[R-390] Official specs
Larry Haney
larry41gm2 at gmail.com
Fri Oct 25 10:54:52 EDT 2024
Jim, Since you are referencing dB, I assume that in your statement: 'Then
the signal applied to the 125 ohm input connector will be a tenth of the SG
setting.' is referring to the 'power' delivered to the 390, and not the
voltage. I agree if you are talking about power, but what we need to know
is what the voltage reduction is, not the power. If you are saying that
the uV will be 1/10 of the SG output, then you are way off base. It's a
44.4% reduction, not 90%.
The dB is used by a lot of different groups of people that need a way to
mathematically express a logarithmic relationship (notice that voltage is
listed). Here's a good definition:
*-----------------------------------------------------------------------*
A decibel (dB) is primarily used to measure sound intensity, particularly
in acoustics, but it also finds applications in various fields like
electronics, telecommunications, and even video imaging, essentially
anywhere a *logarithmic scale *is needed to represent *ratios* of *power,
voltage, or pressure levels* across a wide range, making it useful for
comparing large differences in intensity in a manageable way.
Key uses of decibel:
- *Acoustics:* Measuring sound pressure levels (SPL), which is the most
common use of decibel, allowing us to understand how loud a sound is
perceived by the human ear.
- *Electronics:* Expressing *signal strength, amplifier gain, and
attenuation* of signals in circuits, where a *decibel scale* helps
compare different components easily.
- *Telecommunications:* Measuring signal loss or gain during
transmission through cables or wireless mediums.
- *Audio Engineering:* Assessing the volume of audio signals in
recording and mixing processes.
- *Vibration Analysis:* Measuring the intensity of vibrations in
machines and structures
Why is *decibel a logarithmic* scale?
- *Human Perception:*
The human ear perceives sound intensity logarithmically, meaning a large
change in sound power only results in a small perceived change in loudness.
- *Wide Range of Values:*
Using a logarithmic scale allows for a large range of values to be
represented on a manageable scale, from very quiet sounds to extremely loud
ones.
*Important points about decibel: *
-
*Unitless: A decibel is a unitless ratio, meaning it always needs a
reference point to be meaningful. *
- *dB SPL:* When measuring sound pressure level, the "dB" is often
accompanied by "SPL" (Sound Pressure Level).
- *Interpreting Decibel Values:* A 10 dB increase represents a tenfold
increase in sound intensity.
*-----------------------------------------------------------------------*
The point I'm making here is that dB is not restricted to measuring power,
it is used to measure voltage
relations also. When the Collins engineers decided to use a dB meter for
signal strength, there was a good
reason, they wanted a logarithmic scale to represent the input voltage
level. It's all about the uV into the
balanced antenna connection. The difficulty with measuring the 390s is
their choice to use a flexible
input antenna circuit that usually has a 125 ohm impedance instead of 50
ohms (a very common standard).
That would have made life so much easier for techs and users, but that's
our life.
The bottom line is, we get to figure out how to measure sensitivity and SNR
using *microvolts*, *not* power.
Unfortunately, we must use an impedance adapter with the URM-25D and many
other sig gens in order to
get them to read correctly. Enter the DA-121. It is so simple, 2
resistors to do the impedance matching.
Unfortunately, the manufacturers, Navy and Army did not go into detail
about the real reduced uV level at
the 390, so that is up to us.
You keep calling the signal reduction introduced by the DA-121
'insertion loss', it is not. It is the amount of loss introduced by
design. This is the cheapest method to do impedance matching and we
all know there is a built in loss because there is a resistor in series
with the signal path. Insertion losses are those that are a part of the
components that are being used, such as cables, RF transformers,
connectors, etc.
My simple (but real and accurate) test using a DA-121 shows that for 100 uV
out of my SG at 1 MH, 55.6 uV gets to the 390 bal antenna input (measured
it with my 400FL). That is all we need to know to do the
'real' sensitivity and SNR measurements.
So, since you have a sig gen and resistors for a DA-121, why don't you
measure the sig gen uV output and the
uV signal getting to the 390 and let us know what uV level you
measured, not dB? We can not use dB for the sensitivity and SNR
measurements.
