Challenge: 1 MHz FSK shock test on amplifier output – What would your scope show?

[tomsoon]

Hi Everyone,
I am completely new here, and after looking around, I found this forum to be the most appropriate and capable place for this specific topic. It is truly an honor to be here, and it would be a privilege to get answers from the real pros on this board.
I am very curious to hear your expert opinions and see your simulations regarding a test measurement that pushes the boundaries between theory and practice.
We have a harsh test signal from a UNI-T UTG932E function generator, using a demanding FSK (Frequency Shift Keying) modulation:

• Carrier Frequency (CarrFreq): 1,000,000 kHz (that's 1 MHz!)
• Hop Frequency (HopFreq): 100,000 kHz
• Switching Rate (Rate): 10,000 kHz

Let's assume we feed this ultra-dense signal, packed with drastic frequency and phase jumps, into the input of an amplifier, while the output is loaded with a real, brutal 7.5-ohm dummy load.
My question to the experts:
How do you think a classic pure Class A, a Class AB hi-fi, or a modern high-end Class D (PWM) amplifier would react to this shock on the output?

 

[fpitas]

That will depend on the specific amplifier. Some have good band-limiting at the input, some not so much. And BTW a 7.5 ohm load is not brutal at all. If for some reason you need to do this for work, I suggest you buy a suitable amp and, if necessary, put a LPF on the input.

 

[MaxwellsEq]

Hi Everyone,
I am completely new here, and after looking around, I found this forum to be the most appropriate and capable place for this specific topic. It is truly an honor to be here, and it would be a privilege to get answers from the real pros on this board.
I am very curious to hear your expert opinions and see your simulations regarding a test measurement that pushes the boundaries between theory and practice.
We have a harsh test signal from a UNI-T UTG932E function generator, using a demanding FSK (Frequency Shift Keying) modulation:

• Carrier Frequency (CarrFreq): 1,000,000 kHz (that's 1 MHz!)
• Hop Frequency (HopFreq): 100,000 kHz
• Switching Rate (Rate): 10,000 kHz

Let's assume we feed this ultra-dense signal, packed with drastic frequency and phase jumps, into the input of an amplifier, while the output is loaded with a real, brutal 7.5-ohm dummy load.
My question to the experts:
How do you think a classic pure Class A, a Class AB hi-fi, or a modern high-end Class D (PWM) amplifier would react to this shock on the output?

Impossible to know.

An engineer designing an amplifier must assume it will need to handle signals in the 10Hz to 30kHz range. If the designer is sensible they will put a high-pass filter in to block DC and near-DC signals and a low-pass filter to block audio signals out beyond 50kHz. If the amplifiers are stable with audio signals and stable-y filter non-audio signals then a Class A, Class AB and Class D would all act the same.

The issue may come where some form of RF cross coupling takes places - e.g. the signal gets into the amplifier circuit without being directly injected into the inputs.

 

[pma]

How do you think a classic pure Class A, a Class AB hi-fi, or a modern high-end Class D (PWM) amplifier would react to this shock on the output?

Depends on output stage design and transistors used for class A and AB. Wideband class A with fast power transistors will absorb it best, as they will have lowest and most linear output impedance at HF. Class D will behave much worse for the reason of large inductances in the output filter. PFFB designs will not make it as well. And remember that Class D switching frequency is usually about 500 kHz.
One question - why would you do such experiment?

 

[fpitas]

One question - why would you do such experiment?

Kind of wondered that myself.

 

[solderdude]

Would be fun to check this amp with it.
It should do well:

Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier

Rentals Ship Today - Order Online at The EMC Shop - An ISO 17025 Accredited Company - Leading North American Vendor - Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier - The EMC Shop

theemcshop.com

You'll typically will find them in EMC labs.

No idea why that nor that test signal would be of any importance for audio gear.

 

[DonH56]

Hi Everyone,
I am completely new here, and after looking around, I found this forum to be the most appropriate and capable place for this specific topic. It is truly an honor to be here, and it would be a privilege to get answers from the real pros on this board.
I am very curious to hear your expert opinions and see your simulations regarding a test measurement that pushes the boundaries between theory and practice.
We have a harsh test signal from a UNI-T UTG932E function generator, using a demanding FSK (Frequency Shift Keying) modulation:

• Carrier Frequency (CarrFreq): 1,000,000 kHz (that's 1 MHz!)
• Hop Frequency (HopFreq): 100,000 kHz
• Switching Rate (Rate): 10,000 kHz

Let's assume we feed this ultra-dense signal, packed with drastic frequency and phase jumps, into the input of an amplifier, while the output is loaded with a real, brutal 7.5-ohm dummy load.
My question to the experts:
How do you think a classic pure Class A, a Class AB hi-fi, or a modern high-end Class D (PWM) amplifier would react to this shock on the output?

Firstly, 1,000,000 kHz is 1 GHz, but I suspect a decimal vs. comma thing?

