[R-390] Official specs

Larry Haney larry41gm2 at gmail.com
Sun Oct 13 13:19:34 EDT 2024 

Hi Jim,  My thinking in why the second 'balanced' entry is actually the
Unbalanced info is that the Unbalanced antenna input is the high impedance
input and therefore would have the higher microvolt readings.  And in the
numerous other places in the spec where both are listed, the 'balanced' one
is first.

Regards, Larry

On Sun, Oct 13, 2024 at 8:27 AM Jim Whartenby via R-390 <
r-390 at mailman.qth.net> wrote:

> I agree that one list is mislabeled but I would think that the first
> "Balanced" list is the one that is actually the Unbalanced spec.
>
> My reasoning is that the CW input voltage spec is lower in the first list
> and higher in the second for all frequencies.  We may disagree on the
> actual insertion loss of the DA-121 but it would still be higher for all
> frequencies then the DA-124 which uses a series capacitor to mimic a short
> antenna instead of a voltage divider to transform impedance.
>
> Jim
>
> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
>
>     On Sunday, October 13, 2024 at 08:34:21 AM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
>
>  Pag. 6 (there is a typo, the second is “Unbalanced”, as Larry pointed out
> to me) - The document don’t name the dummy devices but describe them. The
> DA-.. are described in the TB SIG 319.
>
> > Il giorno 13 ott 2024, alle ore 15:25, Barry Scott <
> 72volkswagon at gmail.com> ha scritto:
> >
> > Hi Jim,
> >
> > Just curious where you found that document.  The ones I'm finding
> > don't appear to address/explain that dummy antenna and they're not
> > searchable.
> >
> > Thanks,
> > Barry - N4BUQ
> >
> > On Sun, Oct 13, 2024 at 2:53 AM Jim Whartenby via R-390
> > <r-390 at mailman.qth.net> wrote:
> >>
> >> Bob
> >>
> >> The test method for the R-390 acceptance is Mil-R-13947B, the document
> I found on the web is searchable so I found the applicable sections easily
> by searching (Control F) with the term "dummy".
> >>
> >> The spec for the dummy antennas is TB SIG 319 aka Electronic Equipment
> Maintenance Kit MK-288/URM, it contains the schematics for both the
> balanced and unbalanced dummy antennas.  The former simulates the 125 ohm
> balanced dipole antenna and the latter simulates the 50 ohm unbalanced
> input which is a whip antenna.
> >>
> >> The balanced dummy antenna does the conversion from a 50 ohm system to
> a 125 ohm system.  The 9 dBV loss means that the SG is 2.82 times the
> signal applied to the balanced input of the receiver so would this mean
> that one has more of a sensitivity range since the SG is several dBv above
> minimum RF output of the SG?
> >>
> >> The resistive dummy antenna is good for all frequencies covered by the
> R-390.  I am still not sure about the balun.  I don't have enough
> experience to pick the best ferrite material.  The impedance transformation
> is from 125 / 50 or 2.5:1  I know that you said that two primary turns and
> five secondary turns on the balun should do the trick but as I said, I
> don't have much experience with baluns.  Since there are six octaves
> between 500 kc and 32 mc, which ferrite material, in your opinion, will
> have the best response?
> >>
> >> I think that the original confusion that started this thread is that
> the above spec calls out a much higher SG input to the balanced input then
> most believe is the normal R-390 receiver sensitivity.  Granted, the spec
> does not mention that there is a 9 dBv difference between what the SG
> indicates and the actual input to the R-390 balanced input.  But the spec
> doesn't have to explain, it just says if you "do this" and you "get that",
> the receiver passes the acceptance test.  As is often said, Spec is Spec so
> you don't have to read into it.
> >>
> >> Regards,Jim
> >>
> >> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
> >>
> >>    On Saturday, October 12, 2024 at 06:13:24 PM CDT, Bob Camp <
> kb8tq at n1k.org> wrote:
> >>
> >> Hi
> >> A properly done balun should give you a significant improvement in the
> measured sensitivity.