Gianni was right, your Ohm's Law calculation for what the R-390 "sees" is
in error. (50 in parallel with 68) +
100 in parallel with 125 is 63.44 ohms, is it not? No, it is not. When
you calculate the load or impedance that
a unit sees, you do not include the impedance of the unit. What the 390
sees is: 50 (SG) in parallel with 68 +100 = 129 ohms.
If someone has a sig gen that only displays the output levels in dB, then
its doc should explain how to
convert it to uV and what the output impedance is and whether the
impedance needs to be matched or
not.
Respectfully, Larry
On Thu, Oct 24, 2024 at 9:22 PM Jim Whartenby via R-390 <
r-390 at mailman.qth.net> wrote:
> No, we should realize by now that the insertion loss of the DA-121 is 9
> dB, not 5 dB. Remember that the input and output impedances of the
> attenuator are not the same. If one wants to make the SG output level
> calculation easier, use a 50 ohm to 125 ohm attenuator with a 10 dB
> insertion loss. Then the signal applied to the 125 ohm input connector
> will be a tenth of the SG setting. The calculator at:
> https://k7mem.com/Res_Attenuator.html will give the resistor values
> needed for a 10 dB insertion loss.
> Jim
> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
>
> On Thursday, October 24, 2024 at 10:58:51 PM CDT, Jacques Fortin <
> jacques.f at videotron.ca> wrote:
>
> Hi Bob,
>
> The point is that is not a mis-use as far as the two voltage values are
> took from the same values of impedance (like input and output of an RF
> amplifier).
> But the relation 20 log (Vout/Vin) = 10 log (Pout/Pin) ceases to be true
> if the input and output impedances are different.
> Using dB = 20 log (Vout/Vin) is then improper if there is a matching pad
> involved (like the DA-121/U), or an audio amplifier, for ex.
>
> However, in the context of the DA-121/U use, and in the case that the
> output settings of a given SG are only calibrated in dB, the output of the
> SG have to be increased by 5dB to obtain the proper VOLTAGE level at the
> 125 ohms output.
> EX: you need to have 10µV for the output of the DA-121/U in 125 ohms and
> your SG output settings are indicated in dBµV only.
> Because 0dBµV is 1 µV in 50 ohms and 10µV in 50 ohms is 20dBµV, you will
> need to "adjust" the output of the SG at 25dBµV to obtain 10 µV across the
> DA-121/U loaded output (10 exp(25/20) x .5676) = 10µV
> Remember that the SGs have no brain, so they cannot figure that the
> terminal load at the output of the matching pad is not 50 ohms, so you have
> to "think" for them.
>
> 73, Jacques, VE2JFE in Montreal
>
> -----Message d'origine-----
> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
> part de Bob kb8tq
> Envoyé : 24 octobre 2024 20:41
> À : Ing. Giovanni Becattini <giovanni.becattini at icloud.com>
> Cc : r-390 at mailman.qth.net
> Objet : Re: [R-390] Official specs
>
> Hi
>
> You can indeed find standard measures like:
>
> db V = db above / below one volt
>
>
> https://www.everythingrf.com/community/what-is-dbv#:~:text=dBV%20or%20decibel%20Volt%20is,a%20voltage%20below%201%20V
> .
>
> db uV = db above / below one microvolt (and milivolt and …. )
>
> The cable TV folks like voltage based measurements. There are other areas
> that seem to go that way as well.
>
> Is this mis-use of the term? If so, it’s a *very* commonly accepted
> mis-use.
>
> Bob
>
>
>
> > On Oct 24, 2024, at 4:05 PM, Ing. Giovanni Becattini via R-390 <
> r-390 at mailman.qth.net> wrote:
> >
> > Thanks! I think I have understood now. As you understood, my mistake was
> thinking that dB could be used for every ratio of congruent values.
> >
> > Very happy.
> >
> >> Il giorno 24 ott 2024, alle ore 21:57, Jim Whartenby <old_radio at aol.com>
> ha scritto:
> >>
> >> Gianni
> >>
> >> Answer to 1 below: dB is defined as a POWER ratio. The use of voltage
> to calculate dB assumes that the input and output resistances of any
> electrical system being measured are the same. Take the formula that
> defined the dB: dB = 10 log (power out / power in). Substitute (voltage^2
> /R) for each term "power" then simplify the equation. You should end up
> with 20 log (voltage out / voltage in). This simplification only works if
> Rout = Rin. WRT the DA-121, the out resistance does not equal the input
> resistance or 125 ohms does not equal 50 ohms. So the "voltage" form of
> the dB equation results in an error in certain cases where the power form
> never will.
