Re: RETURN LOSS thoughts de k3eui
On Mon, 28 Jun 2021, Luc ON7DQ wrote: It is generating more confusion than is needed. I will also say "my antenna has a return loss of 26 dB" (positive), and people will get what I mean from the context.
But when I WRITE it down in a table for a report, I will note " - 26 dB". I would argue that if you WRITE DOWN "RETURN LOSS -26dB" I would think of that as a double-negative. If it's a loss, as with finances, it's negative. A negative loss is a positive (double-negative!) Just as I would measure a filter, I will SAY that it has a loss of 3 dB ... but in a level diagram I will neatly WRITE next to the filter block : -3 dB Makes perfect sense to me. It would be strange to write +3 dB, how will you then explain the input and output levels of that block ? "Attenuator: 3dB" (no negative). You EXPECT an attenuator to reduce a signal. "Variable pass element: -6dB to +6dB" - can attenuate OR amplify. Sign becomes relevant, and would require the sign to be indicated, or if spoken, then stated, something like "The variable pass element is set for -3dB", or "the variable pass element is set as a 3dB attenuator". Certain words automatically imply sign. amplifier -> gain passive filter -> loss attenuator -> loss active filter -> (indeterminate!). At least, that's how I see it :) 73, VK2DGY.
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Re: RETURN LOSS thoughts de k3eui
Just my 2 cents ...
I agree that it is a convention ... but to me it still is a "historical error" (not sure, but I think the formula with the minus sign was introduced by HP) .
It is generating more confusion than is needed.
Everywhere else in electronics we have formulas to convert something into dB (voltage, power, sound) , and we write 20 log x/y or 10 log x/y, NEVER with a minus sign.
I have been teaching this stuff for over 35 years, and my aim was always to keep things simple for my students, it was already "complex" (pun intended) enough for them.
So, from my textbook :
Return Loss = 20 log |rho|, NO minus sign. (|rho| being 0 ...1)
Then this agrees nicely with |s11| in dB , which is also a negative number (and everyone agrees on that one).
Why have two different signs for two things that are essentially the same ?
Now why is there this confusion all the time ?
One is because IEEE says so ... but do we always have to agree ?
But it comes also from the difference in what you SAY , and what you WRITE.
I will also say "my antenna has a return loss of 26 dB" (positive), and people will get what I mean from the context.
But when I WRITE it down in a table for a report, I will note " - 26 dB".
Just as I would measure a filter, I will SAY that it has a loss of 3 dB ... but in a level diagram I will neatly WRITE next to the filter block : -3 dB
It would be strange to write +3 dB, how will you then explain the input and output levels of that block ?
Or say you measure a bandpass amplifier, will you say in band it has a GAIN of 20 dB, and out of band it has a LOSS of 10 dB, how will you draw a response curve with all positive numbers ... ?
Again, you can say it so, but you will WRITE down +20 dB and -10 dB.
A good thing that Nano VNA Saver at least has the option to select the sign for Return Loss, I think you can guess what I selected ? ;-)
And whatever belief you have, the line of measured RL will still be BELOW the 0 dB line ...
73,
Luc ON7DQ
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Many thanks Roger, for your answer ...
Just pointing-out to the series resonant circuit (at very input before the semi-rigid line) : 47 ohm - 330nH - 220pF !
I cannot understand what is its purpose ... it looks to be resonant around 18.6 MHz ... ??? ...
Is it a filter ? Why it is there ? This one is not my project ... it comes from military labs ...
One thing I realize for sure : INPUT (and Output) impedance measurementmade with Nano-Vna MUST be performed
having Power-Supply & Bias switched-ON !
Thanks to everybody !
Peter
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----- Original Message ----- From: "Roger Need via groups.io" <sailtamarack@...> To: < [email protected]> Sent: Monday, June 28, 2021 6:36 PM Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications On Mon, Jun 28, 2021 at 08:52 AM, Peter wrote: I was asking WHY the INPUT Resonance changed as soon as I apply Power-Supply
(and consequentely the Bias) ....
