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Since you’re playing with transmitters, a dummy load is an indispensable
tool. If I were you, I would stop and make one before continuing with the
pixie.
It’s simple enough to make one if you have some basic parts on hand. If
not, there are very cheap kits available for low power dummy loads. For
example, qrpguys.com has one for $10 which also gives you a dc voltage
proportional to the RF power you feed it. This will give you some
additional soldering practice, but more importantly will give you a way to
safely test qrp transmitters and measure their output power with your
multimeter.
A little dummy load like that will come in handy again and again as you
progress in your hobby. You will not regret it. I built a similar one into
a box with a dc meter movement years ago. Even though I have plenty of
other test equipment, that’s what I reach for when it’s time to smoke test
a QRP transmitter.
Good luck with your project and have fun!

I have measured electrolytic capacitors with my nano and they are woefully bad at high frequencies.? Now that I have acquired a nanoVNA that can go down to 10 kHz I plan to do some testing to see if they are just as bad at that frequency.
Bob

That makes no sense whatsoever.

Never........NEVER.......rely on an electrolytic as a bypass for RF
energy!!!!!! Use series resonance to your advantage. Chip, N6CA learned
that lesson from (deceased) Gary Frey, W6XJ, in the 1970's. The bypass
caps presented in his preamps are chosen to be self resonant at the
frequency of operation of each preamp. I've used the technique many times
over in home brewing. It's a well known fact among (most) design
engineers that tantalum capacitors are not much good above 500 kHz to a
couple of MHz. Most electrolytics are even worse. Go measure them on
your NANOs.

I just grabbed a 1000 ?F / 25 VDC cap from the parts bin and measured it on
a calibrated HP 8753C using the Smith Chart. Even at 1 MHz it measures 78
mΩ with a series reactance 78 nH. Sure, the DC portion is fine, but what
is 78 nH at 50 MHz?

X(L) = 2 x π x f x L = 6.28 x [50 E 6] x [78 E-9] = 24.5 ohms

Sure, the +j24.5 is non-dissipative, but does not make a very good bypass
even at low VHF frequencies.

Take a capacitor better suited as a bypass at HF, a 450 pF dip mica. Same
setup at 10 MHz: 0.1 ohms at 462 pF. It goes self resonant just above 39
MHz.

Which makes a better bypass at HF.

Dave - W?LEV

On Sun, Feb 14, 2021 at 10:14 PM Manfred Mornhinweg <manfred@...>
wrote:

And what's the best bypass cap? Well, a single, plain, cheap aluminium
electrolytic!

Attached is the impedance plot for a 47?F, 25V electrolytic cap, measured
with lead lengths compatible with mounting it snugly on a PCB. Their narrow
pin spacing helps a lot in keeping their ESL low. I kept the same scale to
make comparison easy.

YES, a single 47?F electrolytic is a much better bypass cap than a
parallel combination of two ceramic caps of different values! Even in the
low VHF range!

The problems with electrolytic caps is that their ESR rises with age, and
rises much faster if they run hot, or if they have to carry large ripple
current. So they can't be applied in every situation. But in situations
that are kind to them, they are the cheapest and easiest way to get an
excellent wideband bypass.

--
*Dave - W?LEV*
*Just Let Darwin Work*

On Sun, Feb 14, 2021 at 12:25 PM, Ed Krome wrote:

Thank you to all who replied. But, since the values I was seeing didn't match
prescribed values (and it was driving me a bit nuts), I tried the experimental
approach. I took a single RF Demo Kit board with the same cable. I carefully
calibrated my 3 nanoVNA's (nanoVNA 2.8", nanoVNA H4, nanoVNA SAA-2N), each
50kHz to 300MHz. Then I recorded the values of the components at position 7
(capacitor) and posn. 8 (inductor) at increments over the frequency range.

Ed there are two versions of the RF demo board and at least 3 manufacturers. The test positions are different between the two. Photos of each attached. You can see that positions 7 and 8 on the NWDZ board are a capacitor and inductor respectively. However on the more commonly available DeepElec board 7 is a resistor and cap in series and 8 is an inductor and cap in series.