> >>
> >>
> >> Answer to 2 below: you make the point that Rin is 50 ohms and Rout is
> 125 ohms but then ignore what this means. See Answer 1 above. Also your
> Ohm's Law calculation for what the R-390 "sees" is in error. (50 in
> parallel with 68) +100 in parallel with 125 is 63.44 ohms, is it not?
> >>
> >>
> >> Answer to 3 below: the equation you used is wrong, for voltage it is
> 20 log X not 10 log X so 5 dB would be correct if both input resistance and
> output resistance are the same but you point this out in your 2 below that
> they are not equal resistances. Again, see Answer 1 above.
> >>
> >>
> >> Answer to 4 below: you are correct, the loss in the DA-121 is 9 dB
> when you consider that the input impedance is different from the output
> impedance and calculate accordingly.
> >>
> >>
> >> Your error is in thinking that there are two different definitions for
> dB, power and voltage. There is only power, the voltage form of the
> equation has the caveat that the input resistance must be the same as the
> output resistance. You are not the first one to make this mistake and
> surely you will not be the last.
> >>
> >>
> >> Regards, Jim
> >>
> >>
> >> Logic: Method used to arrive at the wrong conclusion, with
> >> confidence. Murphy
> >>
> >>
> >> On Thursday, October 24, 2024 at 12:30:03 PM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
> >>
> >>
> >> Thanks Jim!
> >>
> >> I try to explain my thought and I would be grateful to whom would tell
> me at which point I make the mistake.
> >>
> >> 1.What dB is? For what I studied, dB is just a ratio between two
> congruent values, so you can define a dB for voltage or for power.
> >>
> >> 2.DA-121 output voltage. Let’s stop to voltage values for now and work
> almost in DC, assuming that the R390 has really a 125-ohm impedance.
> >>
> >> In this case:
> >> the generator "sees" 50 ohms impedance (68 ohms parallel to 100+125);
> >> the receiver "sees" about 125 ohms (100 ohms + 68 ohms in parallel to
> 50 ohms).
> >>
> >> The ratio between the input and the output voltage is easy to calculate
> just with the ohm law. The output voltage is the input voltage times 0.556.
> If you don’t want to make calculations see
> https://ohmslawcalculator.com/voltage-divider-calculator. It’s easy to
> practically check that this is true.
> >>
> >> I believe that so far nobody could disagree.
> >>
> >> 3.Voltage dB value. So, when two voltages has a ratio of 0.556, their
> ratio, expressed in dB, is 10 log (V1/V2) = -2.5 dB. I am just applying the
> dB definition (forget 5 dB that was an error of mine).
> >>
> >> 4.Power dB value. If we want to calculate the power ratio we have
> (using 1V as input value):
> >>
> >> P1 = input power = V^2 / R = 1V / 50 ohm =0,02
> >> P2 = output power = 0.556^2 / 125 ohm = 0.0025
> >>
> >> So the power ratio in db is 10 log (P1/P2) = -9.078 dB. Again, this is
> simply the dB definition.
> >>
> >> Where am I wrong?
> >>
> >> Thanks for your help.
> >>
> >> Gianni
> >>
> >>
> >>
> >>> Il giorno 24 ott 2024, alle ore 18:08, Jim Whartenby <
> old_radio at aol.com> ha scritto:
> >>>
> >>> Gianni
> >>>
> >>> The dB is defined as a measure of the ratio of powers. In
> electronics, it is the power output to power input.
> >>>
> >>>
> >>> Using the formula 10 log (Pout / Pin) will always give the correct
> answer. This formula considers the resistance of both the input power and
> output power separately since power = voltage^2 / resistance.
> >>>
> >>> The 20 log Vout/Vin will only give the correct answer when both the
> input resistance and the output resistance are the same.
> >>>
> >>>
> >>> So the 5 dB calculation using input and output voltage is wrong. In
> this particular case the resistance transformation provided by the
> attenuator was not considered. If the attenuator did not cause a change in
> resistance (the input resistance and output resistance are the same) then
> using 20 log (Vout / Vin) would result in the same answer as 10 log (Pout /
> Pin).
