What has been overlooked is that there is an impedance transformation from 50
to 125 ohms. Any calculation that ignores this transformation is in error.
The only solution that accounts for different impedances is by looking at the
respective powers at both input and output.
1 volt into the DA-121 gives 0.556 volts out. Looking at the power-in verses
power-out using the respective impedances:
Power = voltage squared / resistance
Pin = 1 volt ^2 / 50 ohms = 0.02 watts
Pout = 0.556 volt ^2 / 125 ohms = .00247 watts
dB = 10 Log ^ (Pout / Pin)
dB = 10 Log ^ (.00247watts / 0.02 watts)
dB = 10 Log ^ 0.1235
dB = -9.083
To convert the SG voltage output into the voltage actually seen by the R-390,
multiply the SG reading by 0.1235 or divide the SG reading by 8.097, either way
works.
Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Wednesday, October 23, 2024 at 09:39:45 AM CDT, Ing. Giovanni Becattini
<giovanni.becatt...@icloud.com> 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 <larry41...@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_ra...@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:
https://k7mem.com/Res_Attenuator.html
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 <
larry41...@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_ra...@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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