LPF frequency curves #lpf
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I was playing around on the bench and decided to check the charicteristics of a couple LPF boards I have put together. I used my Fox Delta Antenna Analyzer to feed the In port of the LPF with a 50 ohm dummy load on the Out. Was surprised to see a pretty much flat line all across the band spectrum. The FD analyzer measures reflected power, is this where I am going wrong? I see on the website others have used feed thru testing, which I was just too lazy to set up on my HP 8935. Any thoughts from you folks who have done some tech measurements on these little guys?
-- Wes AE6ZM Hereford, AZ ? |
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Wes,
The antenna analyzer is only looking at the SWR (reflection) and will not give you an accurate indication of the transmission characteristics. Since you have a good test set, use the tracking generator in the HP 8935 and look at the transmission. I normally use my MiniVNA or an Array Solutions VNA-2180 to look at the "as built" filters. For higher frequencies, I have an older HP spectrum analyzer that is good to 3 GHz (nice when setting up my UHF repeater filters). If I did not have those available to me, I have used an oscilloscope with a 50 ohm load. Feed the filter with a steady (or at least, known) signal level and record the voltage to the load into a spreadsheet. Do the math in the spreadsheet and generate a graph. Much more time consuming than setting up your instrument and doing a direct measurement... --------- Paul KE7HR |
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On Mon, Dec 25, 2017 at 03:50 am, David Bowman wrote:
http://www.foxdelta.com/products/AAZ-0217MX/aaz-0217mx.pdfPaul, thanks for your insight. Yeah, the 8935 has built in software to perform this kind of test. I am building a N2PK VNA (slowly) so that would be an option too. David - that is the newer version of the FD unit. I have model 0713A, purchased a few years ago. I have used it to check antennas, but never for filters before. The manual shows the setup I used, and a sample trace which looks as I would expect for a LPF. The trace I got was pretty much flat, rising slowly with increasing freq. I have examined the boards, check continuity and for shorts, and for correct components. It all looks good, so will have to play some more. Thanks again guys. Will post further when I get some more done. ? -- Wes AE6ZM Hereford, AZ ? |
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I measured one of my 40 meter LPF from QRP-Labs (I am working on another filter project anyway) with the VNA-2180.
In the transmission mode the filter looks like it should. In the reflection mode, with a good 50 ohm load, the filter has a SWR curve that is not flat but rather what Wes was expecting. I guess I better stop replying to postings at 3 am... Screen shots attached. --------- Paul KE7HR |
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I put a couple of documents about filters and design software, in the files section. I worked through the modeling and simulation of the stock 40 meter filter and an optimum design. Then, I constructed both designs and did measurements - for those curious about how the filters work and possibly why the parts variance has caused some to have low power output. The optimum design might be a solution for you (or not).
--------- Paul KE7HR |
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On Wed, Dec 27, 2017 at 10:19 PM, Bob Sutton via Groups.Io <zl1rs@...> wrote:
The output from the synth is squarewave-ish, driving different PA designs in the qrp-labs product line, so not looking the same after amplification. on top of that we have coupling in the pcb, grounding and cables that leaks a bit more of the higher order harmonics past the filters. That said, who knows what questions/problems may have arised if this optimized filter was the design in the qcx (: If/when I rebuild my U3S to include the 5W PA, I may try these designs in the LPF modules as they're a lot easier and cheaper to mess with rather than on the qcx. Also ordering the latest design on the relay-switched board. |
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The synth puts out a more or less square wave which is worked on by the subsequent stages of amplification then filtering. A square wave is the fundamental and all the harmonics. I recall one guy that posted a spectrum plot of the raw output from the Si5351 with evenly spaced spurs out to around 1 GHz. Just what was expected from a square wave.
Most really useful amplifiers have some non-linear properties. Kit built stuff can be quite variable. The harmonic content has to be dealt with to keep from interfering with others. A LPF reduces the harmonics and just leaves the fundamental as a, hopefully, nice sine wave. The design example that I did and the filters constructed and measured were not dealing with a non-linear waveform - the test gear has a nice signal. A spectrum analyzer will need to be used to see the output from a real amplifier using the filters. It may be quite possible that the amplifier has some "self harmonic suppression" in the way that the signal is being amplified and the LPF has less to do and will therefore have a lower "spike" at the harmonics even though the filter has less suppression at the harmonic frequency. Tradeoffs are everywhere! The large combination of tests reference on the QRP Labs harmonics page attests to how different parts placements and ordering have an effect on the performance. I am impressed by the constructed filter matching what the software said it should be doing. When you play with parts values in the software, it is easy to see what becomes critical to the filter shape. The software is free, it just takes a bit of time to learn what is going on. --------- Paul KE7HR |
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I just put the rest of the low pass filter things that I did, which were on different parts of the site, all in the same LPF folder now. The original LPF simulations were from 4 years ago. Sadly, the EMRFD software is no longer (easily) available that I did the original plots in. The Tonne Software programs have become my "go to" for most things now.
--------- Paul KE7HR |
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On Thu, Dec 28, 2017 at 1:39 PM, Al Holt <grovekid2@...> wrote: On Wed, Dec 27, 2017 at 10:49 pm, Paul Jorgenson KE7HR wrote: AFAIK, these single transistor amplifiers (class C, E) have a non symmetric output, looking more like half wave recitified AC even when driven with a square wave, thus contains large even harmonics. Of course it will be a mix of both, but it's not free from the even ones (: |
Wes WB7BR
Paul Jorgenson KE7HR
Daniel Ekman SA2KNG