> >> Ok since we’ve been around a bit on this … .what test method?
> >> 1) Test the radio straight out of a typical signal generator with no
> “stuff” between the radio and the generator
> >> VS
> >> 2) Test things the same way, but put in the balun.
> >> If the balun is properly done *and* it’s hooked up right … 2 should be
> noticeably better than 1.
> >> One note: some of these “adapters” are resistor based rather than
> having a balun in them. That’s not the same thing.
> >> Keep in mind that this may not be the best thing to do overload wise,
> it only is “best” if you are after sensitivity.
> >> Bob
> >>
> >>
> >> On Oct 12, 2024, at 2:43 PM, Ing. Giovanni Becattini <
> giovanni.becattini at icloud.com> wrote:
> >>
> >> Hi Jim,I am better and did the test you suggested me. The reduction is
> exactly 2 dB on the R-390A meter, with or without the DA-124 on the
> unbalanced antenna.
> >> Regarding the balun theme, who sold me the 390 gave me also a little
> balun: practically a twinax to bnc adapter with a fatter body. It is marked
> as specific for the 390, but with it things go much worse: more noise and
> less sensitivity.
> >>
> >>
> >> Il giorno 12 ott 2024, alle ore 01:13, Jim Whartenby <old_radio at aol.com>
> ha scritto:
> >> Jacque , Gianni and Bob Sorry for being a bit late in continuing the
> discussion.  Life gets in the way.
> >> In my zeal to solve for the attenuation of the DA-121, I should not
> have combined the SG impedance with the DA-121.  That would be fine for DC
> circuit analysis but not for a transmission line.  My apologies.
> >> I found this nifty calculator that will solve for the resistor values
> for every type of resistive attenuator.  See:
> https://k7mem.com/Res_Attenuator.html
> >> I entered the following values: Z1 = 50 ohms, Z2 = 125 ohms and an
> attenuation of 9 dB.  It spit out the exact values for the resistors used
> in the DA-121.  The series resistor on the SG side of the shunt resistor is
> less then a 1/4 ohm so it has little effect so it was obviously omitted in
> the DA-121.  The 5% resistor values given are again exactly those used in
> the DA-121.  The calculator also solves for 1% resistor values.
> >> 9 dB is the minimum attenuation that returns positive resistor values
> and does the conversion from 50 ohms to 125 ohms.  Any attenuation below 9
> dB returns a negative resistor value for R1, the series resistor on the SG
> side of the shunt resistor.
> >> As Bob mentioned in a separate email, a balun would be the modern
> solution if the ferrite core is capable of operating over the whole range
> of the R-390.  My balun experience is limited so I do not know which
> ferrite material will do the job.Regards,Jim
> >> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
> >>
> >>    On Tuesday, October 8, 2024 at 10:30:33 AM CDT, Jacques Fortin <
> jacques.f at videotron.ca> wrote:
> >>
> >> Hello Gianni,
> >> PLEASE READ MY COMMENTS IMBEDDED IN YOUR TEXT BELOW.
> >> I USE THE UPPERCASE TEXT BECAUSE EVERY OTHER FONT ATTRIBUTE IS CLEARED
> UP BY THE FORUM SERVER.
> >>
> >> 73, Jacques, VE2JFE in Montreal
> >>
> >> Hi Friends, forgive the savage who tries to understand how the clock
> works :)
> >> What you Bob say is exactly what I would like to understand, i.e., if
> the incredibly high sensitivities I heard are real or fantasy.
> >> Let's imagine that we are in a perfect word and neglect the frequency
> effect.
> >> All the discussion started, from my side, to determine the attenuation
> factor of the DA-121. So I try to better focus my questions:
> >>
> >> 1) if I have a siggen with 50 ohm output and an rx with 50 ohm input,
> is it correct to say that the voltage at the receiver’s input is that
> pointed out by the generator’s knob?
> >>
> >> EXACT: MOST GENERATORS INDICATES THE OUTPUT VOLTAGE WHEN THE EXTERNAL
> LOAD IS EQUAL TO THE INTERNAL OUTPUT IMPEDANCE.