Are you using the NWDZ board and are you using the short, open and load on that board to calibrate?

Roger

Here is the phenomenon I referred to in a previous post in this thread.
[image: image.png]

This paper is written by Cadence and can be accessed at:

Good to see that they show the parallel resonance between two different bypass caps! But too many circuit designers aren't aware of this.

And what's the best bypass cap? Well, a single, plain, cheap aluminium electrolytic!

Attached is the impedance plot for a 47?F, 25V electrolytic cap, measured with lead lengths compatible with mounting it snugly on a PCB. Their narrow pin spacing helps a lot in keeping their ESL low. I kept the same scale to make comparison easy.

YES, a single 47?F electrolytic is a much better bypass cap than a parallel combination of two ceramic caps of different values! Even in the low VHF range!

The problems with electrolytic caps is that their ESR rises with age, and rises much faster if they run hot, or if they have to carry large ripple current. So they can't be applied in every situation. But in situations that are kind to them, they are the cheapest and easiest way to get an excellent wideband bypass.

Here is the phenomenon I referred to in a previous post in this thread.
[image: image.png]

This paper is written by Cadence and can be accessed at:




Take only the 470 nF unit - the left in each plot. Below 15 MHz, the
device is capacitive, although decreasing in value as frequency is
increased. At resonance, it is purely resistive. Above resonance it
becomes inductive. The phase would also have to be shown to confirm that
last statement, but this is typical performance of a capacitor as it goes
through self resonance. Inductors act in a similar manner.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

And since it's sunday and I have time for playing with the NanoVNA, I made measurements to show all of you the effect of placing bypass caps of different value in parallel. Attached are the impedance plots for 100nF alone, 1nF alone, and both in parallel. Whoa! Which option would you prefer?

Note that the 100nF cap alone provides a bypassing impedance below 3? from about 700kHz to about 70MHz. that's pretty good, I would say. The 1nF is bad on low frequencies, and the parallel combination is a total disaster! Yet that's what you will find in equipment designed by people who have never thought about this point, and are just following intuition, which often is wrong...

Dave,

We warmed up the HP impedance meter of the time (the one that had a
tunable drum as a frequency indicator and topped out at 110 MHz). Sure
enough, *EVERY* CK05 capacitor went purely resistive between 1.4 and 1.6
MHz and inductive above that.

What lead length did you use for that test?????

After my last post I grabbed my box of 100nF capacitors, fired up the NanoVNA (works well even without warming up!), and measured two dozen of them, with lead lengths typical for PCB mounting. Their resonant frequencies all fell in the range of 6 to 8.4MHz. As was to be expected, the smallest ones (leaded ceramic chips) had the highest resonance, and the largest foil capacitors had the lowest, inside that range.

I then measured with full length wires. My longest-legged one resonated at 1.85MHz. That one has 35mm long legs (each), of strongly magnetic material, which probably contributes to add lead inductance.

To get those low resonant frequencies, you must have had very long-legged capacitors, like 5cm, and you must have measured them with full lead lengths. Of course nobody would mount a bypass cap with full-length leads! So what you were getting on those boards must have been much better. Resonating around 7MHz, and producing acceptable bypassing to 30MHz or so, depending on the impedance requirements. In a great many situations that's good enough, specially with older electronics.

It has been stated, by multiple members of this group, that due to hardware
deficiencies (port 2 is not a perfect 50Ω) and lack of 12 term error
correction,
the "S21 method" can't be used to measure high impedance reliably.
I decided to test that and see how inaccurate is it really.

On the first graph the red and green traces are the measured values (R&Z)
and the blue and black traces are a simulation of a 10K resistor with 50fF
of parasitic
capacitance (figure from some Vishay paper).

[image: image.png]

The DUT is a resistor of unknown origin from China that measures (at DC)
9.995K ± 10Ω.
The VNA (Nano-H4) was calibrated using the DIY "standards", no compensation
of the test
jig during calibration, offset delay of -4.5ps was used during measurement.
Test jig was made from cheap SMA connectors from AliExpress, cables (RG405)
also.

