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Bandpass filter measurement anomalies

Steven Dick
 

Hi all. Have any of you observed any strange wiggles on the left edge of bandpass filter measurements?  This had been bugging me for awhile and I had attributed it to bad cabling, non-linearities in toroids, etc but I think I found the cause.  See writeup in the K1RF folder, file "bandpass Filter Measurementr Anomalies".  I think harmonics generated in the ERA-3+ output stane are causing wiggles and jiggles I see on the left side of a bandpass filter response. Has anyone else seen a similar problem?

Also see, in the writeup how the output amplifier is not needed if the type II DDS with its push-pull transformer drives the antialias filter directly with no attenuator, providing a 6dB power increase compared to ther single ended version as shown in the AD9851 data sheet, page 11. With the antialias filter driven directly bu the transformer output and a single 6dB attenuator, you can get anout -9dBm output with no amplifier, eliminating any harmonics created in the output stage. I will try this and report back.

Regards,
"Digital Steve", K1RF


Nick Kennedy
 

That's a very interesting observation, Steve.  

I've not done a good job of keeping plots I've made where I can find them, nor having them properly labeled. Also, most filter plots were done only wide enough to see the shape down to a certain floor, so the half-frequency point wasn't covered.

However!  I do have one plot of a 14 MHz filter that goes below 7 MHz and I see the effect you mention. I see a "mini-me" response in the half frequency area.

It's interesting to try to quantify the harmonic output with this. My peak at 14 MHz is at about -3 dBm and my 7 MHz peak is at about -45 dBm so it looks like my 2nd harmonic at 7 MHz is 42 dB down.

This plot is also wide enough for me to view the 1/3 frequency point, but I don't see evidence of any 3rd harmonic content.

73-

Nick, WA5BDU

On Mon, Jun 20, 2016 at 7:15 AM, sbdick@... [PHSNA] <PHSNA@...> wrote:
 

Hi all. Have any of you observed any strange wiggles on the left edge of bandpass filter measurements?  This had been bugging me for awhile and I had attributed it to bad cabling, non-linearities in toroids, etc but I think I found the cause.  See writeup in the K1RF folder, file "bandpass Filter Measurementr Anomalies".  I think harmonics generated in the ERA-3+ output stane are causing wiggles and jiggles I see on the left side of a bandpass filter response. Has anyone else seen a similar problem?

Also see, in the writeup how the output amplifier is not needed if the type II DDS with its push-pull transformer drives the antialias filter directly with no attenuator, providing a 6dB power increase compared to ther single ended version as shown in the AD9851 data sheet, page 11. With the antialias filter driven directly bu the transformer output and a single 6dB attenuator, you can get anout -9dBm output with no amplifier, eliminating any harmonics created in the output stage. I will try this and report back.

Regards,
"Digital Steve", K1RF



Steven Dick
 

Thanks Nick. The Type II PHSNA design drives the ERA-3+ slightly more than the Type I PHSNA so it may show up the effect more. About 2.5dB more drive, which will increase the harmonics in the Type II design.
 
“Digital Steve”, K1RF
 

Sent: Monday, June 20, 2016 9:12 AM
Subject: Re: [PHSNA] Bandpass filter measurement anomalies
 
 

That's a very interesting observation, Steve. 
 
I've not done a good job of keeping plots I've made where I can find them, nor having them properly labeled. Also, most filter plots were done only wide enough to see the shape down to a certain floor, so the half-frequency point wasn't covered.
 
However!  I do have one plot of a 14 MHz filter that goes below 7 MHz and I see the effect you mention. I see a "mini-me" response in the half frequency area.
 
It's interesting to try to quantify the harmonic output with this. My peak at 14 MHz is at about -3 dBm and my 7 MHz peak is at about -45 dBm so it looks like my 2nd harmonic at 7 MHz is 42 dB down.
 
This plot is also wide enough for me to view the 1/3 frequency point, but I don't see evidence of any 3rd harmonic content.
 
73-
 
Nick, WA5BDU
 
On Mon, Jun 20, 2016 at 7:15 AM, sbdick@... [PHSNA] <PHSNA@...> wrote:
 

Hi all. Have any of you observed any strange wiggles on the left edge of bandpass filter measurements?  This had been bugging me for awhile and I had attributed it to bad cabling, non-linearities in toroids, etc but I think I found the cause.  See writeup in the K1RF folder, file "bandpass Filter Measurementr Anomalies".  I think harmonics generated in the ERA-3+ output stane are causing wiggles and jiggles I see on the left side of a bandpass filter response. Has anyone else seen a similar problem?

Also see, in the writeup how the output amplifier is not needed if the type II DDS with its push-pull transformer drives the antialias filter directly with no attenuator, providing a 6dB power increase compared to ther single ended version as shown in the AD9851 data sheet, page 11. With the antialias filter driven directly bu the transformer output and a single 6dB attenuator, you can get anout -9dBm output with no amplifier, eliminating any harmonics created in the output stage. I will try this and report back.

Regards,
"Digital Steve", K1RF


 

Virus-free

Rudolf Reuter
 

Hello Steve,

Thank you for the very interesting post.

Because I did not like a wide band amplifier because of the
lack of qualified measuring equipment, I designed a passive
output filter for the DDS generator. Please look at:
http://www.rudiswiki.de/wiki9/AmateurRadioDDSgenerator#Filter_AD9850_from_DL5FA

With a symmetric transformer I reach about -3 dBm output level.
If I increase the DDS DAC output current within a allowed range,
0 dBm output level is achievable.
http://www.rudiswiki.de/wiki9/AmateurRadioDDSgenerator#Output_level_0_dBm

73, Rudolf DL5FA

Steven Dick
 

Thanks Rudolf. Good info.

Regards,
"Digital Steve", K1RF

N5IB
 

A recent check with a spectrum analyzer on a Type-2 DDS module, whose on-board LPF had been removed, mounted on an Experimenter Board with the "push-pull" output transformer and a 50 ohm termination, showed the 2nd harmonic (of 10 MHz) down about 30 dB from the fundamental. This board does not include the wide band amp.

I'll see about pulling out another Type-2 DDS on a Type-2 PHSNA board with the MMIC amp and see what contribution the amp makes.

I saw the same effect when testing a bunch of our club's BPFs a couple of years ago with a Type-1 PHSNA. See the file "BRARC BPF Sweeps 17 Dec 2013.pdf" in the Files area. On those plots it looks like the 2nd harmonic was 28 dB or so down.

This is one of the few places where the interchangeable LPFs on the Type-2 PHSNA could be a useful feature. You can have a selection of them to use when testing filters at frequencies well below their pass band, where harmonics will give ghost responses. You can usually get another 20 dB or so of harmonic suppression.

Jim, N5IB



---In PHSNA@..., <sbdick@...> wrote :
Hi all. Have any of you observed any strange wiggles on the left edge of bandpass filter measurements?  This had been bugging me for awhile and I had attributed it to bad cabling, non-linearities in toroids, etc but I think I found the cause.  See writeup in the K1RF folder, file "bandpass Filter Measurementr Anomalies".  I think harmonics generated in the ERA-3+ output stane are causing wiggles and jiggles I see on the left side of a bandpass filter response. Has anyone else seen a similar problem?
Regards,
"Digital Steve", K1RF


Steven Dick
 

Yup, the file "BRARC BPF Sweeps 17 Dec 2013.pdf" in the Files area shows the same effect I was posting about - the weird duplicated filter shape on the left side of the bandpass filter plots.

"Digital Steve" K1RF