> >> (MAXIMUM POWER TRANSFER CASE) WHICH MEANS 50 OHMS FOR MOST OF THE
> AMERICAN STUFF BUT 75 OHMS FOR THE OLD BRITISH ONES.
> >> SOME HAVE ALSO (LIKE MY OLD TRUSTY HP 8657A) A SETTING THAT INDICATES
> THE "UNLOADED" OUTPUT VOLTAGE AS THE EMF (ELECTROMOTIVE FORCE) VALUE.
> >> TO BE CLEAR, WITH A "NORMAL" RF GENERATOR SET TO DELIVER 10mV IN A 50
> OHMS LOAD, THE INTERNAL "EQUIVALENT" GENERATOR IS A 20mV SOURCE IN SERIES
> WITH A 50 OHMS RESISTOR (THE THEVENIN EQUIVALENT OF THE SOURCE, IF YOU
> LIKE).
> >>
> >> 2) in this specific case, I have a black box, constituted by the DA-121
> (68/100 ohm) and the R-390. It seems that from the generator’s point of
> view it is equivalent to a rx with 50 ohm input impedance. So, if I set the
> generator for 10 mV output, I have 10 mVrms at the 68 ohm resistor. Right?
> >>
> >> NOT EXACTLY: IF THE OUTPUT OF THE DA-121 IS UNLOADED, 11.525 mV WILL
> APPEAR ACROSS THE 68 OHMS RESISTOR AND THE THEVENIN EQUIVALENT OF THE
> DA-121 OUTPUT WILL BE A 11.525 mV SOURCE IN SERIES WITH A 128.81 OHMS
> RESISTOR.
> >> WITH THE DA-121 OUTPUT LOADED BY A 125 OHMS RESISTOR, THE VOLTAGE
> ACROSS IT WILL BE 5.676 mV.
> >> COMPARED TO THE GENERATOR "SET" VALUE OF 10mV, THIS IS 4.919 dB lower.
> >>
> >> 3) if above is right, the effective voltage at the receiver’s input
> (125 ohm) is the voltage at the 68 ohm reduced by the divider made by
> 100+125 ohm. Right?
> >>
> >> RIGHT, IN ALL CASES.
> >>
> >> If so, the loss factor of the DA-121 is 3 dB ( Vin x .56).
> >> 20 log (.56) = - 5 dB, not 3 dB....
> >>
> >> This can be practically tested, and the voltage at the 125 ohm resistor
> I am using instead of the R-390 for simplicity is exactly 0.56 Vin (this is
> true also for DC).
> >>
> >> PRETTY CLOSE TO THE THEORICAL VALUE, RIGHT.
> >>
> >> This above is what I can suppose by myself but I understand you are
> explaining to me that it is wrong.
> >>
> >> But If you are right, the DA-121 should have a 9 dB loss factor, and
> this seems to be completely denied from the practical experience.  Again,
> forget the frequency effect until this point.
> >>
> >> SAY 4.9dB LOSS.
> >> BUT IF THE DA-121 68 OHMS RESISTOR IS REPLACED BY A 50 OHMS ONE, THE
> GLOBAL ATTENUATION (REAL OUTPUT AT THE 125 OHMS LOAD VS THE SG SETTING)
> WILL BE 6 dB LOWER (0.5) AND THE DA-121 OUTPUT IMPEDANCE WILL BE 125 OHMS
> ALSO (MAXIMUM POWER TRANSFER CASE).
> >>
> >> Thanks for your patience :)
> >>
> >> Gianni
> >>
> >>
> >>
> >>
> >>> Il giorno 8 ott 2024, alle ore 14:58, Bob Camp <kb8tq at n1k.org> ha
> scritto:
> >>>
> >>> Hi
> >>>
> >>> Another wrinkle in this:
> >>>
> >>> If your generator is set to 1 uV, that should mean you get 1 uv across
> a 50 ohm load on the generator. The impedance at that point is 25 ohms (50
> ohms for the load in parallel with 50 ohms from the generator).
