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avatar_Targhan

Anyone to make sound measurements with an oscilloscope?

Started by Targhan, 10:51, 10 April 21

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Bryce

If I get time later I'll simulate the same frequency on an expensive signal generator and compare the noise floor in that range.

Bryce.

Targhan

I don't understand half of what you're talking about :). But the conclusion seems to be straightforward:

Quote from: Bryce on 14:44, 01 May 21The CPC produces an extremely clean, as square as possible (for the electronics in a CPC) wave

You hear that @roudoudou ? Like SuperSylvestre had hinted with his measurements.

Thanks a lot Bryce!
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Bryce

Quote from: RetroCPC on 19:52, 01 May 21
Hi Bryce,

I'm not convinced that the noise at the bottom end of the band is due to the lower limit of your SA. Sure the close in Phase noise of an "affordable" SA local oscillator is not going to be anything to write home about, but I doubt its as bad as seen in the sweep - I think much of what your seeing is really the SNR of the CPC.


For sure the HF noise is due to the lack of Ground plane, decent PSU decoupling etc. If I had spare time on my hands I'd love to redesign the PCB for improved video / audio :) With modern Hi-bandwidth LCD monitors you can see a lot of noise on the CPC's RGB output - even with the HF BW limiting filter capacitors fitted...

Its not in anyway a criticism of the CPC :) , its just the reality of 30 years of engineering progress in consumer grade electronics...

Ok, here's a comparison. Same frequency and amplitude, but produced by a high-end signal generator. As you can see the even harmonics are gone, but the rise in noise floor at the lower end of the scale is still there (ie: it's a component of the SA's filtering). The slightly higher peaks are due to the fact that I have to use a different cable to connect the equipment to each other. On the oscilloscope screenshot the differences are much more obvious, the wave is perfectly square with no rounding on the leading edges. This is due to the fact that the wave is being produced by high end equipment that can produce signals up to >100MHz and has high frequency output transistors.

Bryce.


Targhan

@Bryce when and if you have time, could you record a sound with a period of 0? I know the Basic allows that but I hope it won't implicitly raise it to 1. It should produce the same period as 1 (like what you previously recorded), but according to Zik experiments some years ago, the volume should be lower because of a "filter" (please don't ask me for the details, I don't know them) that is produced by the PSG with this very period. It would be interesting to know the reason...
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RetroCPC

Bryce,


Thanks for the extra tests - I'm really surprised that the Close-in LO phase noise of the Rigol is so poor - I wonder what they are using as the LO synthesizer?

What CPC machine are you using & are you Measuring via the 3.5mm audio output?
I have a CPC6128, you now have me interested to run a few measurements, whats the basic commands to generate a 1KHz / full level signwave via the CPC sound chip?

Bryce

@Targhan : Sound 2,0 doesn't produce any signal, the output stays a 0V.

@RetroCPC : I have used much more expensive analysers at work (Agilent E and N series) and they all do this. It is not due to the local oscillator, rather the fact that the closer you get to DC, the harder it is to filter noise without attenuating the signal too much. That's why almost all SA's start at 9kHz. Usually this doesn't matter though, because they aren't really designed to analyse single signals this low down.
By the way. I should mention that the RF Pre-Amp was turned off for these measurements. With the Pre-Amp turned on the overall noise floor drops a few dB more and the low frequency noise floor is flattened slightly more (see below).

I used a standard German 6128. Measurements were taken at the 3.5mm output. To make a sound you use the command Sound 2,x,y,z
2 is the channel
x is the tone (1kHz is around 61 or 62)
y is the length of time it plays for (30000 gives you about 5 minutes)
z is the volume (I think 15 is full volume)


Bryce.

Animalgril987

According to the 464 User Manual, tone period is 125000÷frequency, so 1kHz should be a tone period value of 125.

RetroCPC

Bryce,

Thanks for the sound commands - Ill give it a go.


Spectrum analyses sweep the frequency band by mixing there internal Local oscillator with the input signal - the sloping noise floor that trends upwards towards DC is the result of the close-in carrier phase noise of the SA's local oscillator (and to a smaller degree any active internal gain stages).

Its true that all oscillators exhibit increasing close in phase noise towards DC – but a good design will reach the flat portion of its phase noise noise floor before say 1KHz.. or as quickly as possible – the higher the Q of the oscillator the narrower the oscillators "side skirts". Tunable oscillators by there very nature have lower Q hence worst close-in phase noise.

https://www.anritsu.com/en-US/test-measurement/solutions/ms2840a-066/index

Its explained very "simplistically" in the above link - see Figures 1 and 2... where the (marketing departments) curde graphs give "text book" examples of LO close-in phase noise...

WRT SA filters (RBW & VBW) in older units they where "simple" Analogue filters which placed practical limits on the minimum resolution B/W  - the brillent HP3585A/B has a 3Hz B/W for its narrowest filters - but modern SA supplement wider B/W analogue filters with FFT digital filter techniques to achieve sub Hz resolution bandwidths with comparative ease.

Bryce

I had RBW and VBW set to 100hz, otherwise the scan takes forever. Of course it's unfair to compare the Rigol with an Agilent or Anritsu. My Rigol "only" cost around €5000, whereas the price of the Agilents we use at work is on the wrong side of €70,000 !!

Bryce.

RetroCPC

Amstrad CPC6128 Audio Line output 1KHz test tone:-

Sound 2,62,xxxx,15

0dB = 137mV RMS into 20Kohms

16K FFT

Well the Amstrad Sound chip produces Squarewaves - rather then "clean" low THD Signwaves

Lots of LF noise, but could be worst...

GUNHED

Quote from: RetroCPC on 19:29, 02 May 21
Lots of LF noise, but could be worst...
Only the internal speaker, not the 3.5 mm jack.  :)

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RetroCPC

Judging by the whinny noise coming from my CPC6128 speaker, the internal audio circuit must be much worst, I'll repeat the test measuring across the internal speaker...

RetroCPC

Quote from: GUNHED on 19:32, 02 May 21
Only the internal speaker, not the 3.5 mm jack.  :)

The above FFT is measured from the line output...

GUNHED

Your ears work better.  :laugh:  (Didn't see the picture before)
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RetroCPC

Well apart from the interesting noise floor, I had no idea that the CPC sound chip produced Square waves rather then Signwaves...

Bryce

I'm sure the capacitance and inductance of the end stage will ensure that they are anything but square :D

Bryce.

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