I was just a question ...
When you apply bias to the transistors Q1 and Q2 the input impedance looking into the base of the transistors changes. This affects the overall input impedance of the network (including any observed resonances) seen at the junction of the 1 nF and 120 pF capacitors. The semi-rigid coax is attached at this point so the impedance seen at the input to this coax will also change. The input resonant circuit formed by the 47 ohm resistor, 330 nH inductor, 220 pF cap, 47 pF cap in parallel with this coax input impedance will now have a different resonant frequency. Roger
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Peter,
it helps to think of the active devices, the PA power transistors, as horribly non-linear devices. The base-emitter junction is a forward biased diode, whose small-signal parameters vary over an RF cycle. The collector-base junction is a reverse biased diode (think varicap diode <>), whose capacitance (Ccb) depends on the instantaneous value of the collector-base voltage. That capacitance is then subject to the Miller effect ( <>), and can provide positive feedback from output to input.
Generally, the stage voltage gain is kept quite low to avoid any danger of the apparatus becoming a power oscillator rather than a power amplifier.
And since the base emitter junction is a diode, it obeys (more or less) the diode equation for AC resistance, r'= .025/Ib?¦¸, where Ib is the base current. There is a lot more going on, for instance see <>.
All that capacitance has an impact on the tuning of the input and output networks. At this point, you might be thinking that any functioning RF amplifier is a triumph of engineering ingenuity over the Laws of Physics. You would not be far wrong.
73, Stay Safe,
Robin, G8DQX
PS: It was a good question, but with a complex answer, depending on how much detail one can cope with!
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On 28/06/2021 16:52, Peter wrote: Hi William ... of course the "squared BIAS" I Drawn is just the one made with the BY1-1F ... there is no any other Bias ...
I was asking WHY the INPUT Resonance changed as soon as I apply Power-Supply (and consequentely the Bias) ....
I was just a question ...
Peter.
----- Original Message ----- From: "William Smith" <w_smith@...>
To: <[email protected]>
Sent: Monday, June 28, 2021 5:46 PM
Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications
Aside:? not sure why you are biasing the input, looks like the transistors are already biased on thru BYI-1F, and the input is capacitively coupled.
At the very least the B-E junction of a transistor changes its¡¯ capacitance when you bias it on, no?
William Smith
ComputerSmiths Consulting, Inc.
w_smith@...
On Jun 28, 2021, at 9:57 AM, Peter <yliroma@...> wrote:
?Thanks to eveyone gave an answer ...
Of course I realize the Bias is affecting the Input (and output) impedance of any device ... BUT WHY ???
I intend to highlight my P.A. is a A/B Class RF Amplifier equipeb by two parallele transistor coupled somehow:
Attached the schematics : it is a surplus (ex milititary) unit which was working originally in AM mode ... the related
Bias is obtained via a paritcular DIODE ...
For those who think DC Power-Supply introduces some extra impedance: this not true .. all +Vcc Supply lines are grounded by the CPs capacitors and RF is blocked by the JAFs...
See the attached schematics.
At any rate I have to connect my NANO-VNA when the system is Power-supplied ... ISN'T IT ???
Peter
----- Original Message ----- From: "schweppe" <schweppe@...>
To: <[email protected]>
Sent: Monday, June 28, 2021 1:38 PM
Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications
Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output)
impedance due to bias. So input (and output) impedance of impeadance
matching network as shown in your picture will also change when bias to
the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value? associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ???? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
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Re: anti-stati foam sheets for lining case
On 6/28/21 12:07 PM, Bill Higdon via groups.io wrote: I might use one of the anti static envelopes I have as an added feature. I can see for what JPL is involved with any particle shedding causing issues. But with my experience with the pink foam in other applications it didn't cause any problems. Just in case, it's not a concern about "space hardware" per se, it's the particles and coating from the sheet material the pink poly bags are made from getting into the electronics and causing "mystery not quite shorts". The containment of "foam falling apart" is just gravy.