[image: 2021-02-14 19.56.05.jpg]

Draw your own conclusions.

Clyde,

CAPACITANCE AND INDUCTANCE ARE SPECIFIED AT A GIVEN FREQUENCY. See the
example.

Yes, often the datasheet states the measurement frequency. But this is invariably a "low" frequency, low enough to keep parasitic effects negligible. The inductance or capacitance of a component won't vary significantly from that frequency down to DC.

The part value starts changing significantly at frequencies high enough to make parasitic effects significant.


Alan,

I have carefully measured normal HF range inductors with a dip oscillator and SDR at working frequencies comparing with a meter that measures
in the 100Khz range. I noticed no significant difference.

When the inductors are built in such a way that in the HF range the parasitics are low, then this is indeed the normal situation. But if you measure them at VHF, the values will very likely change. And at UHF they surely will. It's a simple matter of going high enough in frequency, to make the values of any part change dramatically.

At HF you can often get away with taking the low-frequency values and assuming they will hold true at your working frequency, But not always. And at VHF that becomes rarer, and at UHF it becomes very rare.


Dave:

Since I enjoy challenging you, I will do it again! ;-)

Now, ask yourself: "is an inductor good at bypassing rail noise
as the capacitor was intended to accomplish?" NO!

Well, it depends! A DC-blocked inductor can be a pretty good bypass element, no worse than a capacitor! It just depends on its impedance at the frequency in question. Such a "nasty" 100nF bypass capacitor is resonant at 1.5MHz when its equivalent series inductance is 112nH. A foil-wound capacitor might indeed be that bad. A ceramic capacitor only if mounted with very long leads. Anyway, assuming it has indeed 112nH and thus is resonant at 1.5MHz, how would it behave at 3MHz? Well, it would have a reactance of less than 2?! That's still a pretty good bypass, despite being inductive. At 100MHz it would be bad.

If you replace that nasty 100nF capacitor by a 10nF one, would it be better? NO, if you keep those long leads! It will be much worse bypassing low frequencies, it will be good at its resonant frequency near 5MHz, but at 100MHz it will be almost as bad as the 100nF one.

And what happens if you follow that old rule of putting the 100nF capacitor in parallel with a 1nF one? Well, at some frequency you get a might parallel resonant circuit, with the 100nF capacitor acting as the inductor, and at that frequency you get infinite impedance, and thus NO bypassing! Of course, only if the capacitors have high Q at that frequency. So the important point with bypass capacitors is: They should have enough capacitance for the low frequencies, low enough ESL for the high frequencies, and they should be bad! I mean, they should have a low Q. A high loss factor. That largely pevents getting unbypassed frequencies due to bypass caps happily parallel-resonating with each other.

There is a long-standing myth about electrolytic caps needing a parallel-connected ceramic cap to provide bypassing over a wide frequency range. Using a parallel ceramic cap is indeed useful if this is a chip capacitor. But placing something like a an old-fashioned disc ceramic cap in parallel with an electrolytic of comparable path length doesn't help much, since both have roughly the same ESL.

In some equipment I often see real collections of 6 or more different capacitors in parallel, placed there by some designer who thinks that each frequency will then take the path it likes best. The only problem is that physics don't work like that. Those nice showcases of six different capacitors in parallel are mainly good for one thing: Getting a good laugh!

I also often laugh about that old rule of "one bypass cap per IC". When using slow ICs, often a single bypass cap is enough for the entire board, and in other cases one cap every so much distance is enough. One can save quite a bit of money in series production by leaving out unnecessary parts. Of course without overdoing that...

On Fri, Feb 12, 2021 at 07:25 PM, gmendenh1947 wrote:

Roger, Thanks for the suggestions. I have updated the firmware in my nanoVNA
to v1.0.45 and everything is working fine now.
I would like to try the new NanoVNA app (Windows) by OneOfEleven, but I'm
having trouble locating the Win 64 installation file.
I would appreciate it you could send me the link. Thanks, Geoff

This is the only publicly available version I have found. Works on my Win 64 computer...
You want to download the .rar file and decompress it with something like 7 zip...

