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.
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?
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.
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?
Both read close to zero ohms?
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?
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. 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.
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
Il giorno 5 ott 2024, alle ore 05:13, Jim Whartenby via R-390
<r-390@mailman.qth.net> ha scritto:
BobI don't know why you are speculating "Since you can use any signal
generator" when MIL-R-13947B(SigC) on page 20 specifies the Measurements Corp
Model 82 signal Generator, or it's equal. Your concern with matched impedances
between the generator and the R-390 may appear to be valid but this seems to
have been accounted for in the R-390 spec with higher input voltages to account
for the mismatch losses. Collins and the Signal Corps specified the 125 ohm
input impedance of the R-390 and they were surely aware that the standard
impedance of high end signal generators was normally 50 ohms.
Check out http://hparchive.com/Boonton/BRC-The-Notebook-03.pdf which is a 1954
Boonton Radio explanation for the use of Dummy Antennas. Everything you need
is in the first four pages. It explains why the mismatch between 50 ohms and
the receiver input impedance reduces the loading on the receiver input tuned
circuits and recovers the Q of the receiver RF input tuned circuits. This also
affects favorably the S+N/N measurement. There is a reason for the apparent
madness.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Friday, October 4, 2024 at 07:23:24 PM CDT, Bob Camp <kb...@n1k.org>
wrote:
Hi
Based on a *lot* of interaction with DOD source inspectors over the years …
it’s very much a “that depends” sort of thing. The guy you get this month may
be *very* different than than the guy who comes in next month.
Since you can use any signal generator, there is room for “trouble” with
minimal specs. Typically the way this works out is a request to clarify things.
Unfortunately that stuff does not get into the official specs. Yes, some of us
have pointed that out as a problem …. never got addressed AFIK.
Bob
On Oct 4, 2024, at 5:19 PM, Jim Whartenby via R-390 <r-390@mailman.qth.net>
wrote:
Bob
You are over thinking the testing. The generator is terminated but not in it's
characteristic impedance. Collins and the agency letting the contract agreed
on a test method and wrote it down in the test procedure. Anything that would
affect the testing like VSWR has already been considered and accounted for.
That Hams don't use the proper termination for the signal generator, according
to the spec, which affects the perceived sensitivity is another issue. Spec is
spec as they say.
Not all Government Source Inspectors are knowable about the equipment that they
are reviewing and putting their inspection stamp on. Not all Signal Corps
equipment is inspected by Army GSIs. Every once in a while, Army equipment
will have an anchor stamp on it and vice-versa. Sometimes certain specs are
wavered if not considered critical, most often they are not. Again, spec is
spec.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Friday, October 4, 2024 at 02:43:36 PM CDT, Bob Camp <kb...@n1k.org>
wrote:
Hi
Ok …. but …..
If it is simply a resistor, then the generator is unterminated. It’s designed
and calibrated to run into a 50 ohm load. If all that’s there is a series
resistor the generator is not properly set up (and thus not calibrated ….). Yes
some generators deal with this better than others.
My guess is that there’s more to the load circuit than just that resistor.
Without a schematic …. who knows ….
Bob
On Oct 4, 2024, at 12:53 PM, Jim Whartenby via R-390 <r-390@mailman.qth.net>
wrote:
Bob
But we do know what the interface is between the signal generator and antenna
input. On document page 20, the balanced RF input is in series with a 125 ohm
non-inductive resistor and the unbalanced RF input is in series with a 50 pF
capacitor. Both interfaces do not include the generator's output impedance.
As for the sensitivity not being what the R-390/URR or the R-390A/URR is
capable of, well there are perhaps 100's of tests that the receiver must
successfully pass before it is accepted. Tightening any of the specs to
exactly what the receiver may be capable of passing will guarantee that no one
receiver will ever pass all of the acceptance tests. Then there is the added
problem of your test equipment's error tolerance and being traceable back to
the NIST standards. Is your 10 microvolts from the signal generator really 10
microvolts?
Unless otherwise stated, the specification calls out a value that the receiver
must do better than. A sensitivity of just under 6.5 microvolts for a 10 dB
S+N/N with an audio power output of 10 milliwatts seems reasonable over the
range of 2 to 32 MHz for either balanced or unbalanced RF inputs. This is
perhaps typical for just about all HF receivers built for the military, at
least for what I am aware of, but I am sure that there will be the rare
exception.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Friday, October 4, 2024 at 09:39:26 AM CDT, Bob Camp <kb...@n1k.org>
wrote:
Hi
The gotcah here is that we really don’t know what the interface between the
signal generator and radio looked like for these official tests. Despite the
document going into a lot of detail, they did not include a schematic or a part
number. (or at least not one that I could find). There are a *lot* of ways they
might have been doing things …..
Bob
On Oct 4, 2024, at 10:19 AM, Ing. Giovanni Becattini via R-390
<r-390@mailman.qth.net> wrote:
But when we perform the classical sensitivity test with a modulated signal, do
we get the AM or CW value?
Following the instructions I got a value of -104 dBm, i.e. 1.41 uV. Because I
used the DA-121, the real value should be even better, far from 5 uV. And
surely my R-390A is not as good as it could be .
Is my reasoning correct?
Il giorno 4 ott 2024, alle ore 16:13, Barry <n4...@knology.net> ha scritto:
I don’t think of 1uV as “bad” but most of these radios will beat that.
Thanks,
Barry
On Oct 4, 2024, at 8:55 AM, Ing. Giovanni Becattini via R-390
<r-390@mailman.qth.net> wrote:
Wow, thanks!
By the way, do you know why the sensitivity values were so bad? Are we the ones
who are taking measures that are too lenient or are they the ones who were too
conservative?
Thanks again
Gianni
Il giorno 4 ott 2024, alle ore 15:37, Larry Haney <larry41...@gmail.com> ha
scritto:
Hi Gianni and Barry, Tom Marcotte and Al Tirevold (SK) obtained and cleaned up
the specs for the 390A and put it on our website. Here's the link:
mil-r-13947b– (r-390a.net) <https://www.r-390a.net/mil-r-13947b.pdf>.
Unfortunately, the entry in References page indicates it is for the R390(), but
it is also the the 390A. I will be changing that shortly.
Regards, Larry
On Fri, Oct 4, 2024 at 4:49 AM Ing. Giovanni Becattini via R-390
<r-390@mailman.qth.net <mailto:r-390@mailman.qth.net>> wrote:
Have ever been released official specs for the 390A?
The manuals report few data and neither too correct, for what I can understand
(e.g., sensitivity)
Thanks
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