> >>>
> >>> If your receiver happens to be a high impedance input (as many are ….)
> you get 2 uV at the input to the radio and the impedance is 50 ohms.
> >>>
> >>> Maybe take out some random load resistor and you just doubled the
> sensitivity.
> >>>
> >>> Why would you do this? (though maybe not at HF …)
> >>>
> >>> When you go from 25 ohms to 50 ohms, the thermal noise from the
> resistor(s) goes up by 3 db. Your signal went up by 6 db. You are now 3 db
> further above the thermal noise floor.
> >>>
> >>> Not a big deal on the typical HF setup. It is a big deal as you go up
> in frequency. Yes this makes some other things you do up there “fun” ….
> Even at HF, folks racing to get that 0.001 uV sensitivity number (or some
> equally absurd number) probably are doing this as well.
> >>>
> >>> So: counting on this or that radio to supply a matched input …. maybe
> not a good idea without testing out the specific radio at the operating
> frequency.
> >>>
> >>> Bob
> >>>
> >>>
> >>>
> >>>> On Oct 8, 2024, at 3:31 AM, Ing. Giovanni Becattini via R-390 <
> r-390 at mailman.qth.net> wrote:
> >>>>
> >>>> Hi Jim,
> >>>>
> >>>> I am not sure I can follow you. As I told, I am not using the R-390A
> but just a 125 ohms resistor as a terminator.
> >>>>
> >>>> So:
> >>>> 68 // 225 = 52.21 (seen from the generator)
> >>>>
> >>>> The voltage on the rx is then the siggen’s output voltage divided by
> 100+125, i.e. Vin x 0.556. In dB, 3 dB, that is what I wanted to know.
> >>>>
> >>>> I tested that also in DC, with the same results. Luckily for this
> time practice and theory are completely in accord.
> >>>>
> >>>> Things could be different increasing the frequency, obviously, and
> using the real R-390A, but in my case, @ 7.5 MHz, they are rather similar.
> >>>>
> >>>> Am I wrong?
> >>>>
> >>>>> Il giorno 7 ott 2024, alle ore 22:28, Jacques Fortin <
> jacques.f at videotron.ca> ha scritto:
> >>>>>
> >>>>> Jim, I do not know if I can follow you correctly in your
> explanations below.
> >>>>> How does this DA-121 adaptor is made ??
> >>>>>
> >>>>> 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 Jim Whartenby via R-390 Envoyé : 7 octobre 2024 12:42
> >>>>> Cc : r-390 at mailman.qth.net Objet : Re: [R-390] Official specs
> >>>>>
> >>>>> Well, mailman not only strips photos but also any changes in
> typeface so the larger type and bold letters are stripped as well.
> >>>>> Logic: Method used to arrive at the wrong conclusion, with
> >>>>> confidence.  Murphy
> >>>>>
> >>>>> On Monday, October 7, 2024 at 10:53:50 AM CDT, Jim Whartenby via
> R-390 <r-390 at mailman.qth.net> wrote:
> >>>>>
> >>>>> GianniComments in BOLD
> >>>>>
> >>>>> On Monday, October 7, 2024 at 02:43:53 AM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
> >>>>>
> >>>>> Hi Jim and thanks for your patience, but I still don’t understand.
> >>>>> The generator sees 50 // (100+125), 50 // 225 =  52.22 ohm  No, this
> >>>>> should be 40.9 ohms not 52.22 ohms! R in parallel = 1/ (1/50 + 1/
> >>>>> 225)  so 1/ (1/50 + 1/225) = 40.9 ohms  or if you prefer
> >>>>>
> >>>>>
> >>>>> product over sum = (50 X 225) / (50 + 225) = 11,250 / 275 = 40.9
> >>>>> ohms
> >>>>>
> >>>>>
> >>>>> The result of two resistors combined in parallel will always be a
> lower value then the lowest of the two combined resistors.
