[R-390] Official specs
Bill
bmarx at bellsouth.net
Mon Oct 7 16:22:42 EDT 2024
This list doesn't...I monitor several mailman lists, and have added that
capability to send HTML and other pics.
Bill W2CQ
On 10/7/2024 12:54 PM, Barry wrote:
> Well, this one came right back into my other email account (I'm subscribed from two accounts). I now wonder if it doesn't like the HTML. I know one list I've been on doesn't like them and perhaps it was this one.
>
> I'll resend my earlier post in plain text and see how that goes.
>
> Thanks,
> Barry - N4BUQ
>
>> Does mailman accept HTML posts or just plain text? I ask because lately,
>> some of my posts either don't make it back to me or are very, very
>> delayed. I sent one around 8:00 CDT this morning and it had two words
>> italicized in it and I still have not seen it come back. Now I'm wondering
>> if the italics could be a problem.
>>
>> Thanks,
>> Barry - N4BUQ
>>
>>
>> On Mon, Oct 7, 2024 at 11:42 AM Jim Whartenby via R-390 <
>> r-390 at mailman.qth.net> wrote:
>>
>>> 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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