Rigol DS1074Z-S Function Generator Help

[dellsam34]

I have a vintage audio gear that I need to adjust some audio level values such as recording, playback ...etc. The service manual instructs me to connect a 1KHz audio tone with -10dB level to the line IN of the deck being worked on, How do I set the Oscilloscope's output level to -10dB ?

[ledtester]

Function generators deal with amplitudes, and by "-10 dB" my guess is that they mean "-10 dBV".

dBV is defined as:



(the log is base 10) and VRMS is Vpeak/sqrt(2).

The final answer is Vpeak = sqrt(2)/sqrt(10) V = 0.447 V -- that's the amplitude of the sine wave you want to generate.

A online conversion calculator for these things:

https://www.analog.com/en/design-center/interactive-design-tools/dbconvert.html#

[rvalente]

Also, you need to check the impedance input of your amplifier and the impedance output of your generator.

Of the generator outputs at 50 or 600Ohms and the amplifier input is High impedance, just check back with the scope if the level matches the voltage you want.

[dellsam34]

Here is exactly the test I'm doing as illustrated below, If I have to use a calculator to convert the values every time is there a stand alone function generator that has the output dB reading built in?

[TurboTom]

You can use any (universal) generator that's capable of selecting the putput level as dBm if it's possible to specify the impedance it's attached to. Select the connected impedance to be 1kOhm and your generator will output the displayed value as dBV (provided there's actually a 1kOhm impedance driven). But even a different impedance (infinite to maybe 500 Ohm) won't affect accuracy too much. If necessary, you can compensate by slightly modifying the specified 1kOhm termination impedance entry in the generator setup.

Currently, the best bang-for-buck AWG that's capable of this is Rigol's DG811. There may be special audio generators that provide direct dBV specification but I guess this kind of equipment (unless its an old, beaten-up boat anchor...) cannot compete price-wise.

[dellsam34]

I've already dropped $700 for the oscilloscope, I don't think I'm willing to get a separate generator for another $250 or so, I may look for an analog function generator that is less than $50 used or figure out a way to use the built in my oscilloscope one.

[TurboTom]

Sorry if you didn't like my reply  .

You can very well get the job done with your scope's internal generator. The scope "talks" SCPI and this includes the [:SOURce] functions so use your PC to do the dBV -> Vpp conversion for you and automate the procedure. I assume that you are frequently doing these adjustment jobs so this may make sense.

Otherwise, I'ld just print out a chart that references the typically required dBV figures to Vpp values and pin it to the wall so I could dial in the numbers. Btw, manually dialing in the numbers on Rigol's DS....S "embedded" AWGs is a PITA of its own. I'ld once again think about using a computer to control the AWG...

[dellsam34]

I posted two more posts here and I can't see them anymore, are the mods deleting posts without notice? Anyway, No there is nothing wrong with your post, I just don't have the budget to buy another function gen. Thanks

[dellsam34]

Function generators deal with amplitudes, and by "-10 dB" my guess is that they mean "-10 dBV".

dBV is defined as:

(the log is base 10) and VRMS is Vpeak/sqrt(2).

The final answer is Vpeak = sqrt(2)/sqrt(10) V = 0.447 V -- that's the amplitude of the sine wave you want to generate.

A online conversion calculator for these things:

https://www.analog.com/en/design-center/interactive-design-tools/dbconvert.html#

If I have to use this conversion scheme, So that means I have to enter the -10dB value in the calculator and take the resulting voltage reading and use it to adjust the amplitude in the oscillo function gen, Right? The question is which dB value should I enter dBm, dBu or dBV? and which voltage should I note the Vrms or the Vpeak? And what ohms I enter 600Ω by adding an attenuator or just measure the internal resistance of the pre-amp from the RCA line input when no power applied to it?

[ledtester]

After carefully reading the instructions and looking at the diagram I've come to the following conclusions:

1. Note where it says "1V = 0 dB" so it's clear they are talking about dBV - not dBu or dBm (which is a power measurement anyway).

2. From the excerpt I posted in my earlier reply, dBV means RMS -- so 0 dBV means 1V RMS.

3. The gist of the test is to record say 30 secs of a known signal level onto the tape. Then erase the last 15 secs. Then play the tape and compare the output level of the two segments using a VTVM. The ratio of the two levels should be -55 dB which is 10^(-55/20) ≈ 0.002.

4. The first step is to create a "0 dB" signal on the "VU meter" by which I suppose they mean the VTVM that's connected to the left channel line out but perhaps the recorder also has a VU meter? This signal is a combination of the output of the signal generator and the record volume level so it seems the signal generator doesn't need to be set to a precise level. As long as you can use the record volume control to get the desired strength you'll be fine.

