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Hi,
Hi, I am trying to scale an existing front end receiver (butterworth bandpass) filter to a different frequency range. Unfortunately, it has a transformer in the original design, so I'm stuck. I also don't know how to handle the load presented by the active front end component other than it's probably not significantly reactive. The existing filter is for a 7 Mhz receiver, I'd like to have a similar filter design for 50 to 200 Khz. Notice that the primary to secondary (total) turns ratio is almost 1:1, that both shunt capacitors in the band-pass filter are of the same value and that the path resistance on either side of the secondary, through the FST3126M to the low-pass filters is around 20-ohms. So what you are looking at (to a crude approximation) is a design impedance of 50-ohm all the way through. As for the toroid, for optimum Q you might want to look for a mix more suitable for the frequency range you quoted (maybe 15?) and increase the diameter to take the extra wire as you will need to scale the impedances for use at 50kHz. The FT series, I think, need less turns for the same inductance and so should be easier to wind. Finally, a simple low-pass filter would probably be all you need here as the transformer primary impedance will be a limit the low end. Filter response is dependant on source as well as load impedance and most antennas that you would use at those frequencies will be quite reactive and difficult to design a fixed input network for. You may be aware that a lot of HF receivers use a separate hi-Z antenna terminal for LF/MF. Having said that, if you do want to use a simple low-pass configuration, remember that a symmetrical 'Pi' filter with source and load impedances of 1-ohm and a cut-off of 1-rad/sec would use two 1-Farad capacitors and a 2-Henry inductor. To scale to new impedances and frequency, divide C (and multiply L) by the design impedance and then divide both C and L by the new cut-off frequency in rads/sec. Cheers - Joe |
#2
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Notice that the primary to secondary (total) turns ratio is almost 1:1, that both shunt capacitors in the band-pass filter are of the same value and that the path resistance on either side of the secondary, through the FST3126M to the low-pass filters is around 20-ohms. So what you are looking at (to a crude approximation) is a design impedance of 50-ohm all the way through. OK, that's about what I thought, but I was estimating closer to 15 ohms for ac resistance between the secondary winding and ground. So, I was having a hard time believing it could be 50 ohms in and out. It probably makes little difference though in the filter values or passband response. As for the toroid, for optimum Q you might want to look for a mix more suitable for the frequency range you quoted (maybe 15?) and increase the diameter to take the extra wire as you will need to scale the impedances for use at 50kHz. The FT series, I think, need less turns for the same inductance and so should be easier to wind. I was looking at the CT series, specifically a CT-50-57, which is a little bigger, but probably can contain all the wire easily. Finally, a simple low-pass filter would probably be all you need here as the transformer primary impedance will be a limit the low end. Filter response is dependant on source as well as load impedance and most antennas that you would use at those frequencies will be quite reactive and difficult to design a fixed input network for. You may be aware that a lot of HF receivers use a separate hi-Z antenna terminal for LF/MF. I'm aware of the separate antenna inputs some LF radios have. The lowfer group has quite a bit of information on antennas, and it seems that making a 50 ohm antenna or something approaching 50 ohms from a large single or multiturn loop isn't that difficult. Making a 15 or 20 ohm antenna is even easier, perhaps I should think about 15 ohm input impedance and a 15 ohm output impedance since it's actually easier to handle the antenna step up. Having said that, if you do want to use a simple low-pass configuration, remember that a symmetrical 'Pi' filter with source and load impedances of 1-ohm and a cut-off of 1-rad/sec would use two 1-Farad capacitors and a 2-Henry inductor. To scale to new impedances and frequency, divide C (and multiply L) by the design impedance and then divide both C and L by the new cut-off frequency in rads/sec. Most likely it's probably best to keep it at 50 ohms and not to worry about the slight mismatch in that frequency range. Thanks to you and to all who commented, it was just he sort of information I needed to put me back on track. Regards, T |
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