Re: QRP TX LPF illustration

Hi,

?

A while back, I noticed the familiar double-pi filter circuit that’s often used at output of transmitters. One day, I did some calculations and realized the connection between the values of L and C’s (each “pi” filter has an inductance L in series and two C’s in shunt at each end). For each pi filter, at the frequency at which a tank of L and C (mounted in parallel) meaning L*C*(w^2) = 1, the impedance as seen at the input is equal to the inverse of the impedance of the load at the output, times L divided by C. Stacking two of these pi filters in series, the input impedance is equal to the load impedance (again, this is at the frequency which L and C resonates).

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I made a video (poor quality, sorry about it), it describes the gists of the above explanation:

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When an inductor L is substituted by a parallel L’/C’, as long as the impedance of the L’/C’ is equal to that of the original L, the transformation of impedance (from load to input) at the frequency of interest still stays the same. However, the parallel L’/C’ would block the frequency at which they resonate at. This can be used to reduce substantially a harmonic of the frequency of interest.

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Values of L and C are chosen with considerations of maximum of voltage across the capacitor at the middle (2 of C’s) and the currents that flow through the inductors L’s and C’s. Higher currents would make the filter having larger ohmic loss. Too high of a voltage mike blow capacitors.

I simulated in LTSpice and described this half-wave filter here (the article is in Vietnamese but you can ask Safari to auto-translate to English, it’s not 100% but close enough):?

?

Thang Le

AA6SV

Re: QRP TX LPF illustration

#9529

Hi Goody,

I built the W5USJ 80M LPF from the chart just now and tested it; I used T37-2 toroids, 22 and 23 turns, inductances set with the DE-5000 LCR meter, silver mica caps except polystyrene for the 1200 pF center cap. Tested with the transmitter adjusted for 45 Vpp (5W) output at 3550 kHz. I then reversed the LPF with the higher impedance side to the TX--this gave a reduced output of 37.7 Vpp (3.55W) once more to demonstrate to myself the newly discovered property these trap filters are not symmetrical and the lower impedance side needs to go to the TX output. (I then reversed the filter and obtained the same output as before.)

73,

Steve AA7U

Show quoted text

On 5/1/2025 10:50 AM, Anthony Good via groups.io wrote:
Hi Steve,

Thank you for the information!? I was wondering how one designed these as I wasn't finding any direct design tools online. The chart will be enough to get me started; I'll build?it and do some cut-and-try with the NanoVNA.

73
Goody
K3NG

80M W5USJ LPF.png

Re: QRP TX LPF illustration

#9528

Hi Steve,

Thank you for the information!? I was wondering how one designed these as I wasn't finding any direct design tools online. The chart will be enough to get me started; I'll build?it and do some cut-and-try with the NanoVNA.

73

Goody

K3NG??

Show quoted text

On Wed, Apr 30, 2025 at 4:30?PM Steve Ratzlaff via <ratzlaffsteve=[email protected]> wrote:

Hi Goody,

For my example I used an existing LPF and simply used each inductor to calculate what resonating cap worked at the 2nd and 3rd harmonics and picked the closest standard value cap to use. But once you do that you also need to determine the filter's input and output impedance with the new trap caps installed--the new filter is not symmetrical with one end having a higher impedance and one end having a lower impedance. You want to install the filter such that the lower impedance side goes to the transmitter output and the higher impedance side goes to the output/antenna. I entered this filter I showed into the old? LC filter designer, where you can enter an existing design, then analyze it, which includes selecting either input impedance or output impedance, checked at the fundamental frequency of the LPF. I've only recently discovered this property that trap filters are not symmetrical, and verified by trying them both ways with a 5 watt TX--installed the right way (with lower impedance to the TX) they put out full power; installed the wrong way (higher impedance to TX), output is much less! I've never seen this mentioned anywhere before.

If you'd like, you could give me the schematic for your existing filter and I can find the in/out impedances of the new filter after you've selected the two trap caps (or I could determine the cap values too).

W5USJ did some TX LPFs for qrpme some years back--I've built and used the ones for 40M and 20M, and also determined the input and output impedances of these filters and marked them on the sheet--those filters work very well--which is attached.

73,

Steve AA7U

On 4/30/2025 12:49 PM, Anthony Good via wrote:

I have a homebrew 80m CW rig I built 20 years ago that was out of spec and I'm re-doing the final PA and LPF.? Did you use an online calculator to determine the values?? I'd like to try this harmonic trap technique.

73

Goody

K3NG

On Wed, Apr 30, 2025 at 1:43?PM Steve Ratzlaff via <ratzlaffsteve=[email protected]> wrote:

We're all aware of the current FCC regs for harmonics and spurious
outputs from a transmitter needing to be -43 dBc or greater below the
fundamental output. Here's a 40 M LPF with the original response of the
filter when the current -43 dBc wasn't in effect vs. with adding the
trap capacitors across the two inductors, showing how the filter easily
surpasses -43 dBc--quite a dramatic difference. (I used a 40 dB tap for
these pictures.)

