[R-390] Official specs

Wed Oct 23 10:39:26 EDT 2024

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 <larry41gm2 at 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_radio at 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 <
>> larry41gm2 at 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_radio at 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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