Crystal Bypass Filter

Crystal Bypass Filters

Installing a crystal filter in the IF section of a transceiver or receiver can be challenging, especially for CW reception. 

The conventional method involves disrupting the signal path to insert the filter, requiring a switching mechanism. Additionally, the filter's insertion can diminish sensitivity if the IF strip lacks sufficient gain. 

The presented crystal bypass filter addresses these issues and is particularly suitable for SSB transceivers, which demand better CW selectivity than the SSB filter alone provides. 

The SSB filter's skirt selectivity complements a single crystal filter centered within the transceiver's filter passband, enabling effective CW reception. The crystal bypass filter and SSB filter operate synergistically.

While this approach may not match the performance of a six-pole, 400-Hz CW filter, it significantly improves CW reception in congested band conditions compared to using the SSB filter alone. 

A simpler alternative would be an outboard audio filter. However, the primary drawback of this method is that the AGC remains functional in modern transceivers, allowing strong signals entering the SSB filter to influence AGC behavior despite being rendered inaudible by the audio filter.

Crystal Bypassing:

The concept of crystal bypassing is illustrated in Figure A. 

The emitter resistor's bypass capacitor offers no selectivity and is purposely chosen to ground the emitter for AC signals at frequencies the stage is designed to amplify.  Allowing the transistor to provide maximum gain.

Equivalent Circuit:

The equivalent electrical circuit of a crystal is depicted in Figure B. It exhibits two resonance modes: one formed by the parallel LC combination (high impedance) and the other formed by the series LC combination (low impedance). The two resonant frequencies are very close to one another. While the series-resonant condition cannot be influenced externally, the parallel-resonant condition is readily affected by wiring and crystal holder capacitance.

Crystal Bypass Operation:

Replacing the emitter bypass capacitor with a crystal grounds the emitter for AC signals at only one frequency, the crystal's series-resonant frequency. 

At all other frequencies, the stage undergoes significant degeneration, resulting in reduced gain. 

The extent to which the crystal grounds the emitter determines the loss introduced by the crystal.

 With high-Q crystals, characterized by very low equivalent resistance at their series-resonant frequency, the loss is minimal. I

n fact, it's possible that if an IF stage were inadequately bypassed to begin with, the crystal bypass could increase stage gain at the crystal frequency. On the other hand, if IF stage gain is unaffected by the bypass capacitor, the crystal bypass wouldn't provide substantial selectivity.

Practical application 

The bypass capacitor should first be lifted from ground to determine whether the crystal bypass method might be effective with a particular amplifier stage. 

If a considerable reduction in gain occurs and the bypass capacitor is fairly large (0.1 uF or so), indicating the circuit is of low impedance then crystal bypassing will work well.

Conclusion:

If you're seeking to enhance the CW selectivity of your SSB equipment, the crystal bypassing method presented here is an excellent option. While this circuit may not be compatible with all SSB gear, its compatibility can be easily determined using the previously described method. 

A simple, non-invasive modification can then be implemented to provide genuine CW selectivity with minimal component investment.

Ceramic Resonators as Alternatives:

In addition to crystals, (455 kHz) ceramic resonators can also be employed as effective substitutes in crystal bypassing circuits. These resonators offer several advantages over traditional crystals, including their smaller size, lower cost, and greater resistance to mechanical shock and vibration. 

Homebrew Radio Applications:

The concept of crystal bypassing holds significant value for designers of homebrew radio circuits. By incorporating this technique, hobbyists can enhance the selectivity and performance of their custom-built radios without introducing undue complexity or cost. The ability to seamlessly integrate crystal bypass circuits into existing SSB equipment further expands the versatility of this approach.

Overall, crystal bypassing presents a practical and cost-effective solution for improving CW selectivity in SSB radios. Its applicability to both commercial and homebrew radio circuits makes it a valuable tool for both radio enthusiasts and hobbyists alike.