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Re: anti-stati foam sheets for lining case
I might use one of the anti static envelopes I have as an added feature. I can see for what JPL is involved with any particle shedding causing issues. But with my experience with the pink foam in other applications it didn't cause any problems.
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Re: RETURN LOSS thoughts de k3eui
You've received many good replies, but I can't help to contribute mine as
well, so:
Quote: " So the FORWARD voltage is always positive and the RETURN signal
is attenuated.."
But if all we measure is voltage, it well could be static with no real
power involved. A charged cloud may be at a high voltage, but nothing
happened unless a lightning discharge occurs. Current flows from the cloud
to earth or to another cloud. That current produces all kinds of
electromagnetic energy. If we have just voltage on the transmission line,
no power is conveyed. Just voltage alone is inadequate to measure the flow
of RF energy. Both voltage AND current are required. S-Parameters are
truly a measure of vector power - both voltage and curent.
Dave - W?LEV
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On Mon, Jun 28, 2021 at 6:13 AM Barry K3EUI <k3euibarry@...> wrote: I have been wondering for months why I object to RETURN LOSS being a
"positive value".
Yes, we are talking about a LOSS.
And of course the formula shows RL = -20 log (rho)
Since rho is a number between zero and one, the log (rho) is itself a
negative number.
And -20 times a negative number is positive.
OK OK.... I get the math.
RL is positive - You "lose" 20 dB or whatever.
Now, what if we just rename the term to RETURN voltage (drop the "loss"
in the label).
The return voltage is less than the forward voltage. I hope that is true.
So now the RETURN signal is attenuated by -X dB.
Now that feels right.
It's the word "loss" in RETURN LOSS that makes it confusing (I'm not an
engineer).
So the FORWARD voltage is always positive and the RETURN signal is
attenuated.....
Now I can say correctly that the reflected signal is - 10 dB compared
to the forward signal.
In fact, that's the way I think of it, even if I don't say those words.
When I plot SWR of an antenna, the low numbers (close to 1) are "good".
Bad numbers for SWR are 3 and above.
When I plot the RETURN voltage on the same antenna, it feels that a DIP
(negative number) is also good, in the sense that the graphs have the same
overall shape. Why is that pleasing?
Or is this just a play on words?
It's not a big deal, as long as we realize what the graph means.
I want a low SWR. I want a big return loss (I want no return signal at
all if possible).
If my antenna-feed line match is good, there is very little power
returning to the source.
So why is it called RETURN "loss" ?
de k3eui Barry
--
*Dave - W?LEV*
*Just Let Darwin Work*
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Re: RETURN LOSS thoughts de k3eui
On 6/28/21 8:24 AM, Maynard Wright, P. E., W6PAP wrote: Carey,
You are exactly right.? Terms such as "return loss," "reflection loss," "reflection coefficient," "SWR," etc. all have precise meanings that are very important.? Many of them are defined in IEEE, ITU, and/or ANSI standards.?? Telephone engineers who work on voice frequency or low carrier frequency cable pairs, incidentally, generally work in terms of return loss and reflection loss rather than reflection coefficient and SWR because the last two measures are not easy to work with when the characteristic impedance is complex rather than approximately real.
And be careful about the assumptions that are made for RF calculations above a few hundred kHz.? For purposes of the characteristic impedance, the primary constants G and R and usually assumed to be zero so that the resulting calculation of Zo is real. But a seemingly contradictory assumption is made for the complex propagation constant, where calculation of the attenuation is important.? For that calculation, G is usually assumed to be zero, but R, L, and C are not.? It makes sense to do it that way, but it has proved confusing in some contexts.