Like everybody else has said, it depends upon the amplifier's input circuit, specifically how well it filters RFI. Most amplifiers have a fairly large (for RF) input capacitor network to filter out RFI. An "ultrawideband" amp could have issues with it. You say at the end of your post about reacting to this "on the output"; do you mean to apply it to the input, output, or both? I would find the output more interesting as a number of amplifiers in the past have had relatively poor RFI rejection at the output terminals.

 

[tomsoon]

Depends on output stage design and transistors used for class A and AB. Wideband class A with fast power transistors will absorb it best, as they will have lowest and most linear output impedance at HF. Class D will behave much worse for the reason of large inductances in the output filter. PFFB designs will not make it as well. And remember that Class D switching frequency is usually about 500 kHz.
One question - why would you do such experiment?

Hi,
Thank you for your excellent, deeply technical insights. You hit the nail on the head regarding the limitations of Class D output filters and switching frequencies at 1 MHz.
To answer your question about why anyone would perform such a test:
The goal of this experiment was to evaluate structural phase linearity, bandwidth stability, and the pure speed of an analog design under a brutal, dynamic stress test. Standard 1 kHz sine wave tests are too "comfortable" and tell us very little about how a topology handles extreme, real-time transient energy.
I wanted to see if an analog topology could handle this 1 MHz FSK shock wave under a real 7.5-ohm load without losing control, without phase lag, and without overheating.
In a short time, I will share the actual scope captures of this experiment. I think the results will surprise many of the experts here

 

[tomsoon]

Impossible to know.

An engineer designing an amplifier must assume it will need to handle signals in the 10Hz to 30kHz range. If the designer is sensible they will put a high-pass filter in to block DC and near-DC signals and a low-pass filter to block audio signals out beyond 50kHz. If the amplifiers are stable with audio signals and stable-y filter non-audio signals then a Class A, Class AB and Class D would all act the same.

The issue may come where some form of RF cross coupling takes places - e.g. the signal gets into the amplifier circuit without being directly injected into the inputs.

Hi,
Thank you for your response and for highlighting the classic engineering principles regarding low-pass and high-pass filtering. It is absolutely true that in standard commercial designs, blocking anything above 50 kHz is a fundamental rule for ensuring system stability [AS INDEX].
The core intent of this theoretical challenge is precisely to look beyond those traditional protective boundaries. I wanted to open a discussion on what happens if we imagine a topology that doesn't rely on heavy input filtering, but instead possesses the native speed and linearity to handle a 1 MHz FSK transient directly into a 7.5-ohm load.
To take this theoretical scenario a step further, I would like to ask the experts on this board:
What would happen to the phase linearity and circuit parameters if we tested such a topology at drastically different rail voltages—for example, at +/-5V, +/-8V, and then up to +/-30V? In a conventional design, changing the voltage so drastically would typically shift the operating points and affect the transistor capacitances. But is it theoretically possible to create an analog architecture that remains completely immune to these voltage changes, maintaining identical phase behavior and tracking across all levels? Does such an approach have a solid physical foundation in your view?
As for the Deming quote—I completely agree. Data is the ultimate truth [AS INDEX]. While this topic is presented here as a fascinating theoretical question, it is always real-world performance that settles the debate [AS INDEX].
Thank you for sharing your expertise, and I look forward to your thoughts on this voltage scaling aspect!

 

[tomsoon]

Would be fun to check this amp with it.
It should do well:

Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier

Rentals Ship Today - Order Online at The EMC Shop - An ISO 17025 Accredited Company - Leading North American Vendor - Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier - The EMC Shop

theemcshop.com

You'll typically will find them in EMC labs.

No idea why that nor that test signal would be of any importance for audio gear.

Hi,
Thank you for your thoughts! That Amplifier Research model is indeed a magnificent piece of laboratory equipment. You are spot on—those are exactly the machines built to handle this kind of high-frequency energy [AS INDEX].
But the real, fascinating question for an audio designer is this: is it theoretically possible to achieve this level of structural phase linearity and stability within a pure, analog audio topology, without relying on military-grade lab gear?
Thank you again for bringing that EMC perspective into the discussion, it really highlights the scale of the challenge!
Best regards,

---

 

[tomsoon]

That will depend on the specific amplifier. Some have good band-limiting at the input, some not so much. And BTW a 7.5 ohm load is not brutal at all. If for some reason you need to do this for work, I suggest you buy a suitable amp and, if necessary, put a LPF on the input.

Hi,
Thank you for your advice! You are completely right that a 7.5-ohm load on its own is standard and not brutal at all for any normal audio amplifier.
However, the real challenge in this theoretical scenario is driving that 7.5-ohm load at a continuous 1 MHz frequency with constant, drastic FSK phase and frequency modulation. At 1 MHz, the behavior of output transistors and parasitic elements changes completely compared to the standard audio band.
The core of this challenge is to see if a topology can stay linear and stable under these specific conditions without using an input low-pass filter (LPF) to choke the signal.
Thank you again for sharing your perspective!
Best regards,

 

[DavidEdwinAston]

Would be fun to check this amp with it.
It should do well:

Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier

Rentals Ship Today - Order Online at The EMC Shop - An ISO 17025 Accredited Company - Leading North American Vendor - Amplifier Research 350AH1A 10 Hz - 1 MHz Amplifier - The EMC Shop

theemcshop.com

You'll typically will find them in EMC labs.

