Again: "FG servo" what is it?

[Montmorency]

I read in often encounter in various sources (both turntable-related and cassette deck-related) mentions of different speed control systems, including the oft compared "FG servo" systems and "quartz-locked" systems. Quartz-locked systems (or "quartz PLL systems") are presented as more modern and superior to "FG servo" systems.

I know very well what quartz PLL speed control system is: a stable quartz-based oscillator generates a reference waveform, while a FG coil in the motor provides a speed-dependent feedback waveform from the actual motor. A comparator performs phase comparison of the two waveforms, extracts the phase difference and turns it into the speed control signal, adjusting the speed of the motor. This negative feedback system strives to keep the two waveforms in-phase.

But what is "FG servo" system then? How does "FG servo" system work? How is it different from "quartz PLL" system?

I found this thread in this very forum: https://audiokarma.org/forums/index.php?threads/what-does-fg-servo-system-mean.403440 But the people who answer there don't seem to care much about the difference between "FG servo" and "quartz PLL". They attempt to answer the question about "FG servo" system, but instead drift into the description of "quartz PLL" system, as if the two are actually the same.

So, would someone please illuminate me on this very specific and focused matter: what is the distinguishing feature of "FG servo" speed control that makes it different from "quartz PLL"?

I presume there's no quartz in "FG servo", right? But apparently there's an FG coil and some sort of feedback from it. Is there? And if so, where does that feedback go and how is it used then?

 

[KentTeffeteller]

Frequency Generator- Servo. Not Quartz Locked. Motor controlled by an electronic servo circuit using a frequency generated reference. Some use a transistor or transistors, many use a IC. Better if direct drive. The belt drive motor uses an IC, if the frequency reference gets too far off due to electrolytic caps takes out the IC which is the Frequency Generator which is also unobtainable as is the motor.

 

[tnsilver]

what is the distinguishing feature of "FG servo" speed control that makes it different from "quartz PLL"?

You seem to have a good grasp of it already so here goes the nutshell version. It boils down to feedback as a voltage vs. feedback as a frequency. Both of these systems use a tacho to provide feedback from the motor. Feedback, however, can be encoded/decoded in several ways.

With the 'servo' system feedback is a variable voltage generated by a frequency to voltage converter to represent the motor speed. That feedback goes into a differential amplifier to be compared with the reference voltage. When differences are found they're translated to speed correction voltages that are feeding the motor. It's a voltage based feedback control circuit where the input is DC voltage feeding the motor and the output is also voltage - for comparison with reference.

The quartz lock PLL system is very similar in principal, except it doesn't use voltage for feedback - it uses frequency instead. The motor is still fed DC voltage but the feedback is encoded differently and it's represented as a frequency. It is then compared to the reference frequency of a quartz crystal. A phase detector replaces the differential amplifier - but the result is the same as before - it generates DC voltage to feed the motor.

Please note this is just a simplified version - in reality quartz lock PLL circuits are more complex and typically just handle error correction with another circuit supplying motor power.

 

[Wolfie62]

A few FG Servos use a crystal oscillator to generate the stable frequency; but they don’t use PLL to generate the error signal. Both QL and DC FG Servo are great systems. Generally, the FG Servos are used in belt drive motors.

 

[ripblade]

My understanding is that the FG servo uses an oscillator to generate a reference frequency for the motor. This oscillator frequency can drift over time. Quartz crystal has a very specific and constant frequency which doesn't drift. An analogy can be found in vintage FM tuners that used AFC (Automatic Frequency Control) based on a similar oscillator to control the tuning frequency. They still drifted over time. Modern tuners use the Quartz PLL and never drift. Another analogy is the quartz watch, which because of its reliance on the quartz crystal frequency, can keep very accurate time.

 

[Montmorency]

Well, thank you all for the answers, but it seems that we have three different answers now.

* tnsilver's answer states that "FG servo" is a feedback-based system, except that the feedback mechanism is not based on phase comparison, but rather on plain DC voltage comparison.

* Wolfie62 seems to state that "FG servo" is not a feedback-based system at all.