> >>>
> >>>
> >>> Regards, Jim
> >>>
> >>>
> >>> Logic: Method used to arrive at the wrong conclusion, with
> >>> confidence. Murphy
> >>>
> >>>
> >>> On Thursday, October 24, 2024 at 02:26:08 AM CDT, Ing. Giovanni
> Becattini via R-390 <r-390 at mailman.qth.net> wrote:
> >>>
> >>>
> >>> Hi Guys,
> >>>
> >>> sorry I was not considered - I hate to be tiring, but an answer would
> be great to help me better following the discussion.
> >>>
> >>> It seems to me that 5 dB in voltage and about 8.9 dB in power are very
> easy to demonstrate (keeping out the frequency effect), either
> arithmetically, either practically, either with LTSpice.
> >>>
> >>> At least on this point …. do the parties agree? I could not understand
> this. Perhaps, if we don’t agree on this, it is hard to go further.
> >>>
> >>> No problem if you don’t want to answer...I'm not touchy 😁
> >>>
> >>> Gianni
> >>>
> >>>> Il giorno 24 ott 2024, alle ore 02:18, Barry Scott <
> 72volkswagon at gmail.com <mailto:72volkswagon at gmail.com>> ha scritto:
> >>>>
> >>>> GR-1001A. Even funner.
> >>>>
> >>>> Thanks,
> >>>> Barry - N4BUQ
> >>>>
> >>>> On Wed, Oct 23, 2024 at 4:46 PM Jacques Fortin <
> jacques.f at videotron.ca <mailto:jacques.f at videotron.ca>> wrote:
> >>>>>
> >>>>> Bob, I see your point, but we know everything about that too !
> >>>>> That's bring me back to the end of the '70s when my workbench signal
> generator was a Marconi Instruments TF955/5.
> >>>>> Output calibrated in Volts EMF (open circuit voltage) and 75 ohms of
> output impedance !
> >>>>> Using it, you had to compute what was the real load voltage you got
> all the time...
> >>>>>
> >>>>> 73, Jacques, VE2JFE in Montreal
> >>>>>
> >>>>> -----Message d'origine-----
> >>>>> De : Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> Envoyé : 23
> >>>>> octobre 2024 16:49 À : Jacques Fortin <jacques.f at videotron.ca
> >>>>> <mailto:jacques.f at videotron.ca>> Cc : Ing. Giovanni Becattini
> >>>>> <giovanni.becattini at icloud.com
> >>>>> <mailto:giovanni.becattini at icloud.com>>; r-390 at mailman.qth.net;
> >>>>> Larry Haney <larry41gm2 at gmail.com <mailto:larry41gm2 at gmail.com>>
> >>>>> Objet : Re: [R-390] Official specs
> >>>>>
> >>>>> Hi
> >>>>>
> >>>>> If you start with a Spice model and put in a 2V source with some
> series resistance. You loose 6 db when you get to 1.0 V. That’s how Spice
> looks at things.
> >>>>>
> >>>>> If you start with the signal generator convention you have a “spice”
> 2V source and a 50 ohm resistor “inside the box”. Put on a 50 ohm load and
> you have 1V. That’s your zero db point with the signal generator.
> >>>>>
> >>>>> Start one way and you are 6 db down.
> >>>>>
> >>>>> Start the other way and you are at zero db.
> >>>>>
> >>>>> Bob
> >>>>>
> >>>>>> On Oct 23, 2024, at 1:03 PM, Jacques Fortin <jacques.f at videotron.ca
> <mailto:jacques.f at videotron.ca>> wrote:
> >>>>>>
> >>>>>> Hello Bob,
> >>>>>>
> >>>>>> I 100% agree that the convention on a RF source is that the
> displayed output voltage is valid only when the SG is loaded with the
> correct termination impedance.
> >>>>>> I disagree however that the outcome "depends" on what can be used
> to provide the answer.
> >>>>>> If anyone is ignorant enough to use a Spice model that not include
> a source output impedance, it is sure that the result of such "simulation"
> will be different from what is obtained with properly set up test equipment.
> >>>>>> A properly used simulation software results will not be different
> that what can be obtained with "real" instruments.
> >>>>>> If ever this is the case, the inputs to the simulation program are
> faulty.
> >>>>>>
> >>>>>> Bob Pease (RIP) once fell in that trap: he took revenge by throwing
> his computer from the top of the building into the parking lot below.