73,

Steve AA7U

Re: QRP TX LPF illustration

#9527

Wow that is an amazing difference!? Next time I need one I will try to use this.? Sounds like the design is a bit more involved though.

?

Thanks!

Re: 1W and 5W build guides

#9526

8 have never seen the videos and would appreciate if they were available again!

Thanks ??

Re: How good is the 1N4004 diode as a varactor

#9525

Like Dave mentioned, there is info in the old qrp group from the 25th anniversery NC-40 build...

Here is what I wrote (message #14308):

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"I am running a 1n4007 in mine, works great.
I am using Chuck's crystals so my VFO is at 2.9mHz but I get 60KHz range out of it.
Don't think it is as stable as Chuck's but I tested from cold room start and for longer
time... ?about 100Hz.
Didn't buy the specified varactor but tried LEDs, 1N4001, 1n9148, etc. ?best was the
1N4007."

?

73,

Gary

WB6OGD

?

Re: How good is the 1N4004 diode as a varactor

#9524

If anyone has a project that needs an MV209 (40pF at 1V to 10pF at 9V) or an MV2101 (10pF at 1V to 5pF at 9V), I have some extras. But to be clear I'm not moving them from my spare stock to your spare stock. I will let you have 1 at a time for a specific project or repair.

You can contact me at thomas.b.seeger(at)gmail.com

73 Tom

Re: 60 Meter homebrew rig IF frequencies

#9523

For 60 meters there are channel bandwidth requirements.

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"Amateurs are permitted to operate on five frequency channels, each having an effective bandwidth of 2.8 kHz."

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I believe operating on the steep side? of the skirt would? be best practice (At least on 60 meters).

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All of the filters I've designed so far are 6 pole as that? provides a steep enough skirt to get the carrier rejection below -40db from the filter passband (With the help of the doubly balanced mixer).

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Steve

WA6ZFT

Re: How good is the 1N4004 diode as a varactor

#9522

I'm assuming you've seen this Doug, in the old QRP-Tech group?

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/g/qrp-tech/topic/1n4004_vs_mv209_tuning_range/6523696

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Dave

AA7EE

Re: 1W and 5W build guides

#9521

If I put a keyer into a project, I use the one that Diz used in the 1W and 5W series.
It is the same keyer for both W's, just a different output on power up.? One did the 1W and the
other did the 5W in Morse.

<>

is the source and the hex.? I just got in the mail today an UNO R3 for programming
some ATTiny45s.? The 1W for 12m was missing its keyer chip.? I assume that I
yanked it out for some project and forgot to put it back.? You know the drill.

I wish to thank Mike MAIORANA, KU4QO, for keeping the legacy alive at KitsAndParts.com.
All the documentation is there in the retired kits section and he still has the 5W
PCBs for $5 each.? I loved the 1W and used it for CW training classes and introductory
kit building.? Don't know if Mike can bring it back at a price to compete with
todays market.

FYI,

On 4/30/25 13:12, Daniel KK4MRN via groups.io wrote:
Thanks Chuck!
I may celebrate and build my Organic 5W CW Transceiver Kit for 40m finally...
I have an extra PCB and keyer chip in case I screw up.?? But just so others know - the keyer chip and PCBs are still available.? The kits are no longer are.
And you know where to get the toroids too...? ;)
I think I will be able to do the SMT parts now with a better digital microscope I got on sale.? I few years ago - it would have been $100.? Now, this model runs about $65 with a small 7 inch screen.?? And I have gotten better at soldering and desoldering.? Good thin solder is key.? I have been reading what the experts are doing on here...
It was the SMT parts that held me up.?? Plus, I was afraid of the #34 magnet wire used in the toroids that easily break when I go to solder them.
But I remember the 1Watter, and I think it is was great kit with an awesome keyer.?? So, I look forward to the 5Watter being a great kit with an awesome keyer as well.
73 Daniel KK4MRN
_._,_._,_--chuck adams, aa7fo
QRP WAS 17m started Apr 2, 2025
WAS-17 QRP --> TX,FL,SC,GA,OK,AR,KS,AL,AZ,MN
               PA,CA,CO,WI,NE,ID,IL,NC,NY,WV
               MA,OH,OR,RI,MT,WY,WA
DXCC           W,VE,JA,PY,ZL

Re: How good is the 1N4004 diode as a varactor

#9520

Hans provides a circuit to measure its varying capacitance. Of course you can also install candidate diodes into your vfo that may be easier.?

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I realize in the old days folk sought a huge range. But note the venerable nc20 used a variable cap in parallel to expand the range. Or switching in a fixed cap. I am confident Doug has seen all these approaches.?

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My all time favorite may be the vcxo in the 38S.?

?

Curt

?

