter (low pass or high pass) is marked not by the number of poles used but the output measured in 'db'. Filters are categorised in strength values like 1db, 0.1db, 0.01 db, 0.001 db etc. Here the difference is at the value of inductors and capacitors used. This is based n the pass band ripple in that circuit. Before designing a filter circuit, confirm the level of attenuation that the circuit requires (in the case of High pass filters, attenuation to sub-harmonic frequencies). Then decide the maximum passband ripple that can be let through the circuit. Further, find out the VSWR (Voltage standing wave ratio). The VSWR will be 2.66 for 1db, 1.36 for 0.1db, 1.10 for 0.01db and 1.03 for 0.001 db. These basic information is enough to refer a chart and find out the capacitor values and inductor value necessary in a circuit.
In C-13/3 the details of a satisfactory front end filter is given with components values recommended at various meter bands.
3.5 MHz - L1 - 12T of 28 SWG, L2, L3 - 59 Turns of 28 SWG, tap at 40 the turn from ground. C1-130 PF, C2, C4 - 15 PF, C3 - 100 PF and C5- 115 PF.
7 MHz - L1 - 5T 24 SWG, L2, L3- 23 Turns of 24 SWG, L4 - same as L3 with tap at 17th turn from ground.
14 MHz - L1 - 3T of 22 SWG, L2, L3 - 16 Turns of 22 SWG, tap at 10th turn from ground. C1, C5 - 120PF, C2, C4 - 3.3 PF, C3 - 100 PF and C5- 115 PF.
21 MHz - L1 - 2T of 22 SWG, L2, L3 - 10 Turns of 22 SWG, tap at 7th turn from ground. C1, C3, C5 - 120PF, C2, C4 - 3.3 PF
28 MHz - L1, L2 and L3 are the same as in 21 MHz table except wire gauge is 24. C1, C3, C5 - 60PF, C2, C4 - 2.2 PF,
In many situations the values could change at trial and error experiments. It is better to wind the coils on toroid formers. Amidon T-50-6 Toroids are used for 80 and 40 meters while T-50 for 20, 15 and 10 MHz.
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