Maynard Wright, W6PAP On the other hand, if someone quotes a Return Loss as either positive or negative, the *meaning* is generally understood, unless the circuit returns more power than it receives (a ferrite circulator with an amplifier, perhaps).? So beating someone up for choosing the "other" convention probably isn't worth it.? If you're publishing a paper, or better, writing a procurement spec or test procedure) - make sure it's right.? In the latter situations, I tend to not use the term return loss, rather, I'd specify |S11| < -10 dB.
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
On Mon, Jun 28, 2021 at 08:52 AM, Peter wrote: I was asking WHY the INPUT Resonance changed as soon as I apply Power-Supply
(and consequentely the Bias) ....
I was just a question ...
When you apply bias to the transistors Q1 and Q2 the input impedance looking into the base of the transistors changes. This affects the overall input impedance of the network (including any observed resonances) seen at the junction of the 1 nF and 120 pF capacitors. The semi-rigid coax is attached at this point so the impedance seen at the input to this coax will also change. The input resonant circuit formed by the 47 ohm resistor, 330 nH inductor, 220 pF cap, 47 pF cap in parallel with this coax input impedance will now have a different resonant frequency. Roger
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Re: Firmware version 1.0.39
Art, if you're using defuse to update the software, read my post several messages back.? Defuse uses confusing terms. Upload means to copy from the device to the PC.? Read my post and install the modified defuse I made, then delete the firmware file you've been trying to install and download and install a new copy.? On Mon., 28 Jun. 2021 at 10:45 a.m., aparrisjr@...<aparrisjr@...> wrote: I reloaded thee software and it shows the device. I uploaded the new firmware and the program shows device 00 updated and the screen shot is like the one you provided. When I check the firmware version on the Nano it is still 1.0.39. I tried the another version with the same results. Thanks for your help. Art
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Re: RETURN LOSS thoughts de k3eui
On Mon, Jun 28, 2021 at 05:13 AM, Barry K3EUI wrote:
I have been wondering for months why I object to RETURN LOSS being a
"positive value".
So why is it called RETURN "loss" ?
de k3eui Barry
We had a topic on this subject a few months ago. I posted a link to a paper by an IEEE editor on the "Definition and Misuse of Return Loss" which details the history and why it is a positive number. It makes for interesting reading. /g/nanovna-users/message/18708?p=,,,20,0,0,0::Created,,IEEE,20,2,0,78100772 Roger
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Hi William ... of course the "squared BIAS" I Drawn is just the one made with the BY1-1F ... there is no any other Bias ...
I was asking WHY the INPUT Resonance changed as soon as I apply Power-Supply (and consequentely the Bias) ....
I was just a question ...
Peter.
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Show quoted text
----- Original Message ----- From: "William Smith" <w_smith@...> To: < [email protected]> Sent: Monday, June 28, 2021 5:46 PM Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications Aside: not sure why you are biasing the input, looks like the transistors are already biased on thru BYI-1F, and the input is capacitively coupled. At the very least the B-E junction of a transistor changes its¡¯ capacitance when you bias it on, no? William Smith ComputerSmiths Consulting, Inc. w_smith@... On Jun 28, 2021, at 9:57 AM, Peter <yliroma@...> wrote:
?Thanks to eveyone gave an answer ...
Of course I realize the Bias is affecting the Input (and output) impedance of any device ... BUT WHY ???
I intend to highlight my P.A. is a A/B Class RF Amplifier equipeb by two parallele transistor coupled somehow:
Attached the schematics : it is a surplus (ex milititary) unit which was working originally in AM mode ... the related
Bias is obtained via a paritcular DIODE ...
For those who think DC Power-Supply introduces some extra impedance: this not true .. all +Vcc Supply lines are grounded by the CPs capacitors and RF is blocked by the JAFs...
See the attached schematics.
At any rate I have to connect my NANO-VNA when the system is Power-supplied ... ISN'T IT ???