> >>>>>>
> >>>>>> But, at the end of the day, nothing is more practical than a good
> theory.
> >>>>>>
> >>>>>> 73, Jacques, VE2JFE in Montreal
> >>>>>>
> >>>>>> -----Message d'origine-----
> >>>>>> De : r-390-bounces at mailman.qth.net
> >>>>>> <mailto:r-390-bounces at mailman.qth.net>
> >>>>>> <r-390-bounces at mailman.qth.net
> >>>>>> <mailto:r-390-bounces at mailman.qth.net>> De la part de Bob Camp
> >>>>>> Envoyé : 23 octobre 2024 12:26 À : Ing. Giovanni Becattini
> >>>>>> <giovanni.becattini at icloud.com
> >>>>>> <mailto:giovanni.becattini at icloud.com>> Cc :
> >>>>>> r-390 at mailman.qth.net; Larry Haney <larry41gm2 at gmail.com
> >>>>>> <mailto:larry41gm2 at gmail.com>> Objet : Re: [R-390] Official specs
> >>>>>>
> >>>>>> Hi
> >>>>>>
> >>>>>> The very basic issue here turns that into a “that depends” answer:
> >>>>>>
> >>>>>> If you are playing with a Spice model, and starting from the
> voltage on your ideal source, you get one answer.
> >>>>>>
> >>>>>> If you are working at RF and using a signal generator (and doing
> things properly) you get a very different answer.
> >>>>>>
> >>>>>> The convention on an RF source is that the “starting voltage” is
> measured with the correct termination in place.
> >>>>>>
> >>>>>> Since we’re talking about RF …. that’s how it would be done.
> >>>>>>
> >>>>>> Bob
> >>>>>>
> >>>>>>> On Oct 23, 2024, at 10:39 AM, Ing. Giovanni Becattini via R-390 <
> r-390 at mailman.qth.net <mailto:r-390 at mailman.qth.net>> wrote:
> >>>>>>>
> >>>>>>> Hi,
> >>>>>>>
> >>>>>>> I find this topic very intriguing, so I cannot help but return to
> this discussion.
> >>>>>>>
> >>>>>>> As I told you, I don't consider myself an RF expert or simply a
> 390 expert, but I have had to solve complex engineering problems many times
> in my life. So I think we should first create a mathematical model that is
> as simple as possible, i.e. without taking into account the frequency
> effect. Once the model works, we can try to make it more real with the
> right corrections.
> >>>>>>>
> >>>>>>> So I would like to ask a question to see if we are on the same
> page:
> >>>>>>> Do you agree that if the R-390A were a perfect 125 ohm resistor
> and we were working at 1 kHz, the DA-121 would attenuate 5 dB in voltage
> and 8.98 in power?
> >>>>>>>
> >>>>>>> Greetings
> >>>>>>>
> >>>>>>> Gianni
> >>>>>>>
> >>>>>>>> Il giorno 23 ott 2024, alle ore 16:15, Larry Haney <
> larry41gm2 at gmail.com <mailto:larry41gm2 at gmail.com>> ha scritto:
> >>>>>>>>
> >>>>>>>> Jim, I read your referenced post a few times looking for the
> >>>>>>>> answer we're all looking for, what the microvolt level is that
> >>>>>>>> is going into the 390 for a given level going into the DA-121,
> >>>>>>>> but I couldn't find it. All I read was a bunch of db numbers
> >>>>>>>> that don't make any difference. We need to know about the uV
> >>>>>>>> levels. You can talk about insertion losses all you want, but
> that does not tell us what the uV level is that is going into the 390.
> >>>>>>>>
> >>>>>>>> I used my URM-25D to generate a 1 MegaHertz 50 uV signal into
> >>>>>>>> the
> >>>>>>>> DA-121 and got 28 uV going into the 390 (that's a 44% reduction
> >>>>>>>> of signal from the 25D as measured with my HP 400FL RMS RF AC
> >>>>>>>> voltmeter). Nothing else matters. The calculation is very
> simple:
> >>>>>>>> 50 - 28 = 22, 22 / 50 = .4444 or 44.44%. That means that
> >>>>>>>> 55.55% of the signal from the SG is getting to the 390. The
> >>>>>>>> accuracy of my 400FL is +/- 1%. All my signal measurements were
> in RF RMS volts measured with my HP 400FL.