> >>>>> Going a step further, 40.9 ohms in parallel with the Signal
> Generator impedance of 50 ohms results in an impedance of 22.5 ohms so the
> SG now sees about half of the expected impedance.  You can think of the SG
> as being a current source feeding a 50 ohm resistor.  If the current source
> now sees half the expected impedance, the voltage output of the SG will now
> be half of the set voltage.
> >>>>>
> >>>>> Now applying the voltage divider rule to the series 100 ohm and
> shunt 125 ohm resistors, the voltage across the 125 ohm resistor will be
> the SG voltage X (125 / 225) = SG voltage X 0.55  We already know that the
> signal generator output is half of what the SG attenuator says so 0.5 X
> 0.55 = 0.27 so the output of the DA-121 is now approximately one fourth of
> the SG dial setting.
> >>>>> I checked with the VOM using a 125 ohm terminator instead of the
> R-390A and read 52.4 ohm.  You must have a wiring error!  The DA-121 should
> read approximately 40.9 ohms at the SG terminals when the DA-121 output is
> terminated with 125 ohms.  This is what was calculated above.  If you now
> put a 50 ohm resistor across the DA-121 input, the resistance of the input
> to the DA-121 should measure approximately 25 ohms.
> >>>>> To be sure that I was not tricked by the cables, I made the same
> test at 100 kHz with 10 mV and that below is what I read, again using the
> 125 ohm terminator on the oscilloscope side.
> >>>>> Probably I am doing something wrong, but what?
> >>>>> Your experimental data should closely agree with the math, it does
> >>>>> not.  There is at least a simple wiring error or the BNC to TWINAX
> >>>>> adapter is not wired properly.  As I mentioned in the email below,
> >>>>> one of the TWINAX pins should be directly connect to the BNC center
> >>>>> conductor, the other TWINAX pin should be directly connect to the
> >>>>> shell of the BNC connector.  There should not be any measurable
> >>>>> resistance, ideally a short circuit for both ohmmeter readings.  Can
> >>>>> you verify this? Thanks again Gianni Regards, Jim
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> Il giorno 6 ott 2024, alle ore 17:05, Jim Whartenby <
> old_radio at aol.com> ha scritto:
> >>>>> Gianni
> >>>>>
> >>>>>
> >>>>> There is something wrong with your measurements.  They do not agree
> with the mathematical analysis.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> Reducing the resistances of the DA-121 with the input resistance of
> the R-390 to a single resistance results in the total resistance seen by
> the SG of 25 ohms.  So the generator output should fall from 10 mV to 5 mV
> which you confirm although there is an error of some 14% ((5.7 mV - 5 mV) /
> 5 mV).  But as you say, the resistors are not perfect.
> >>>>>
> >>>>>
> >>>>> What is apparently the problem is that your adapter from BNC to
> TWINAX does not measure correctly.  One TWINAX pin should connect to the
> BNC center pin and the other TWINAX pin should connect to ground.  If this
> does not happen, the second voltage divider, the 100 ohm in series with the
> 125 ohm is not connected to ground.  This error would give you the voltage
> that you measure.
> >>>>>
> >>>>>
> >>>>> There is agreement between us that when the 68 ohm resistor is
> connected to the SG that the output will fall from 10 mV to about 5 mV.
> Putting the two remaining resistors into the circuit results in a series
> 100 ohm resistor and a parallel 125 ohm resistor.  Applying voltage divider
> analysis to this we have (5 mV X 125 ohms) / 225 ohms) which equals 2.28
> mV.  2.28 mV divided by 10 mV gives a ratio of 0.23 which is in agreement
> with the DA-121 reducing the SG output from 10 mV to 2.5 mV or 4:1.
> >>>>>
> >>>>>
> >>>>> The above analysis agrees completely with figure 3, the analysis of
> a T-pad, which was done in the 1950s.  It changes the SG impedance of 50
> ohms to the receiver impedance of 72 ohms with a voltage loss of 4:1 which
> I again enclose in this email.
