[R-390] Official specs

Barry n4buq at knology.net
Wed Oct 23 13:17:31 EDT 2024 

Stretching this a bit more, I'm still wondering whether my HP-8640B(s) need a 50-ohm terminator when doing this.  I ask because unless it's terminated, then the output is higher than what shows on the generator's output meter as well as what I get with my HP-410B's AC probe.  Granted, the HP-8640B struggles to output anything with enough amplitude to use the HP-410B to measure it, but I've run the HP-410B against my meter calibrator and it seems to be pretty close.

With the DA-121/U already having the 68 ohm paralleled with the SG's output, I'm thinking an additional 50-ohm terminator isn't necessary but still wondering.

Basically, if the manual calls for, say, 150uv, I'd like to somewhat confidently dial that in on the SG.

Thanks,
Barry - N4BUQ

> Hello Bob,
> 
> I 100% agree that the convention on a RF source is that the displayed output
> voltage is valid only when the SG is loaded with the correct termination
> impedance.
> I disagree however that the outcome "depends" on what can be used to provide the
> answer.
> If anyone is ignorant enough to use a Spice model that not include a source
> output impedance, it is sure that the result of such "simulation" will be
> different from what is obtained with properly set up test equipment.
> A properly used simulation software results will not be different that what can
> be obtained with "real" instruments.
> If ever this is the case, the inputs to the simulation program are faulty.
> 
> Bob Pease (RIP) once fell in that trap: he took revenge by throwing his computer
> from the top of the building into the parking lot below.
> 
> But, at the end of the day, nothing is more practical than a good theory.
> 
> 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
> Bob Camp
> Envoyé : 23 octobre 2024 12:26
> À : Ing. Giovanni Becattini <giovanni.becattini at icloud.com>
> Cc : r-390 at mailman.qth.net; Larry Haney <larry41gm2 at gmail.com>
> Objet : Re: [R-390] Official specs
> 
> Hi
> 
> The very basic issue here turns that into a “that depends” answer:
> 
> If you are playing with a Spice model, and starting from the voltage on your
> ideal source, you get one answer.
> 
> If you are working at RF and using a signal generator (and doing things
> properly) you get a very different answer.
> 
> The convention on an RF source is that the “starting voltage” is measured with
> the correct termination in place.
> 
> Since we’re talking about RF …. that’s how it would be done.
> 
> Bob
> 
>> On Oct 23, 2024, at 10:39 AM, Ing. Giovanni Becattini via R-390
>> <r-390 at mailman.qth.net> 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 <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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