[R-390] Official specs

Wed Oct 23 13:21:04 EDT 2024

Hi Jacques,

I was thinking that any effects of the spiral cutting in the modern resistors isn't much of a factor unless they're used at frequencies a lot higher than 32 MHz but thought I'd check.

Thanks!
Barry - N4BUQ

> Hi Barry,
> Carbon composition for sure, but using metal or metal oxide films will do no
> harm.
> There is an urban legend about the carbon composition resistors, stating that
> they are not inductive and that the metal or mox ones are.
> This is not absolutely true: the carbon comps turns capacitive at high
> frequencies and the metal + mox ones did not exhibit more inductance than the
> equivalent length of wire.
> 
> 73, Jacques, VE2JFE in Montreal
> 
> -----Message d'origine-----
> De : Barry <n4buq at knology.net>
> Envoyé : 23 octobre 2024 12:46
> À : Jacques Fortin <jacques.f at videotron.ca>
> Cc : Jim Whartenby <old_radio at aol.com>; Larry Haney <larry41gm2 at gmail.com>;
> R-390 Mailing List <r-390 at mailman.qth.net>
> Objet : Re: [R-390] Official specs
> 
> 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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