> >>>>>
> >>>>>
> >>>>> Regards,
> >>>>>
> >>>>> Jim
> >>>>> Logic: Method used to arrive at the wrong conclusion, with
> >>>>> confidence.  Murphy
> >>>>>
> >>>>> On Sunday, October 6, 2024 at 01:46:02 AM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
> >>>>>
> >>>>> Hi Jim,Setting rge SG to 10 mV I have1) with no terminator
> >>>>> oscilloscope side: 20 mV2) with 50-ohm terminator: 10 mV3) with
> >>>>> DA-121 no terminator: 11.4 mV4) with DA-121 and 125 ohm terminator
> >>>>> (which simulates the receiver): 5.7 mV
> >>>>>
> >>>>> exactly as I would expect. Now I am going to pickup another
> generator to see if it behaves like the 8640. In the afternoon I tell you
> the result of the test.
> >>>>> YoursGianni
> >>>>>
> >>>>> Il giorno 6 ott 2024, alle ore 00:00, Jim Whartenby <
> old_radio at aol.com> ha scritto:
> >>>>>
> >>>>> Giovanni
> >>>>>
> >>>>> If you measure twice the voltage with no load on the SG then the
> actual voltage when the SG is properly loaded with a 50 ohm termination,
> what does the meter read when you put a 25 ohm resistor on the SG output?
> It should now read a third of the unterminated SG voltage.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> Enclosed is page 51 of the Measurements catalog.  Figure 3 shows a T
> pad to match 50 ohms to 72.  The resistor values are chosen to reduce the
> SG output voltage by half at the input to the T pad and to 1/4 at the
> output of the T pad when the T pad is terminated with a 72 ohm resistor.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> The same is done with the DA-121 but the impedance transformation is
> now from 50 to 125 ohms.  Can you measure the voltages at the output of the
> SG with an oscilloscope?  It should be 2X of the SG meter reading with no
> load on the SG, 1X with a 50 ohm load and 1/4X of the SG meter at the
> output of the DA-121 when the DA-121 is terminated with a 125 ohm non
> inductive resistor in place of the R-390A.  If you do not terminate the
> DA-121 with a 125 ohm load then what you report as 0.56 of the SG meter
> reading would be correct.
> >>>>>
> >>>>> Regards,
> >>>>>
> >>>>> Jim
> >>>>>
> >>>>>
> >>>>> Logic: Method used to arrive at the wrong conclusion, with
> >>>>> confidence.  Murphy
> >>>>>
> >>>>> On Saturday, October 5, 2024 at 03:14:58 PM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
> >>>>>
> >>>>> Thanks for replying, I am very intrigued by this theme.
> >>>>> See below please and tell me your opinion.
> >>>>>
> >>>>>
> >>>>> Il giorno 5 ott 2024, alle ore 20:33, Jim Whartenby via R-390 <
> r-390 at mailman.qth.net> ha scritto:
> >>>>> Giovanni
> >>>>>
> >>>>> I need some clarifications.
> >>>>>
> >>>>> 1) You said: "It attenuates the signal voltage of 0.56 V, i.e. 5
> dB."So the Signal Generator (SG) meter indicates that the output voltage is
> 0.56 volts or are you are measuring 0.56 volts at the output of the
> DA-121/U when the SG meter reads 1 volt?  If so, how are you measuring this
> voltage?  Is it peak or peak to peak or RMS?  The assumption here is that
> it is RMS.
> >>>>> I wrote wrongly; I meant that the DA-121 is a voltage divider that,
> considered 125 ohm the input impedance of the receiver, multiplies the
> siggen voltage x 0.56.
> >>>>>
> >>>>> 2) The DA-121/U contains two resistors, a 68 ohm resistor in
> >>>>> parallel with the signal generator output and a series 100 ohm
> >>>>> resistor to the center pin of the BNC output connector.  You are
> >>>>> then adapting the BNC output connector of the DA-121/U to TWINAX and
> >>>>> then connecting it to the balanced RF input connector on the back of
> >>>>> the R-390A, correct?  Yes
> >>>>>
> >>>>> 3) What are the two resistor values in the DA-121 when you measure
> with your DMM?  How close are they to what is expected?  I am guessing that
> these two resistors are carbon composition and are a bit off in value.  It
> is interesting to note that carbon composition resistors will change value
> when soldered into a circuit. No, it is not the original, I built it with
> new components.