Thank you to all who replied. But, since the values I was seeing didn't match prescribed values (and it was driving me a bit nuts), I tried the experimental approach. I took a single RF Demo Kit board with the same cable. I carefully calibrated my 3 nanoVNA's (nanoVNA 2.8", nanoVNA H4, nanoVNA SAA-2N), each 50kHz to 300MHz. Then I recorded the values of the components at position 7 (capacitor) and posn. 8 (inductor) at increments over the frequency range. Results showed remarkably good correlation over reduced frequency range on all three nano's. Pos. 7 showed the capacitor to be 100pF from 0.1 to 100 MHz. Posn 8 showed the inductor to be 700nH from 0.1 to 30 MHz. Things went rapidly askew at higher frequencies, although trends were similar. The capacitor lines followed each other reasonably well all the way to 300 MHz on all 3 nano's, but the inductor curves, while trending similarly, showed marked differences in values between the 3 different nano's. As a reality check, I measured the inductor on an AADE L/C meter IIB and read 700nH. This wasn't as precise a setup as with the nano's. Curves attached
I think I'm getting a handle on this now. Thanks again.

Ed K9EK

Excellent presentation thanks for sharing! :-)

73!
Greg / W4GAP

I once loaned a (VERY CHEAP) analog multimeter to some technicians that used it to check the resistance of the power lines. When they brought it back, the inside of the case was copper clad.. They reimbursed me for my $3 multimeter.
Ed

I say go for it, 1.2W is nothing and the soldering lesson you will get
afterwards
will be a great learning experience.

On Fri, Feb 12, 2021 at 06:33 PM, Ed Krome wrote:

Concerning how to measure capacitance and inductance on the RF Demo board (or
any capacitor or inductor), reference
/g/nanovna-users/wiki/16592. I can get the curves shown in
this demo just fine.. but how to read the actual values off of those curves
eludes me. On the item 8 inductor example, if I vary the frequency, I can make
that component read about any value I want. I must be missing something. Help,
please.

Ed,

There have been a number of good posts by others describing the characteristics of inductors and capacitors versus frequency. Parasitic capacitance is a big factor and needs consideration.

This post will hopefully be a direct answer to your question of how to read the inductance value on the NanoVNA.

The first step is to make sure that you have calibrated correctly using the open, short and load that are on the test board. This is cumbersome because those little u.fl connectors are not easy to work with. Once you are sure that your calibration is OK you can measure the component. The Smith chart has a readout that will give you the resistance and inductance or capacitance for the marker frequency. As you move the marker you will see the resistance increase on the inductor due to the "skin effect" . The inductor value shown at the marker is calculated by dividing the reactance by 2*pi*frequency. You have to be careful interpreting the results because parasitic capacitance will affect the reactance measured and the estimate of L by this method will get worse as the frequency increases.

I made some tests today on a small air-core inductor that measured 243 nH on a DE-5000 inductance meter at 100 kHz. I then connected it to a calibrated test jig and made some measurements. I attached a series of annotated screenshots showing the estimate of L by the NanoVNA at various frequencies up to 150 MHz. .

I then used the NanoVNA app by OneOfEleven to plot L, R and X versus frequency. You will note that at 50 kHz. the estimate of L is poor compared to my DE-5000 inductance meter. This is because the value of X is very small at this frequency and not in a reasonable measurement range for the NanoVNA. As the frequency increases to about 1 MHz. the accuracy of the estimate improves because the reactance is now a little over 1 ohm. Note the slight rise in calculated L with frequency due to the parasitic capacitance of the inductor.

Roger

Well, the radio is a QRP Pixie kit and it doesn't put out very much power; I just wanted to test if I soldered the connections well. I have an analog multimeter I could use as well. Eventually I plan to get (or make) a dummy load, but I'm still practicing my soldering for now.

The second one is pretty much what I was thinking of. Apparently he is making the Gerbers available.