| Ten-Tec Model 224 Audio CW Filter |
This filter was used in the Ten-Tec ARGOCY transceiver, Model 525. When installed, it provided two selectivity positions centered at 750 Hz, selected by two push button switches on the front of the transceiver. The basic selectivity position has a single stage in series with the audio. The higher selectivity position has all four filter stages in series with the audio.
According to the manual, the filter has a center frequency 750 Hz.
- Switch position ONE has 450 Hz bandwidth down 10 dB at 400 Hz and 1,500 Hz.
- Switch position TWO has 150 Hz bandwidth, down 40 dB at 400 Hz and 1,500 Hz.
In switch position ONE only a single stage is in the audio circuit. As shown, a single stage of the filter has a F0 = 750 Hz, -3db BW = 326 Hz (587 - 913 Hz), Q = 2.3-2.6, Gain = 1. Below the -3dB Bandwidth edges, the filter response should roll-off at a rate of -6dB per Octave. Keeping the Gain low and the Bandwidth relatively wide, prevents the filter from ringing, when strong signals are encountered. So the manual specification seems to be pretty reasonable.
In switch position TWO, four stages of filter are connected in series. Each filter stage is exactly the same as the the filter described in the previous paragraph. By cascading similar filter stages, the filter -3dB Bandwidth is narrowed and the roll-off, beyond the -3dB points, gets increasingly steep. This aids in rejecting adjacent interference. Again, the manual specification seems to be pretty reasonable.
| Ten-Tec Model 245 Audio CW Filter |
Below is the schematic for the Ten-Tec Model 245 Audio CW Filter. It is essentially the same as the Model 224 with only a few differences. They are mostly physical differences and none of them change the frequency response characteristics.
- The Model 245 uses two LM747 Dual Op-Amps (14 Pin Dip) to replace the RC4136 Quad Op-Amp package. Note that the LM747 has two VCC pins, but only one Ground pin, for each Op-Amp package. So, in the drawing, the Ground pin is show twice for each package.
- The LM747 has two Null pins for each Op-Amp, but they are not shown on the drawing.
- The Model 224 uses a 22KΩ resistor for biasing the non-inverting input of each Op-Amp. But the Model 245 uses a 15KΩ and 6.8KΩ resistor in series, which is equivalent to 21,800 Ω. Why? I don't know.
- The Model 245 has four AF OUT pins as opposed to two on the Model 224. But note that AF OUT2 and AF OUT3, on the Model 245, do not have output coupling capacitors and output load resistors. These may be required if you are using these outputs.
- The Model 245 does not have a filter capacitor on the power (+12V) input. Whereas, the Model 224 has a 33uF filter capacitor (C13) on the power (+REG) input.
- The schematic that I obtained for the Model 245 did not have any Reference Designators. I used matching Reference Designators from the Model 224.