> >>>>>>>>
> >>>>>>>> The ONLY DC measurements I made were to measure the resistances
> >>>>>>>> in the
> >>>>>>>> DA-121 and mine are a 70 ohm shunt and a 100 ohm in series.
> >>>>>>>> These are close to the documented values of 68 ohms and 100 ohms.
> >>>>>>>>
> >>>>>>>> So, what uV level of signal do you MEASURE (not calculated or
> >>>>>>>> theorized) going into the DA-121 and going into the 390. Let's
> >>>>>>>> keep it simple and stick to *MICROVOLTS* because that is what
> >>>>>>>> the sensitivity and signal to noise ratio measurements use, NOT
> db.
> >>>>>>>>
> >>>>>>>> By the way, since this test is all about the DA-121, you should
> >>>>>>>> be using the documented resistor values in it for testing (68 and
> 100 ohms).
> >>>>>>>>
> >>>>>>>> And contrary to what you said, my DC circuit calculations
> >>>>>>>> (resistance and estimated signal loss) do agree with my RF
> >>>>>>>> measurements. The resistance calculation is: 100 ohms / (100 +
> 125 ohms) = .4444. That's a 44% loss.
> >>>>>>>> To get the signal level at the 390, multiply the SG output by 56%.
> >>>>>>>> And I did not calculate any db loss, the 5 db loss is what my
> 400FL says it is.
> >>>>>>>>
> >>>>>>>> For anyone wanting to make their own DA-121, use what's
> >>>>>>>> documented in it, a
> >>>>>>>> 68 ohm shunt and 100 ohm series resistor. Otherwise you will
> >>>>>>>> get a different answer from those that use a real DA-121.
> >>>>>>>>
> >>>>>>>> Regards, Larry
> >>>>>>>>
> >>>>>>>> On Tue, Oct 22, 2024 at 3:36 PM Jim Whartenby <old_radio at aol.com
> <mailto:old_radio at aol.com>> wrote:
> >>>>>>>>
> >>>>>>>>> Larry
> >>>>>>>>>
> >>>>>>>>> I built a test fixture that is essentially two DA-121's
> >>>>>>>>> connected back to back. Photos and drawing are enclosed.
> >>>>>>>>> This does the conversion from 50 ohms to 125 ohms and then
> >>>>>>>>> back to 50 ohms. I used 1% resistors to make the attenuator
> circuit with the values close to those found here:
> >>>>>>>>>
> >>>>>>>>> L, U, PI, T, and H Attenuators - K7MEM
>
>
> |
> |
> | |
> L, U, PI, T, and H Attenuators - K7MEM
>
> Martin E. Meserve - K7MEM
>
> Calculator for L, U, PI, T, and H Attenuators.
> |
>
> |
>
> |
>
>
>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> The closest I could come to the 64.18 ohms result from the
> >>>>>>>>> attenuator calculator was 63.9 ohms. This is from the
> >>>>>>>>> parallel combination of 3 each
> >>>>>>>>> 237 ohm in parallel with a 1k, in parallel with a 499 ohm
> resistor.
> >>>>>>>>> Five resistors in parallel, all 1% resistors. The result was
> >>>>>>>>> 63.85 ohms, a 0.5% error. The sub for the 96.83 ohm resistor
> >>>>>>>>> is a 100 ohm 1% resistor (3%
> >>>>>>>>> error) and the sub for the R-390's 125 ohm impedance was a 121
> >>>>>>>>> ohm 1% resistor (3% error). This is still much better then
> >>>>>>>>> the 5% resistors used in the original DA-121.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> For a test oscillator I used a Helper SM-1000 signal generator
> >>>>>>>>> and measured the insertion loss with a Stoddart NM-25T
> >>>>>>>>> frequency selective voltmeter. The insertion loss was
> >>>>>>>>> measured at 10 MHz using two 4 foot BNC
> >>>>>>>>> RG-58 coax cables from Pomona Electric. 4 foot of coax from
> >>>>>>>>> the
> >>>>>>>>> SM-1000 to the test fixture and another 4 feet from the test
> fixture to the NM-25T.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> The SG was set for a reading of 30 dB on the NM-25T signal
> >>>>>>>>> strength meter when measuring a BNC through connection and
> >>>>>>>>> then measured 11 dB when the test fixture was installed in
> >>>>>>>>> place of the BNC through. The insertion loss for the test
> >>>>>>>>> fixture is 19 dB. Dividing this by two since there are
> >>>>>>>>> essentially two DA-121s back to back gives an insertion loss
> >>>>>>>>> of about 9.5 dB for a single DA-121. This closely agrees with
> the attenuator calculator findings.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> So it seems that your DC circuit calculation do not agree with
> >>>>>>>>> the RF measurements. Transmission lines behave differently
> >>>>>>>>> then DC circuits. You calculate a 5 dB insertion loss, I
> measure a 9.5 dB insertion loss.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Here is an experiment that you can try. Insert a 50 ohm
> >>>>>>>>> resistor in parallel with the 50 ohm coax. What do you think
> will happen?
