#CRYSTAL FILTER DESIGN SERIES#

At the same time, the multiplicity of crystals will also insure very steep skirts outside the desired passband.В These kinds of filters are often optional filters and are usually found in more complex, communications grade receivers.

It would be made of several crystals spaced in frequency to give a fairly flat topped response over a 2.5 to perhaps 6 or 8 kHz bandwidth to allow the sidebands of the SSB signal or AM signal to be passed without muffling the sound. There might be a design that leaves the top a little flat to avoid ringing in the filter.ДЄ multi pole filter designed for Single Side Band or AM might be made quite differently. A filter designed predominantly for cw operation might be made up of crystals very close in frequency to give a very sharp, narrow filter with steep skirts. The purpose of the filter determines how the lattice is set up. These filters make use of more than one crystal and are sometimes referred to as ' crystal lattice' filters or ' multi pole' filters. More complex filters can be built to shape that response. The closer in the skirts as the response goes down, the sharper the selectivity. The lower parts of the curve are referred to as the 'skirts'. The 60 DB down figure indicates the frequency separation at which all but the strongest signal would be practically gone.ДЄ graph of the response can be either very 'pointed' or sharp, or slope down gently. The 6 DB down figure indicates the frequency separation at which the signal strength is half what it is on the peak. The difference between the two gives some idea of the sharpness of the filter or tuned circuits in the radio.

The points usually used are 6 DB down and 60 DB down. The selectivity figures for receivers are usually referred to in how many DB down the signal is from the resonant frequency. The center frequency is the resonant frequency with response falling off in a curve. The peak on a single, simple crystal filter is fairly sharp. The crystal will pass frequencies on its frequency fairly well, with response falling off rapidly on either side of its resonant frequency.

#CRYSTAL FILTER DESIGN SERIES#

The simplest form of crystal filter consists of one crystal cut for the Intermediate Frequency and at some point in the IF chain in series with the signal path. This was considered a significant expense to add to receivers in that time period and would boost the price considerably.В Now because of collectors looking for even the lower level, or entry level receivers, things have kind of evened out. The next selectivity enhancing feature is the crystal filter. We have covered the Q multiplier in a previous post.

CW reception on the amateur bands would be possible but digging out the real DX would be difficult because of interference from neighboring stronger signals. This is not to say that two broadcast stations 5 kHz apart could not be identified, but that if one was particularly stronger than the other, there would be considerable 'slop' between the two of them and listening would not be comfortable. When it gets down to it, almost all boat anchor receivers that have two IF stages with a 455 kHz Intermediate Frequency ( which covers about 95% of those built in theВ 50's, 60's and 70's) the selectivity of all of them are pretty much equal.В About allВ you can count on is about 10 kHz selectivity without some kind of additional selectivity enhancement. After the usual ' default ' requirements of at least one tuned RF amplifier stage and at least two IF amplifier stages comes some means of tightening selectivity. While looking at older receivers for SWL DX purposes, there are a few features that are particularly helpful.

Crystal filter design