Peter
----- Original Message ----- From: "schweppe" <schweppe@...>
To: <[email protected]>
Sent: Monday, June 28, 2021 1:38 PM
Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications
Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output)
impedance due to bias. So input (and output) impedance of impeadance
matching network as shown in your picture will also change when bias to
the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ?? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
|
Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Aside: not sure why you are biasing the input, looks like the transistors are already biased on thru BYI-1F, and the input is capacitively coupled.
At the very least the B-E junction of a transistor changes its¡¯ capacitance when you bias it on, no?
William Smith
ComputerSmiths Consulting, Inc.
w_smith@...
toggle quoted message
Show quoted text
On Jun 28, 2021, at 9:57 AM, Peter <yliroma@...> wrote:
?Thanks to eveyone gave an answer ...
Of course I realize the Bias is affecting the Input (and output) impedance of any device ... BUT WHY ???
I intend to highlight my P.A. is a A/B Class RF Amplifier equipeb by two parallele transistor coupled somehow:
Attached the schematics : it is a surplus (ex milititary) unit which was working originally in AM mode ... the related
Bias is obtained via a paritcular DIODE ...
For those who think DC Power-Supply introduces some extra impedance: this not true .. all +Vcc Supply lines are grounded by the CPs capacitors and RF is blocked by the JAFs...
See the attached schematics.
At any rate I have to connect my NANO-VNA when the system is Power-supplied ... ISN'T IT ???
Peter
----- Original Message ----- From: "schweppe" <schweppe@...>
To: <[email protected]>
Sent: Monday, June 28, 2021 1:38 PM
Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications
Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output)
impedance due to bias. So input (and output) impedance of impeadance
matching network as shown in your picture will also change when bias to
the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ?? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
|
Re: RETURN LOSS thoughts de k3eui
Carey,
You are exactly right.? Terms such as "return loss," "reflection loss," "reflection coefficient," "SWR," etc. all have precise meanings that are very important.? Many of them are defined in IEEE, ITU, and/or ANSI standards.?? Telephone engineers who work on voice frequency or low carrier frequency cable pairs, incidentally, generally work in terms of return loss and reflection loss rather than reflection coefficient and SWR because the last two measures are not easy to work with when the characteristic impedance is complex rather than approximately real.
And be careful about the assumptions that are made for RF calculations above a few hundred kHz.? For purposes of the characteristic impedance, the primary constants G and R and usually assumed to be zero so that the resulting calculation of Zo is real. But a seemingly contradictory assumption is made for the complex propagation constant, where calculation of the attenuation is important.? For that calculation, G is usually assumed to be zero, but R, L, and C are not.? It makes sense to do it that way, but it has proved confusing in some contexts.
Maynard Wright, W6PAP
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Show quoted text
On 6/28/21 6:45 AM, Carey Fisher wrote: The term "Return Loss" wasn't invented yesterday. It's an engineering term
that's been around for decades. It's just like the concept that electrical
current flows from positive to negative. These are conventions that help
everyone understand things from the same reference point.
73, Carey, WB4HXE
On Mon, Jun 28, 2021 at 8:36 AM William Smith <w_smith@...>
wrote:
Well, since you are being pedantic (don't get me wrong, I like pedantic,
it helps add clarity and reduce confusion):
On Jun 28, 2021, at 8:13 AM, Barry K3EUI <k3euibarry@...> wrote:
RL is positive - You "lose" 20 dB or whatever. Exactly
So now the RETURN signal is attenuated by -X dB.
Now that feels right. Now you've got it backwards again. Return signal is attenuated by some
number of dB, but that number (the amount that it's attenuated) is a
positive number.
You should say "So now the RETURN signal is attenuated by X dB."
You wouldn't say "I drove 20 miles, now my gas tank has been emptied by
MINUS one gallon", would you?
You would say "I'm down one gallon" or, (if you really wanted to be
confusing) "I'm up by MINUS one gallon".
So why is it called RETURN "loss" ? Because I sent a signal into the device, and some amount of it returned.
And (usually) the amount that returned is LESS than the amount I sent, so:
GAIN of the device (how much MORE I got back compared to how much I sent)
is less than one (a negative number).
