I was using coax. I found supply decoupling was critical to reduce the linearity errors.
Interesting, I can imagine it took a while to figure that out. At least if I were to debug something like that, I would probably waste lots of time attempting to improve the transmission line terminations before finding out it was something else altogether.
By the way, the Studer A726 has a 68.18 ns coaxial delay line according to its product brochure.
By the way, the Studer A726 has a 68.18 ns coaxial delay line according to its product brochure.
HI if we try to get back to your original question, I was able to find this text which clarifies.
"This detection process can also be accomplished by combining, in an exclusive-OR (XOR) logic gate, the original FM signal and a square wave whose frequency equals the FM signal's center frequency. The XOR gate produces an output pulse whose duration equals the difference between the times at which the square wave and the received FM signal pass through zero volts. As the FM signal's frequency varies from its unmodulated center frequency (which is also the frequency of the square wave), the output pulses from the XOR gate become longer or shorter. (In essence, this quadrature detector converts an FM signal into a pulse-width modulated (PWM) signal.) When these pulses are filtered, the filter's output rises as the pulses grow longer and its output falls as the pulses grow shorter. In this way, one recovers the original signal that was used to modulate the FM carrier."
One tuner that uses XOR gate, the accuphase t-109V.
thanks
Paba
I’m not going to go into noise density vs IF bandwidth, it is obvious that you didn’t even checked the link I gave and look at the FFT plots taken from a low noise and distortion FM stereo generator and an FM stereo tuner that can take advantage of it. I have the same real measurements taken from actual FM stereo tuners that employ pulse count and PLL detectors. Not simulations and no speculations.
An FM generator has its own internal VCO linearity limit. Using an external modulation source with low THD like the R&S UPV doesn’t change this VCO linearity limit.
The likely reason your old colleagues can get lower than the 0.1% distortion limit of R&S SMB100A FM generator from the DUT discriminator is distortion cancellation.
You talk about colleagues measuring again, so you have very limited or no hands on experience to what you are talking about.
I did look at the link, but I didn't see a single plot of the multiplex spectrum in the 50 kHz...200 kHz range, only audio plots. Of course audio plots are all you need when you just want to determine the performance of the receiver, but it doesn't help much for the discussion about noise around the fifth harmonic of 38 kHz.
It's true that I didn't usually measure FM receivers, I only designed them and then someone else measured them and sent the results to me and the rest of the design team. I only got involved more directly with the measuring when that was needed for debugging.
Your inability to deal with simple theoretical arguments suggests you have very limited experience with designing FM radios, or at least with designing FM radios with an architecture that hasn't been obsolete for decades.
It's true that I didn't usually measure FM receivers, I only designed them and then someone else measured them and sent the results to me and the rest of the design team. I only got involved more directly with the measuring when that was needed for debugging.
Your inability to deal with simple theoretical arguments suggests you have very limited experience with designing FM radios, or at least with designing FM radios with an architecture that hasn't been obsolete for decades.
Here we go, professional jealousy at work. People like yourself gauge others based on your own limitations, i.e. utter lack of hands on experience other than what you already know and cling to that notion. I have met a lot of other engineers who are very smart, but only on a narrow field.
You keep on moving the goal post to suite your comfort zone by throwing irrelevant questions.
I know my theories well and I don’t need to convince anyone of my capacity. I’m skeptical of forum posters who go by statistics and just throw their opinion on a broad subject they clearly have no mastery of. NOBODY knows everything, and forums are not contests.
Back on the off topic subject.
The noise from the 3rd and 5th harmonics are decoded by the regenerated 38kHz subcarrier. If there are noise present +/-15kHz of these, it will show up in the 20Hz-15kHz audio band if the stereo decoder used strictly a 38kHz 50% duty cycle switching. I know very well the theories as to the why but it is not my job to elaborate the theory behind these.
No jealousy at all, at least not from my side, I just wanted to reply in your style for a change.
You still miss the point that noise at 190 kHz +/- 15 kHz at the FM demodulator output should be largely suppressed by the IF filter as long as its bandwidth is well below 350 kHz (which is two times (190 kHz - 15 kHz)) and as long as there is no strong modulation. Therefore there is nothing to convert, and the fifth harmonic of 38 kHz doesn't affect the stereo noise floor much.
Of course this doesn't apply anymore when there is some strong interferer present at 200 kHz distance, an interferer that's too strong to be properly suppressed by the IF filter, but I'm just writing about the stereo noise floor when there is only the desired signal and noise.
In any case, this side discussion has been going on since post #36 and obviously isn't going anywhere, so I suggest we stop.
You still miss the point that noise at 190 kHz +/- 15 kHz at the FM demodulator output should be largely suppressed by the IF filter as long as its bandwidth is well below 350 kHz (which is two times (190 kHz - 15 kHz)) and as long as there is no strong modulation. Therefore there is nothing to convert, and the fifth harmonic of 38 kHz doesn't affect the stereo noise floor much.
Of course this doesn't apply anymore when there is some strong interferer present at 200 kHz distance, an interferer that's too strong to be properly suppressed by the IF filter, but I'm just writing about the stereo noise floor when there is only the desired signal and noise.
In any case, this side discussion has been going on since post #36 and obviously isn't going anywhere, so I suggest we stop.
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I am going to compare the noise specs for the t-105v to my favourite the Yamaha t-85 with its stereo product detector.
This is great tuner info even if it is a bit off topic and we should be nice about it
I’d like to ask questions if we can conduct ourselves
I have some test gear to learn with, hp 11715A, 8901A, 8904A,3325A,8656B, tek dso’s
Cheers
Rick
This is great tuner info even if it is a bit off topic and we should be nice about it
I’d like to ask questions if we can conduct ourselves
I have some test gear to learn with, hp 11715A, 8901A, 8904A,3325A,8656B, tek dso’s
Cheers
Rick
Rick,
Can you measure the FM THD/THD+N of your HP 11715A and HP 8656B at 90MHz, 98MHz, 106MHz with a 1kHz rate and 75kHz peak deviation using your HP 8901A as the reference demod?
I don’t know what you have for distortion analysis so for THD+N, 300Hz-400Hz high pass filter and 15kHz-30kHz low pass filter engaged is a good start. I’m sure you are well aware of the reasoning for these filter choices.
You can also measure residual FM noise with the 8901A.
I never owned either FM generators and I’m curious about your results.
Rick,
I assure you, your apparent 0.1% THD+N limit is not due to your HP 8901A. The 8901A actual THD and noise floor are well below 0.01% and -80dB respectively. Even my much older HP 5210A the precursor to the 8901A, its THD+N/linearity is typically 0.025%.
Both HP gear employ some form of single-shot/monostable pulse count detectors.
I’ll wait for your next round of FM generator measurements.
I assure you, your apparent 0.1% THD+N limit is not due to your HP 8901A. The 8901A actual THD and noise floor are well below 0.01% and -80dB respectively. Even my much older HP 5210A the precursor to the 8901A, its THD+N/linearity is typically 0.025%.
Both HP gear employ some form of single-shot/monostable pulse count detectors.
I’ll wait for your next round of FM generator measurements.
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