> >>>>>
> >>>>> 4) When you measure the BNC to TWINAX adapter, one of the TWINAX
> pins goes to the center pin of the BNC connector and the other TWINAX pin
> goes to ground?  Yes Both read close to zero ohms? each other yes, but they
> are open to ground.
> >>>>>
> >>>>> 5) How old are the coax cables used in your measurements?  In other
> words, how lossy are they?  Coax ages so the cable losses will increase and
> it will have an affect on your measurements.  The coax is 50 ohms? Yes,
> they are normal BNC/BNC, 1 meter long, with 50 ohm cable, bought new ready
> to be used.
> >>>>>
> >>>>> The way I see it, 50 ohms in parallel with 68 ohms = 29 ohms.  29
> ohms in series with 100 ohms = 129 ohms which is approximately your
> impedance transformation needed from 50 to 125 ohms.  Because of the 68
> ohms is in parallel with the SG output, the voltage at this point should be
> half of what the SG meter indicates. I am not sure it is so. The siggen
> indicated voltage is in Vrms and it is true when you have a 50 ohm load. If
> you don’t have the 50 ohm load, the voltage is double. I am sure of this, I
> tested more times with different generators and oscilloscopes.
> >>>>>
> >>>>>
> >>>>> The second voltage divider of 100 and 125 ohms is again reducing the
> SG output voltage by another half so the actual voltage applied to the
> receiver is 0.5 X 0.5 or 0.25 times the SG meter reading.  In other words,
> actual voltage applied to the R-390A receiver is 1/4 of what the SG meter
> indicates or 12 dB down. No, I am sure of 0.56. In the doubt, I built a 125
> ohm terminator and checked with the oscilloscope. Starting with 10 mVrms I
> read 5.7 mVrms because the resistors are not perfect. And thus reduces the
> voltage by 5 dB. Do you agree?
> >>>>>
> >>>>> So what this means to the original discussion is that the 6.5
> microvolt limit in the R-390A specification is actually 1.6 microvolts that
> is applied to the R-390A balance RF input for a 10 dB S+N/N reading when
> all of the losses in the test setup are accounted for.  So the spec has
> simplified the measurement and eliminated all of the above math.  Again,
> spec is spec and those who wrote it knew what they were doing.
> >>>>>
> >>>>> This back of the envelope analysis does not agree with what you have
> measured.  I am interested in what you find when you have a chance to take
> a closer look.
> >>>>>
> >>>>> Jim
> >>>>>
> >>>>> Logic: Method used to arrive at the wrong conclusion, with
> >>>>> confidence.  Murphy
> >>>>>
> >>>>> On Saturday, October 5, 2024 at 01:48:09 AM CDT, Ing. Giovanni
> Becattini <giovanni.becattini at icloud.com> wrote:
> >>>>>
> >>>>> Hi Jim and thanks for your reply. I read the very interesting
> document you pointed out. I did not understand everything, but for my
> practical interest it confirms that the impedance matching is mandatory.
> >>>>> I am using an HP8640B as a signal generator. Let’s suppose it is
> ideally calibrated. I use also the DA-121/U impedance adapter which shows
> 50 ohm to the siggen and 125 to the receiver. It is the fourth type of pad
> of figure 4 of the article.
> >>>>> My practical question is how to take in account the DA-121?
> >>>>> It attenuates the signal voltage of 0.56 V, i.e. 5 dB. So,
> >>>>> - in volts: the voltage value for the 10 dB S/N I read on the
> generator’s scale should be multiplied by 0.56.
> >>>>> - in dBm:  the dBm value for the 10 dB S/N I read on the generator’s
> scale should be reduced by 5 dBm.
> >>>>>
> >>>>> Is this correct?
> >>>>> Thanks
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
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