Re: How good is the 1N4004 diode as a varactor

#9519

Please ignore my last post. I wish these things could be edited after posting, but it doesn't work that way here! I was forgetting the influence of the 270pF cap connected to pin 6 of the 602. My brain is somewhat fried this evening. I need to learn not to post quite so quickly.

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Dave

AA7EE

Re: How good is the 1N4004 diode as a varactor

#9518

I have no experience with the 1N4004 as a varactor, but I did use a 1N4001 as one in a simple NE602-LM386 DC receiver on 40M. The inductor was 3.1?H (27 turns on a T50-7). With a voltage span of 3.2V-7.9V on the 1N4001, I was getting a tuning range of~7000-7054KHz.? That would equate to a (rather small) capacitance variation of ~164-167pF.

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If you're interested, I can open it up, fiddle with a few things, and provide the frequency swing for voltages on the 1N4001 below 3.2V. The write-up on the receiver is here -?

?

Dave

AA7EE

Re: How good is the 1N4004 diode as a varactor

#9517

What circuit did you use to test?

Re: How good is the 1N4004 diode as a varactor

#9516

Thanks for the info.? Very interesting that a 5mm red led works so well.? I'm going to do some testing.? ?Again, I appreciate the help.? Doug

Re: 60 Meter homebrew rig IF frequencies

#9515

In my BITX40 I used the VFO to switch USB/LSB for digital, so high side and low side. Worked great.Could I hear the difference? Yes. The filter skirt was different. Was it equally usable? You bet it was. And that was with just a 4 pole filter. Like this one. Check the plot on it.?

Show quoted text

On Wed, Apr 30, 2025 at 5:45?PM Kees PE7ST via <forum444=[email protected]> wrote:

On Tue, Apr 29, 2025 at 11:48 PM, hwstar wrote:

If only it were so easy (it isn't) Sideband inversion: When the LO is above the IF the sideband gets inverted. The Crystal filter favors the LSB with the steepest roll of on the high side of the skirt.

?

Steve

WA6ZFT

?

But i think it? it is easy because will you hear the difference? I believe not.

I cant give exact figures because I do not have the precission equipment to measure it. Yes the roll off slope is greater on the high side. But many of the published plots do not show that much difference. So I just built the best xtal filter that I could using good parts and best matched xtals. So I have a 8mhz filter, 4 pole using 6 xtals (QER design ). My LO is at 15mhz for 40m so there is sideband inversion. But to my ears for a QPR tansceiver it sounds great. I do not believe I would? hear any difference.

?

73

Kees PE9ST

--

Ryan Flowers - W7RLF

Re: 60 Meter homebrew rig IF frequencies

#9514

On Tue, Apr 29, 2025 at 11:48 PM, hwstar wrote:

If only it were so easy (it isn't) Sideband inversion: When the LO is above the IF the sideband gets inverted. The Crystal filter favors the LSB with the steepest roll of on the high side of the skirt.

?

Steve

WA6ZFT

?

But i think it? it is easy because will you hear the difference? I believe not.

I cant give exact figures because I do not have the precission equipment to measure it. Yes the roll off slope is greater on the high side. But many of the published plots do not show that much difference. So I just built the best xtal filter that I could using good parts and best matched xtals. So I have a 8mhz filter, 4 pole using 6 xtals (QER design ). My LO is at 15mhz for 40m so there is sideband inversion. But to my ears for a QPR tansceiver it sounds great. I do not believe I would? hear any difference.

?

73

Kees PE9ST

Re: How good is the 1N4004 diode as a varactor

#9513

开云体育

Hi Doug,

One normally doesn't run a varactor down to 0 volts (as Han's chart showed), but at minimum 1 volts. I tested two brands of rectifier--1N4003 and 1N4007.? The 4003 at 1V was 10.6 pF and at 12V was 5.2 pF. The 4007 at 1V was 7.8 pF and at 12V was 3.8 pF, so quite a difference, relatively speaking. I don't have a MV209 to test but the specs say it's a nominal 29 pF varactor at 3 volts; the data shows? a graph with about 44 pF extrapolated at 1V. So comparing the 1Nxxx to MV209 show quite different total capacitances which of course would also give a smaller tuning range for the 1Nxxx vs. for the MV209.

73,

Steve AA7U

On 4/30/2025 4:10 PM, Doug Hendricks via groups.io wrote:

Show quoted text

What is your experience with using the 1N4004 diode as a varactor in tuning circuits?? How does it compare to the MV209? In the same circuit, what is the comparison between the 2 diodes in tuning range?

Re: How good is the 1N4004 diode as a varactor

#9512

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Not that particular part but useful info here. Plus a circuit to take some data.?

How good is the 1N4004 diode as a varactor

#9511

What is your experience with using the 1N4004 diode as a varactor in tuning circuits?? How does it compare to the MV209? In the same circuit, what is the comparison between the 2 diodes in tuning range?