> >>>>>>>>> Perhaps nothing since the coax is 50 ohms and the resistor is
> >>>>>>>>> also
> >>>>>>>>> 50 ohms? In reality, the coax has reactive elements, parallel
> >>>>>>>>> capacitance and series inductance that make up the coax
> impedance.
> >>>>>>>>> Neither of which will dissipate the signal carried on the coax.
> >>>>>>>>> The only losses are from the resistance of the conductors that
> >>>>>>>>> make up the coax. Adding a parallel resistor will attenuate the
> signal to the receiver by 3 dB.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> If anyone on this list wants to make their own version of the
> >>>>>>>>> DA-121, I can supply the resistor values I used for a token $2
> >>>>>>>>> plus postage. Just DM me with your address and if you want one
> or two resistor sets.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Regards, Jim
> >>>>>>>>>
> >>>>>>>>> Logic: Method used to arrive at the wrong conclusion, with
> confidence.
> >>>>>>>>> Murphy
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> On Friday, October 18, 2024 at 05:36:08 AM CDT, Larry Haney <
> >>>>>>>>> larry41gm2 at gmail.com <mailto:larry41gm2 at gmail.com>> wrote:
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Hi Jim, I just checked and I only have 1 da-121. As for
> >>>>>>>>> insertion loss, my coax is very short and the connections are
> >>>>>>>>> very good so the loss there would not be possible for me to
> >>>>>>>>> measure. Now for the insertion loss due to impedance mismatch
> >>>>>>>>> (due to resistance
> >>>>>>>>> variations) would also not be possible for me to measure, as I
> >>>>>>>>> don't have the equipment required for that. But, because the
> >>>>>>>>> 3 resistors in the circuit are very close to the required
> >>>>>>>>> values for a perfect
> >>>>>>>>> 50 ohm match to the sig gen, I am sure that the insertion loss
> >>>>>>>>> due to that very slight impedance mismatch is extremely
> >>>>>>>>> small. I have no way to measure that loss as I don't have the
> >>>>>>>>> 3 exact value resistors to compare it to. I could calculate it,
> but I believe that would be a waste of time without being able to measure
> it.
> >>>>>>>>>
> >>>>>>>>> After all the input you have given me and the research just
> >>>>>>>>> done, I'm satisfied with my current measurements and
> calculations (IE:
> >>>>>>>>> the output voltage of the da-121 is 56% of the input voltage
> >>>>>>>>> when the load is
> >>>>>>>>> 125 ohms).
> >>>>>>>>>
> >>>>>>>>> My biggest concern about making snr measurements is for those
> >>>>>>>>> folks that don't have a recently calibrated sig gen or
> >>>>>>>>> calibrated rms AC voltmeter to verify their readings with.
> >>>>>>>>>
> >>>>>>>>> Regards, Larry
> >>>>>>>>>
> >>>>>>>>> On Thu, Oct 17, 2024 at 1:55 PM Jim Whartenby <old_radio at aol.com
> <mailto:old_radio at aol.com>> wrote:
> >>>>>>>>>
> >>>>>>>>> Larry
> >>>>>>>>> No, just one SG and one 125 ohm load. You should be able to
> >>>>>>>>> determine the total loss through two DA-121 attenuators
> >>>>>>>>> connected back to back with an o'scope and then divide the loss
> by two to solve for the insertion loss.
> >>>>>>>>> Jim
> >>>>>>>>> Logic: Method used to arrive at the wrong conclusion, with
> confidence.
> >>>>>>>>> Murphy
> >>>>>>>>>
> >>>>>>>>>
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