GianniComments in BOLD
On Monday, October 7, 2024 at 02:43:53 AM CDT, Ing. Giovanni Becattini
<giovanni.becatt...@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_ra...@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.becatt...@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_ra...@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.becatt...@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@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.becatt...@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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