Hi If you start with a Spice model and put in a 2V source with some series resistance. You loose 6 db when you get to 1.0 V. That’s how Spice looks at things.
If you start with the signal generator convention you have a “spice” 2V source and a 50 ohm resistor “inside the box”. Put on a 50 ohm load and you have 1V. That’s your zero db point with the signal generator. Start one way and you are 6 db down. Start the other way and you are at zero db. Bob > On Oct 23, 2024, at 1:03 PM, Jacques Fortin <jacque...@videotron.ca> wrote: > > 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-boun...@mailman.qth.net <r-390-boun...@mailman.qth.net> De la part > de Bob Camp > Envoyé : 23 octobre 2024 12:26 > À : Ing. Giovanni Becattini <giovanni.becatt...@icloud.com> > Cc : r-390@mailman.qth.net; Larry Haney <larry41...@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@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 <larry41...@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_ra...@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 < >>>> larry41...@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_ra...@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 >>>> >>>> >>> ______________________________________________________________ >>> R-390 mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:R-390@mailman.qth.net >>> >>> This list hosted by: http://www.qsl.net Please help support this >>> email list: http://www.qsl.net/donate.html >> >> ______________________________________________________________ >> R-390 mailing list >> Home: http://mailman.qth.net/mailman/listinfo/r-390 >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:R-390@mailman.qth.net >> >> This list hosted by: http://www.qsl.net Please help support this email >> list: http://www.qsl.net/donate.html > > ______________________________________________________________ > R-390 mailing list > Home: http://mailman.qth.net/mailman/listinfo/r-390 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:R-390@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:R-390@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