LOSS of the device (how much LESS I got back compared to how much I sent)
is a positive number.
Think of measuring through a passive network, your gain is negative, but
your loss is positive.
Does that help?
73, Willie N1JBJ
[Yes, I have the T-shirt: "Does Anal-Retentive have a hyphen?"]
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Re: Firmware version 1.0.39
I reloaded thee software and it shows the device. I uploaded the new firmware and the program shows device 00 updated and the screen shot is like the one you provided. When I check the firmware version on the Nano it is still 1.0.39. I tried the another version with the same results.
Thanks for your help.
Art
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On Sun, 27 Jun 2021 at 20:22, Donald S Brant Jr <dsbrantjr@...> wrote: Jim: The optimal way would be to measure several standard articles on each
test analyzer and compare the S2P files from each instrument
I'm not convinced that tells you anything useful, unless one is known to be good. A lot of these VNAs are copying code from another, so I would not trust that. The classical verification/transfer standards are a Beatty mismatch standard which is a section of 25¦¸ air line between 50¦¸ air line sections,
a 50¦¸ air line and one or more attenuators with different values, typically
one with a relatively low value and one with a higher attenuation. The
air line standards are expensive and easily damaged:
My company produces kits with one verification attenuator. It is not as good as a Beatty line, or airline, but it produces something that can be verified. One customer with an R&S VNA got very different results on the attenuator to those we supplied.
Please note that measuring one or more of the same standards used for
calibration will tell you little about the calibration quality, only the
connection repeatability.
Yes, I once demonstrated to someone that a 22 ohm wire-wound resistor could be used as a calibration standard. If that's defined to be 50 ohm in the calibration routines, then it will show a very high return loss if you measure that same 22-ohm wire-wound resistor. If you then connect a high quality 50 ohm load, it will look awful. I have found that measuring a filter with passband and stopband within the calibrated frequency range as a quick check gives pretty useful information
about the instruments in terms of directivity and transmission dynamic
range.
But instrument verification is an entire specialty within the science of
metrology.
METAS have published a lot on that, as have NPL. 73, Don N2VGU. Dr David Kirkby Ph.D C.Eng MIET Email: drkirkby@... Web: Kirkby Microwave Ltd (Tel 01621-680100 / +44 1621-680100) Stokes Hall Lodge, Burnham Rd, Chelmsford, Essex, CM3 6DT.
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Thanks to eveyone gave an answer ...
Of course I realize the Bias is affecting the Input (and output) impedance of any device ... BUT WHY ???
I intend to highlight my P.A. is a A/B Class RF Amplifier equipeb by two parallele transistor coupled somehow:
Attached the schematics : it is a surplus (ex milititary) unit which was working originally in AM mode ... the related
Bias is obtained via a paritcular DIODE ...
For those who think DC Power-Supply introduces some extra impedance: this not true .. all +Vcc Supply lines are grounded by the CPs capacitors and RF is blocked by the JAFs...
See the attached schematics.
At any rate I have to connect my NANO-VNA when the system is Power-supplied ... ISN'T IT ???
Peter
toggle quoted message
Show quoted text
----- Original Message ----- From: "schweppe" <schweppe@...> To: < [email protected]> Sent: Monday, June 28, 2021 1:38 PM Subject: Re: [nanovna-users] I/O RF Matching net-works measuring with NANO-VNA #applications Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output)
impedance due to bias. So input (and output) impedance of impeadance
matching network as shown in your picture will also change when bias to
the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ?? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
|
Re: RETURN LOSS thoughts de k3eui
The term "Return Loss" wasn't invented yesterday. It's an engineering term that's been around for decades. It's just like the concept that electrical current flows from positive to negative. These are conventions that help everyone understand things from the same reference point. 73, Carey, WB4HXE On Mon, Jun 28, 2021 at 8:36 AM William Smith <w_smith@...> wrote: Well, since you are being pedantic (don't get me wrong, I like pedantic,
it helps add clarity and reduce confusion):
On Jun 28, 2021, at 8:13 AM, Barry K3EUI <k3euibarry@...> wrote: RL is positive - You "lose" 20 dB or whatever. Exactly
So now the RETURN signal is attenuated by -X dB.
