Topics

Low pass filter advice #vfo #40m #80m


Peter GM0EUL
 

Hi All
Can I request a bit of a reality check before I take my next steps.  I've made the vfo kit to add to my one-valve transmitter (this is the one that's the same as Hans's first transmitter, so arguably an ancestor of the QCX!).  The vfo has the 40m and 80m low pass filters and I had a look at their characteristics because I wasn't getting as much output as I expected.  

I've measured the peak to peak voltage output from the 40m filter on a scan from about 4 MHz to 11 MHz (have a look at the 1 min video here https://youtu.be/Lozyyh2OO9w  )  Ignore the actual numbers but look at the amplitude of the wave against the frequency (bottom left readout) The amplitude doesn't vary much from about 4 to about 9 MHz then it shoots up and peaks at 9.2 MHz then rapidly falls off and is almost fully suppressed by about 10 to 11 MHz. 

The 80m filter has similar characteristics and the peak output is at 5.3 MHz and its suppressed by just over 6 MHz 

I think I need to tweak the filters so that the peak output is at the bottom of the bands of interest (i.e pull it down by about 2 MHz by fiddling with the coils) is that right or am I interpreting the data wrong or misunderstanding what its telling me?

Any thoughts or definitive wisdom appreciated.

73
Peter GM0EUL


Alan G4ZFQ
 

The amplitude doesn't vary much from about 4 to about 9 MHz then it shoots up and peaks at 9.2 MHz then rapidly falls off and is almost fully suppressed by about 10 to 11 MHz.
Peter

Your description makes me wonder about the technique you used.
A LPF is flat until it begins to reject HF. There is no peak.

73 Alan G4ZXF


Peter GM0EUL
 

Thanks Alan- that puzzled me, I wasn't expecting a peak and don't know what to make of it or what sort of artefact may have caused it.  I want to try and understand it before I do anything to change it.

73
Peter GM0EUL


Evan Hand
 

Peter,
If I am reading the filter specification right, you might want to start with a 50ohm termination on the output of the filter before the input to your scope for your test.  Rerun the test to see what you get. 

You may also want to verify the impedance of the transmitter to be sure that it is expecting a 50ohm source.  If it is different, then either a resistor pad or a transformer may be needed.

Just a couple of suggestions.
73
Evan
AC9TU


Peter GM0EUL
 

Thanks Evan
That seems to have done it.  With 47 Ohm across the scope it behaves exactly as I would expect i.e the 40m filter is flat to about 8.2 MHz then drops off quickly.  Testing the 80m filter now but I'm expecting that to behave itself too.

Noted about the need for a transformer when coupling the vfo to the transmitter.  It isn't expecting 50 Ohm I don't think, but not sure what it is expecting.  I'll see if I can figure it out.

73
Peter GM0EUL


@CurtisM
 

Peter

Find your way to the website of AA8V, use your search engine to find Digital VFO for Vintage Transmitters.

https://www.frostburg.edu/personal/latta/ee/vfo/vfo.html

Of course the drive impedances are different for thermions. You might enjoy your visit to professor Latta's website.

73 Curt


Steven Dick
 

I assume it was properly terminated at both the load and the source.  Otherwise you will not get the proper filter response curve. You shouldn't feed the output of the filter into a high impedance scope input. It should include a 50 ohm termination at the filter output before it goes into the scope. You can improve the source termination by using a 50 ohm 3dB or 6dB attenuator at the input, rated for the power used.   Depending on the LPF design, it is possible to get a peak.  A Butterworth design  should give a smooth fall-off with no peak. Other designs may have a slight peak near cutoff or a rising response in the bandpass.

-Steve K1RF

------ Original Message ------
From: "Peter GM0EUL" <gm0eul@...>
Sent: 9/13/2020 9:28:14 AM
Subject: Re: [QRPLabs] Low pass filter advice #40m #80m #vfo

Thanks Alan- that puzzled me, I wasn't expecting a peak and don't know what to make of it or what sort of artefact may have caused it.  I want to try and understand it before I do anything to change it.

73
Peter GM0EUL

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Peter GM0EUL
 

Hi All
Thanks for the suggestions.  It does behave much more as I expected when properly terminated with a 50 ohm resistor across the probe.  I believe the tx input impedance is around 47 kOhm so I'll try coupling it with a 1:1000 transformer and a 10nf capacitor in series between the transformer and the tx input roughly where the xtal now is.

Curt- that's for that link.  Great site and lots of stuff I didn't know.

73
Peter GM0EUL