To: |
tentec@contesting.com |
---|---|

Subject: |
Re: [TenTec] Omni VI 1st IF Narrow CW Filter Enable? |

From: |
"Dr. Gerald N. Johnson" <geraldj@weather.net> |

Reply-to: |
geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com> |

Date: |
Fri, 28 Jan 2011 11:30:54 -0600 |

List-post: |
<tentec@contesting.com">mailto:tentec@contesting.com> |

Ah, but describing the circuit with math allows easy analysis and creation without having to assemble a million combinations of parts and measuring to see the results. This filter is a low pass with the cutoff frequency selectable in steps. It uses the same coils for different cutoff frequencies. This filter is designed to have the same shape and terminal impedances in and out of the transformers for the selected frequency chosen by the rotary switch. The transformers give different filter circuit characteristic impedances for different frequencies, that allows the same inductors to present the same reactance proportional to the cutoff frequency and the impedance. The higher impedances between the transformer compared to the speaker impedance of 4 ohms makes these filters practical. To build at 4 ohms impedance the capacitors would be at least 2" x 4" x 6" each, but for a single cutoff frequency only two would be required. The cost of the impedance changes and frequency changes with constant coil inductances is that the capacitor value change twice as fast as the frequency change. So far, good capacitors in the ranges used are cheaper than good inductors, and it take a simpler switch to change the capacitors and the transformer taps than to change capacitors and coils. Though switching would be simpler to change complete filters, just two poles required. To look at it from a filter designer's viewpoint, the basic filter designs are often tabulated for an impedance of one ohm and a cutoff frequency of 1 Hz. Then we can scale them to the frequency and circuit impedance we desire. To scale them for frequency we multiply inductances by the frequency change ratio and divide capacitor values by that frequency change ratio. So to go from 1 Hz to 1 kiloHertz, we multiply the inductance values by 1000 and divide the capcitors by 1000. Then to change the impedance we scale again multiplying the inductance values by the ratio of the impedance change and dividing the capacitors by that same ratio. So to change from 1 ohm impedance to 5000 ohms, we multiply the frequency scaled prototype by multiplying each inductance value by 5000 and by dividing each capacitor by 5000. The transformer taps give a handy sequence of impedances of 8000, 4000, 2000, 1000 and 500 ohms. These transformere are made for selecting the speaker power level on a constant voltage 70 volt line in a PA system with many speakers distributed around the facility where you want more or less sound in spots. And while the power selected depends entirely on the impedance, they aren't rate that way. I use those different impedances to allow scaling to different cutoff frequencies while using the same inductors because good inductors are much more expensive and less commonly available than a wide range of capacitors. This technique of varying the impedance is not a common filter adjusting technique. I may have invented it. BUT IT WORKS WELL. There are four poles of switching, one pole for input transformer, one for the output transformer and two for the shunt capacitor banks. All on one shaft. The table on the first page shows the transformer impedance, the cutoff frequency, and the capacitance values required. Fundamentally, a low pass L/C filter takes advantage of the fact that the reactance of an inductor rises as we go up and frequency and that the reactance of a capacitor goes down as we go up in frequency. So the series impedance rises with frequency and the shunt impedance goes down with frequency so the voltage divider changes more rapidly than either alone to make a more effective low pass filter than either part alone. Adding more stages of each as in this filter makes the cutoff that much more rapid. There are literally dozens of different designs of low pass filters with differing ratios between L and C to give different cutoff shapes that have been derived over a century of higher math applications. There are often good reasons for choosing one over another, such as the slope of the cutoff, the 2nd and 3rd harmonic attenuation, the ultimate harmonic attenuation, the transient (e.g. ringing or lack of ringing) response, and for power filters, the circulating current in the parts. With more complex math it is the standard technique to derive bandpass, high pass, and band stop filters from the same 1 Hz 1 ohm low pass filter prototypes from sub sonic through microwaves made of coils and capacitors, or vibrating metal bits (as in a mechanical filter), vibrating quartz or ceramic, or chunks of waveguide suitably modified by added metal or plastic bits as well a active filters either with op amps or by digital means. 73, Jerry, K0CQ On 1/28/2011 1:09 AM, Richards wrote: > |

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