* ripblade's answer seems to state that "quartz PLL"and "FG servo" are both frequency-based (i.e. phase-based) feedback systems, except that "FG servo" uses a regular oscillator to generate reference frequency (no quartz).

This is basically the same conundrum I started from.

 

[tnsilver]

That's AK. We all have opinions. You could look it up in the electrical engineering forums or in the scientific literature and form your own opinion or just hunt for a hint of confirmation for the theory you trust the most and research it from there. It's rare to get an all around consensus answer around here.

 

[Montmorency]

No, we don't.

Firstly, the question is about a specific technical term. Technical terminology is not a matter of anyone's opinion. It is a matter of factual knowledge. My question here is directed specifically at those who have that knowledge, if any are present here.

Secondly, forming a valid opinion requires quite a bit of base knowledge, hard research and proper aptitude. Having an opinion is akin to belonging to a rather exclusive club. Not everyone has an opinion, even if some attempt to claim otherwise.

 

[tnsilver]

Your questions were actually on a much wider spectrum than the semantics of a technical term:

what is "FG servo" system then? How does "FG servo" system work? How is it different from "quartz PLL" system? ...what is the distinguishing feature of "FG servo" speed control that makes it different from "quartz PLL"? ...I presume there's no quartz in "FG servo", right? But apparently there's an FG coil and some sort of feedback from it. Is there? And if so, where does that feedback go and how is it used then?

We tried to provide the best answers we can to satisfy the essence of it. Sorry to disappoint.

 

[ripblade]

The terms 'servo' and 'phase locked loop' both imply some kind of feedback mechanism. Whatever the exact mechanism, the Quartz succeeded FG because of its greater accuracy and relative imperviousness to frequency drift.

 

[Wolfie62]

The terms 'servo' and 'phase locked loop' both imply some kind of feedback mechanism. Whatever the exact mechanism, the Quartz succeeded FG because of its greater accuracy and relative imperviousness to frequency drift.

I’ve got 4 tables using FG Servo circuits. No frequency drift yet! Even after 38 years!

Perhaps next year?

 

[tnsilver]

^ betcha those are heavy platter belt drives where inertia covers up for electronics. FG servos built with discrete components do drift with temperature fluctuations and components tolerances. Sometimes it's just hard to see. Any of those turntables got a strobe?

 

[Wolfie62]

They all have strobes. That’s how I know they don’t drift. And discrete + ICs. Early 80s-1986. I’m not familiar with FG Servos using all discrete components.

 

[tnsilver]

@Wolfie62, the weak link IMO in many simple FG Servo designs is the reference voltage being regulated by cheap flaky Zener based circuits.

Anyhow... I think what adds to the confusion around the subject is the "PLL" acronym associated with "quartz lock". Phase lock loop is a circuit that is not necessarily associated with quartz crystal oscillators. In fact, very basic early and mid 70's FG Servo designs incorporated a PLL section into the FG servo IC. It is just an error correction circuit that has nothing to do with quartz crystals.

Such is the case, for example, with the proprietary NEC μPC1003C IC that's commonly found in the Technics SL-20 and 23, Rotel's, Kyocera's and many others. The entire speed control PCB humbly looks like this:

Rotel RP-500/Technics SL-23 FG Servo speed control PCB

The little chip in the middle is actually a smart bugger that does, well, so many things...

NEC μPC1003C block diagram

For what I'm about to quote from the service manual - the schematics are at the bottom for reference. Here's how Technics explains what's going on with this circuitry (hard to believe it's all in that 8 legged small chip).

1. A frequency generator (FG) is mechanically coupled to the motor shaft (visible in the schematics as the coils to the left of the motor "M"). It's output is directly proportional to the motor speed.
2. The signal produced by the FG (this is AC voltage) is amplified
   
3. and then converted to a square wave by the pulse shaper circuit
4. the square wave is then converted to into a trigger pulse
5. The trigger pulse is converted into a square wave of fixed amplitude (ES) and width (TS) to determine the square wave frequency. This square wave has a frequency (T) that is directly proportional to the motor speed.
6. The square wave is then converted to a ramp wave (saw-tooth) which is used as the motor control voltage (Ei). It's the commanding voltage and it is higher when the motor spins faster, hence directly proportional too. Enter the phase compensator stage (in blue) - with the voltage comparator first.
   