Now that feels right. Now you've got it backwards again. Return signal is attenuated by some
number of dB, but that number (the amount that it's attenuated) is a
positive number.
You should say "So now the RETURN signal is attenuated by X dB."
You wouldn't say "I drove 20 miles, now my gas tank has been emptied by
MINUS one gallon", would you?
You would say "I'm down one gallon" or, (if you really wanted to be
confusing) "I'm up by MINUS one gallon".
So why is it called RETURN "loss" ? Because I sent a signal into the device, and some amount of it returned.
And (usually) the amount that returned is LESS than the amount I sent, so:
GAIN of the device (how much MORE I got back compared to how much I sent)
is less than one (a negative number).
LOSS of the device (how much LESS I got back compared to how much I sent)
is a positive number.
Think of measuring through a passive network, your gain is negative, but
your loss is positive.
Does that help?
73, Willie N1JBJ
[Yes, I have the T-shirt: "Does Anal-Retentive have a hyphen?"]
-- Carey Fisher careyfisher@...
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
On 6/28/21 5:43 AM, G8DQX list wrote: Peter,
you haven't indicated whether the amplifier is linear (class A or AB or B) or class C (for FM or CW). The circuit operating conditions (class, signal level, bias, Vcc) make a big difference.
Input match
The input match is generally designed to present 50¦¸ to the input socket.
Output match
The output match is generally designed to deliver maximum power to a 50¦¸ load. The PA is *not* a conjugate match, which would reduce the output power by 6dB!
Take care
A nanoVNA (most VNAs for that matter) is not safe anywhere near a 200W amplifier. All it would take is a spurious oscillation in the PA, and the VNA is likely to see rather more than +10dBm (10mW) at a measuring port, with the danger of damaging the VNA electronics. (See also /g/nanovna-users/topic/77980973#18555 </g/nanovna-users/topic/77980973#18555>, for instance.) If it breaks into oscillation (or picks up a spurious signal) it could easily push more than 10dBm *out* of the input port. Even operating as designed, it could have surprisingly high power coming out the input. In order to be stable, the S12 (reverse isolation) just has to be bigger than the forward gain (S21) (slight oversimplification). Let's say the amplifier gain is 10 dB,? S12 is -15 dB, and a mismatched load reflects -20dB, and the amplifier is putting out 100W (+50dBm) - the reflected power from the load is +30dBm, the amplifier (going backwards) reduces that to +15dBm.
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Re: I/O RF Matching net-works measuring with NANO-VNA
#applications
Peter, you haven't indicated whether the amplifier is linear (class A or AB or B) or class C (for FM or CW). The circuit operating conditions (class, signal level, bias, Vcc) make a big difference. Input match The input match is generally designed to present 50¦¸ to the input socket. Output match The output match is generally designed to deliver maximum power to a 50¦¸ load. The PA is *not* a conjugate match, which would reduce the output power by 6dB! Take care A nanoVNA (most VNAs for that matter) is not safe anywhere near a 200W amplifier. All it would take is a spurious oscillation in the PA, and the VNA is likely to see rather more than +10dBm (10mW) at a measuring port, with the danger of damaging the VNA electronics. (See also /g/nanovna-users/topic/77980973#18555 < /g/nanovna-users/topic/77980973#18555>, for instance.) HTH, 73, Stay Safe, Robin, G8DQX
On 28/06/2021 12:38, schweppe wrote: Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output) impedance due to bias. So input (and output) impedance of impeadance matching network as shown in your picture will also change when bias to the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value? associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ???? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
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Peter
Roger Need
Dr. David Kirkby, Kirkby Microwave Ltd
