[R-390] Official specs

[Larry Haney]

Hi Barry,  Yes, they were 5% carbon comps and yes, the metal films should
work fine.  Above 100 MH can be a problem, though, so that does not affect
390s.

Regards, Larry

On Wed, Oct 23, 2024 at 9:46 AM Barry <n4buq at knology.net> wrote:

> Anyone know what type of resistors were used in the DA-121/U?  I almost
> presume 5% carbon comps but not sure if metal film would be okay.  I
> presume so, especially at the frequencies where this is used, but would
> still like to know.  I plan to make one and want to make it as close to
> original as possible.
>
> Thanks,
> Barry - N4BUQ
>
> > Hi Jim,
> >
> > I feel that you went to a unnecessary elaborate demonstration to find
> the (power
> > wise) attenuation of a DA-121/U.
> > I cannot understand why you had not replicated the resistor values used
> in the
> > DA-121/U because the discussion starting point was to evaluate the
> DA-121/U
> > output vs the output setting of the SG connected to it.
> > In your experiment, it is clear that two DA-121/U connected back to back
> will
> > provide a POWER attenuation being the double of what one unit will
> provide.
> > But the initial Larry Haney concern was about the VOLTAGE output of a
> DA-121/U
> > in the R-390A balanced input (125 ohms load).
> > The calculated attenuation IN VOLTAGE measured by Larry of the DA-121/U
> is
> > correct as being 0.56.
> > If all the resistances values are EXACT (68 and 100 ohms) the calculated
> output
> > voltage across the 125 ohms receiver impedance is 0.5676 of the SG output
> > setting.
> > When Larry mentioned "5 dB" below, that is what his AC voltmeter tell
> him from
> > it's dB scale, (as being 20 log 0.56).
> > But what he used is a VOLTMETER, not a POWER meter: calling that -5 dBv
> will be
> > correct, but it cannot be translated as a POWER ratio because the input
> and
> > output impedances are not the same.
> > The real POWER attenuation of the DA-121/U is (calculated) 8.896 dB,
> close
> > enough from the 9dB you expected as the result of your own experiment.
> >
> > When you mention an "experiment" about inserting a 50 ohms resistor in
> parallel
> > with a 50 ohms coax, it will give the same result as loading the output
> of the
> > source SG with a 25 ohms value.
> > One of the 50 ohms resistors used will receive 3.52 dB less signal (10
> log
> > 0.44444) than if it was left alone as the load of the SG.
> > Not even necessary to involve the 50 ohms coax SWR (or the Gamma, for
> that
> > matter) in the demonstration.
> >
> > About your statement:
> > " Transmission lines behave differently then DC circuits"
> > I have a funny conceptual experiment for it, that I used as the
> introduction to
> > the course about transmission lines that I gave to my students at the
> time:
> > Let's figure that you have a PERFECT 50 ohms coaxial line (no power loss
> of any
> > kind inside the line) for which the electrical signal velocity inside is
> 70% of
> > the speed of light (Teflon dielectric) and that this "perfect"
> transmission
> > line is 210000 km long.
> > You take an ohmmeter (DC instrument) and connect it to one side of the
> line,
> > between the center conductor and the braid:
> > a) Does the meter reading will be 50 ohms ?
> > b) If yes, for how much time ?
> > c) If the other side (the end) of the line is shorted, what will be the
> > measurement ?
> > d) And what if the end is open ?
> > e) And what if the end is terminated in 50 ohms ?
> >
> > I will provide the answers later.
> > All the remaining of the course I gave was to explain these answers, and
> more.
> >
> > 73, Jacques, VE2JFE in Montreal
> >
> > -----Message d'origine-----
> > De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De
> la part de
> > Jim Whartenby via R-390
> > Envoyé : 22 octobre 2024 18:36
> > À : Larry Haney <larry41gm2 at gmail.com>; r-390 at mailman.qth.net
> > Objet : Re: [R-390] Official specs
> >
> > 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:
> >
> > LarryNo, 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.JimLogic: Method used to arrive at the wrong conclusion, with
> confidence.
> > Murphy
> >
> >
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