[R-390] Official specs

Wed Oct 23 12:46:23 EDT 2024

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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