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Band Pass Filter High Attenuation
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Greetings all. I seem to have an issue with my Band Pass Filter. This was built using the components from Sunil Lakhani on eBay. The filter was attached to a spectrum analyzer as shown in the attached photo. Sweeps were performed. We're measuring an insertion loss of >30-40db through the filter. This matched our on-air results. The receiver only heard one CW station on 20m while my dad's Yaesu heard dozens of signals on the same antenna. Tuning the BPF did peek it some, but never was the loss less than 30db. The testing setup is correct because we tested the 10MHz crystal filter on my board and it performs beautifully and draws a nice picture on the spectrum analyzer. Any thoughts? Is the BFP terminated on both ends top 50 ohms? The entire schematic is here:? |
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As I recall, this BPF is about 200 ohms in and out. If you used the IF transformers supplied with the kit, suspect first the coils of these transformers.? They are quite fragile and it is easy to get a poor or nonexixtent connection through them.
First, were they oriented right? Second, it is easy to pull one of the pins out of alignment and even break the connection internally. Check them very carefully. Third, there is a specific and quite narrow window which creates the bandpass. Triple check your alignment. The proper final inductance should be about 4 uH (for 20m). Hope this helps. john AD5YE ---In BITX20@..., <marktbaldridge@...> wrote : Greetings all. I seem to have an issue with my Band Pass Filter. This was built using the components from Sunil Lakhani on eBay. ----The entire schematic is here:? |
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Okay. That is helpful information. A couple questions:? 1: Can I assume that I will for the most part tune the inductors together? To make the correct bandpass, they must all be the same value, right? 2: Can I attach a 50 ohm spectrum analyzer as I showed in the link? Or will the impedance mismatch cause the filter to not function properly? On Tue, Jan 3, 2017 at 12:41 AM, iam74@... [BITX20] <BITX20@...> wrote:
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Mark Baldridge 608.561.3853 |
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---In BITX20@..., <marktbaldridge@...> wrote :
Okay. That is helpful information. A couple questions:? 1: Can I assume that I will for the most part tune the inductors together? To make the correct bandpass, they must all be the same value, right? [john]? More or less. Please note that the 82 pF. caps in the original schematic are wrong. Most people get by with about 33-39 pF. Those and 4-5 uH. seems to tune 20m well. Note that the tuning for the middle transformer is a bit different than for the outer ones. This is normal for a LC filter. 2: Can I attach a 50 ohm spectrum analyzer as I showed in the link? Or will the impedance mismatch cause the filter to not function properly? [john]? If it does, a simple 4:1 transformer. or even an inline resistor at each end, will probably straighten it out. Experiment. I assume you are disconnecting the BPF temporarily from the circuit anyway. Include an attenuator to control your input signal. Tune for the LEAST coherent signal level. What I did was build a new BPF based on the one in the BITX20A. It works very well and is a bit easier to adjust. You can also build a new one if you have 3 or 4 FM IF transformers handy. YRMV. Hi. john AD5YE |
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