If you are monitoring the left channel line out with your scope I'd aim for a 1V RMS signal for this step.

5. It then says to increase the output of the signal generator by 10 dB. 10 dB = 10^(10/20) = 3.16 - so this should happen by increasing the amplitude of your signal generator by a factor of 3.16. Although check the strength on the "VU meter" just to be sure.

[dellsam34]

The VU meter they are talking about is the built in the tape deck which has to be calibrated first according to the service manual), once calibrated it will be the reference for the remaining tests.

I'm not monitoring the line out with the oscilloscope, I'm just using its function generator to create the desired signal level which is -10dB.

My concern is only how to use the function generator, I understand the remaining of the test procedure which is to look for any residual signal left after erasing the tape in the same frequency range by isolating tape hiss using a pass thru filter set to 1KHz.

[TurboTom]

Yesterday i typed an extensive reply and published it but unfortunately, while the forum was down for maintenance, it must have been lost  . Also no copy in my local cache... And I won't type all this again, sorry.

To adjust the recorder, you apparently only need two levels (as @ledtester already told). -10dBV = 0.894Vpp; 0dBV = 2.282Vpp. Both of these figures are available with your DS1000Z-S when set to high-Z impedance. It's assumed that the input impedeance of your DUT is high (>5kOhm will result in an absolute error < 1%). If it's lower, you've got to take the generator's 50 Ohm source resistance into account and correct the amplitude setting accordingly, i.e. for an input impedance of 600 Ohms, multiply the specified Vpp figures by 1.083 (factor=(Rin+50)/Rin).

[dellsam34]

According to the calculator if I enter -10dB next to the dBV, the Vrms is 0.3162V and Vpeak is 0.4472V which neither equals to what you have suggested, Even changing the ohm value in the calculator doesn't seem to affect the above voltage values, Again still confused, sorry.

My question is really not about the test itself, it's about using the function generator, and while I understand that dB is a ratio I still have to enter that value into the function gen. which unfortunately speaks a different language, how to translate from dB to voltage is my biggest hurdle.

So if I understand correctly the values used for converting are dBV and Vrms.
-20 dBV = 0.1 Vrms
-10 dBV = 0.3162 Vrms
0 dBV = 1 Vrms (Reference)
3 dBV = 1.413 Vrms
10 dBV = 3.162 Vrms (which doesn't exist in line level world I guess)

[TurboTom]

For a sine wave, 0.4472Vp equals 0.894Vpp (just twice your figure). My "pimped" DS (MSO) 2072A-S AWG permits only the entry of Vpp values as amplitude, I don't know if the same is case on your DS1074Z-S. So the conversion of the 1V-referenced level (Lv) to Vpp is: Vpp = 2 * Sqrt(2) * 10^(Lv/20)

[dellsam34]

What's Vp and Vpp? and how they are different from Vrms. Also how did you come up with 1.083 figure?

Edit: Oh I see why you want to double the reading, Bingo

[Anthocyanina]

What's Vp and Vpp? and how they are different from Vrms. Also how did you come up with 1.083 figure?

Edit: Oh I see why you want to double the reading, Bingo

Vp is peak voltage, from 0 to the highest voltage the waveform gets to, Vpp is peak to peak voltage, from the negative peak to the positive peak. Vrms is ~0.707*Vp and this website explains in depth why that is https://www.electronics-tutorials.ws/accircuits/rms-voltage.html 

[TurboTom]

The 1.083 factor is just the inverse of the voltage divider ratio of the 50 ohm source resistance of your generator and the (assumed) input impedance (Rin)  of 600 Ohms of your appliance that you want to calibrate. I explained that at high input impedances, the correction isn't necessary. The correction factor is (Rin + 50Ohms)/Rin as mentioned above. You will wasily see that at Rin >> 1kOhm, this factor pretty much equals one.

[dellsam34]

Now for the VTVM part, Should I use a true RMS DMM such as the Fluke 115 that I already have or any suggested equivalent or I have to have a VTVM? Some say that the VTVM has very high impedance that's why it's preferred for such measurement, What do you guys think?

[TurboTom]

The Fluke 115 is specified for AC measurements up to 1kHz (2% tolerance) so it would do the job for you. Since the signal is supposed to be measured in parallel with 100kOhm, the 10MOhm input impedance of the meter is neglible. Just use it instead of the VTVM.

But you could also use your scope and enable the RMS measurement function, though this may not be as accurate as the DMM is. Enabling statistics/averaging helps here. Have a look at the scope's maunal if you are not familiar with these functions. Using the scope for this task will especially increase available measurement bandwidth tremendously.

[dellsam34]

I posted about using the oscilloscope in a different thread where I asked if I could use the Oscillo as a function gen and as an AC meter at the same time.