Topics

LPF frequency curves #lpf

Daniel Ekman SA2KNG
 



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:
.......A square wave is the fundamental and all the harmonics.......
I learned recently prepping for the Extra Class exam a sawtooth wave is composed of the fundamental and all harmonics; square waves are going to be predominately made up of odd harmonics as Bob Sutton's measurement bear out.

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 (:

Virus-free. www.avg.com

Al Holt
 

On Wed, Dec 27, 2017 at 10:49 pm, Paul Jorgenson KE7HR wrote:
.......A square wave is the fundamental and all the harmonics.......
I learned recently prepping for the Extra Class exam a sawtooth wave is composed of the fundamental and all harmonics; square waves are going to be predominately made up of odd harmonics as Bob Sutton's measurement bear out.

Many thanks to the folks at HamTestOnline for getting me over the hump!  

--Al
WD4AH

Bob ZL1RS
 

Some measurements just for information (as measured on 14 MHz with a HP8592B spec an).

The QRP-Labs Si5351a synth board 'raw output' contains the fundamental and fairly strong odd harmonics only.  The even harmonics are all well suppressed, by about -50dBc  (perhaps by a push-pull output stage in the IC?)

Si5351a output via a 0.1uF capacitor:
2nd harmonic less than -50dBc
3rd harmonic -10dBc
4th harmonic about -50dBc
5th harmonic -16dBc
6th harmonic about -50dBc
7th harmonic -20dBc

The PA stage does add some non-linearity itself.  The 'raw' output from a pair of BS-170's biased just at their 'turn on' point (more or less Class B) in a single band U3S (i.e. at the input socket of the LPF):
2nd harmonic -24dBc
3rd harmonic -15dBc

After plugging in a standard QRP-Labs 20m LPF, the harmonic content at the U3S RF output is:
2nd harmonic -54dBc
3rd harmonic -52dBc

73, Bob  ZL1RS

Paul Jorgenson KE7HR
 

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

Paul Jorgenson KE7HR
 

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

Daniel Ekman SA2KNG
 

On Wed, Dec 27, 2017 at 10:19 PM, Bob Sutton via Groups.Io <zl1rs@...> wrote:
... but aren't the harmonics from the Si5351a already >10dB down on the fundamental, so the over all spurious emissions after the 40dB suppression of the LPF are already much greater than the 43dB required by your USA regulations?

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.

Bob ZL1RS
 

Hi Paul

The doc says about the stock 40m LPF ...

"The insertion loss across the band is low and the harmonics are suppressed but not as much as the FCC regulations (Part
97.307 says “(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a
station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB
below the mean power of the fundamental emission.”

... but aren't the harmonics from the Si5351a already >10dB down on the fundamental, so the over all spurious emissions after the 40dB suppression of the LPF are already much greater than the 43dB required by your USA regulations?

The 'optlowpass' filter sure is impressive!

73, Bob  ZL1RS



Paul Jorgenson KE7HR
 

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

Paul Jorgenson KE7HR
 

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

Wes AE6ZM
 

On Mon, Dec 25, 2017 at 03:50 am, David Bowman wrote:
http://www.foxdelta.com/products/AAZ-0217MX/aaz-0217mx.pdf
Paul, 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

 

David Bowman <g0mrf@...>
 

Hi Wes.

That looks interesting.... Is it this one you have?
http://www.foxdelta.com/products/AAZ-0217MX/aaz-0217mx.pdf

Unfortunately the manual doesn't show any pics of the unit in use.
However, I would have thought that it should show a flat line (low SWR or high return loss) up to just above the band of interest
and then the graph should start to change as the attenuation of the LPF starts to take effect.

Having the 50 load on the end is exactly right.....they all seem to work nicely here, but it is reassuring to see the plot of frequency if you have the kit to measure it.

Merry Christmas

David  G0MRF

Paul Jorgenson KE7HR
 

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

Wes AE6ZM
 

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