No idea why that nor that test signal would be of any importance for audio gear.

Can I ask if this product would be a perfectly reasonable, albeit, slightly expensive, hi-fi amp?

 

[tomsoon]

Firstly, 1,000,000 kHz is 1 GHz, but I suspect a decimal vs. comma thing?

Like everybody else has said, it depends upon the amplifier's input circuit, specifically how well it filters RFI. Most amplifiers have a fairly large (for RF) input capacitor network to filter out RFI. An "ultrawideband" could have issues with it. You say at the end of your post about reacting to this "on the output"; do you mean to apply it to the input, output, or both? I would find the output more interesting as a number of amplifiers in the past have had relatively poor RFI rejection at the output terminals.

Hi,
Thank you for catching that! Yes, it was absolutely a decimal vs. comma typo on my part—I meant 1.000000 MHz (1 MHz), not 1 GHz. I appreciate the correction! [Technical Articles]
Also, I must say I love your quote by Aldous Huxley. It resonates deeply with anyone who truly appreciates the emotional depth of music. [Technical Articles]
To clarify the measurement setup: the 1 MHz FSK signal is applied directly to the input of the amplifier. The question is specifically focused on how the topology behaves under a real 7.5-ohm load, and what we would see on the output terminals. [Technical Articles]
Your point about poor RFI rejection at the output terminals in past designs is brilliant. That is exactly why this scenario is so fascinating to look at from a theoretical perspective. If an analog topology were fast and linear enough by nature, could it maintain a stable, clean output stage under this 1 MHz dynamic shock without relying on heavy input filtering or chokes? [Technical Articles]
Thank you again for your valuable insights, and I’m honored to have your expertise in this thread! [Technical Articles]
Best regards,

---

 

[fpitas]

Can I ask if this product would be a perfectly reasonable, albeit, slightly expensive, hi-fi amp?

If your pets listen to supersonic tweeters, it's a good choice.

 

[fpitas]

Hi,
Thank you for your advice! You are completely right that a 7.5-ohm load on its own is standard and not brutal at all for any normal audio amplifier.
However, the real challenge in this theoretical scenario is driving that 7.5-ohm load at a continuous 1 MHz frequency with constant, drastic FSK phase and frequency modulation. At 1 MHz, the behavior of output transistors and parasitic elements changes completely compared to the standard audio band.
The core of this challenge is to see if a topology can stay linear and stable under these specific conditions without using an input low-pass filter (LPF) to choke the signal.
Thank you again for sharing your perspective!
Best regards,

It's an unusual test. Can I ask what real-world situation it simulates?

 

[DavidEdwinAston]

If your pets listen to supersonic tweeters, it's a good choice.

Yet again, I display my monumental ignorance!

 

[FrantzM]

Some HEA amplifiers are purported to be "wideband" and able to pass 1 Mhz .. From the top of my head, Spectral, and some early Krell, there are many others who claim the same

from Spectral (not a safe website accoring to Chrome)

Spectral Audio DMA-300 Specifications

 

[wwenze]

Is "audio amplifier class" classification even a thing for RF amplifiers?

 

[fpitas]

Is "audio amplifier class" classification even a thing for RF amplifiers?

None that I've used, at least. Shake tables sometimes use "normal" audio amplifiers.

 

[tomsoon]

Depends on output stage design and transistors used for class A and AB. Wideband class A with fast power transistors will absorb it best, as they will have lowest and most linear output impedance at HF. Class D will behave much worse for the reason of large inductances in the output filter. PFFB designs will not make it as well. And remember that Class D switching frequency is usually about 500 kHz.
One question - why would you do such experiment?

Hi ,
Thank you for your excellent and deeply technical response. I completely agree with your concluding sentence: oversimplification indeed leads to false conclusions [AS INDEX]. That is precisely why this challenge was created—to move away from the "oversimplified" standard 1 kHz sine wave tests that dominate the industry [AS INDEX].
To answer your question about why anyone would perform such an experiment:
The core intent is to evaluate structural phase linearity, bandwidth stability, and the pure transient speed of an analog architecture under extreme, dynamic conditions [AS INDEX]. Standard audio band tests are often too comfortable and mask how a topology truly handles high-frequency, real-time energy modulation [AS INDEX].
I wanted to open a discussion on whether it is theoretically possible for an analog topology to maintain an ultra-low, linear output impedance and stay perfectly stable under a 1 MHz FSK shock wave directly into a 7.5-ohm load—without losing control and without relying on heavy protective input filters [AS INDEX].
Thank you again for sharing your valuable expertise and your brilliant point about Class D filter limitations! [AS INDEX]
Best regards,

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