7. In the voltage comparator stage, the motor control voltage (Ei) is compared with the reference control voltage (Es/n) which is supplied from the reference voltage supply (in red, this is the unstable section IMO) and the divider circuits.
8. The output of the comparator (Ec) is the actual motor voltage which will adjust it's speed. It just needs to be amplified to power the motor and it becomes Em.
   
9. A phase compensator (in blue) is used to feed back the AC component of the motor drive voltage - to the voltage comparator (this is the error correction loop) to improve speed stability.

Technics SL-20 Schematics

OK, so in this μPC1003C based FG Servo, there's a PLL stage that constantly corrects errors. There's no quartz lock here at all. Here's another FG Servo design (hall element based), this time from a Technics SL-B202. It is based on a Matsushita SVIBA808 chip which has no PLL section:

Technics SL-B202 Block Diagram (SVIBA808 based)

I won't go into the details with this one - but you can clearly see the reference voltage used by the standard time generating section and the motor feed (Em) being amplified by an external power transistor.

With a quartz crystal oscillator in a quartz lock design - we don't need the reference voltage any more. We compare frequencies - not voltages and phase lock loops can be implemented regardless. So... I hope this clears things up a little.

 

[Wolfie62]

Tnsilver: Very good explanation! Dead on accurate, too.

The voltage reference is subject to failure. Diodes of all types fail, zeners also. Zener Vref is a very common circuit element.

 

[Montmorency]

I think what adds to the confusion around the subject is the "PLL" acronym associated with "quartz lock". Phase lock loop is a circuit that is not necessarily associated with quartz crystal oscillators.

... which is exactly why I, for one example, always make sure to refer to quartz-based PLL as "quartz PLL".

In fact, very basic early and mid 70's FG Servo designs incorporated a PLL section into the FG servo IC. It is just an error correction circuit that has nothing to do with quartz crystals [...] OK, so in this μPC1003C based FG Servo, there's a PLL stage that constantly corrects errors.

But now you are blatantly contradicting your previous answer. In your first answer you stated that the defining property of "FG servo" is its use of DC voltage as feedback mechanism (as opposed to using PLL - a waveform phase comparison mechanism). You clearly stated that the feedback mechanism uses DC-voltage-vs-DC-voltage comparator, not a phase-difference comparator.

Now here you are stating something completely different: that "FG Servo" actually uses a phase comparator and that "FG Servo" is basically a quartz-less variant of PLL. This is consistent with ripblade's answer.

So, what is it then? Is it a DC-voltage-feedback system or PLL-feedback system? These are mutually exclusive.

With a quartz crystal oscillator in a quartz lock design - we don't need the reference voltage any more. We compare frequencies - not voltages and phase lock loops can be implemented regardless. So... I hope this clears things up a little.

And here you seem to be suddenly flip-flopping back to your original answer and again trying to claim that "FG servo" compares voltages!

Sorry, your answer makes no sense.

---

Note, BTW, that in some sources I see mentions of [supposedly] another approach: "DC servo", which is [supposedly] based on DC voltage feedback. If that's the case, then it is probably safe to conclude that in your original answer you simply confused "FG servo" and "DC servo". You actually described "DC servo" even though the question was about "FG servo".

In that case one might make a preliminary guess that the ripblade's answer might be the correct one: "FG servo" is just quartz-less PLL. However, this is still a rather cautious and preliminary conclusion. One problem I see here is that I know some "DC servo" devices and they don't seem to use any kind of feedback...

Let's agree to do some extra research...

 

[Wolfie62]

Different manufacturers used different methods to execute “Quartz Lock”. Same for FG Servo. Same for DC FG Servo.

Sounds like you’re trying to lump each way of executing the idea in one bucket. Sorry, but that doesn’t work either!

 

[tnsilver]

And here you seem to be suddenly flip-flopping back to your original answer and again trying to claim that "FG servo" compares voltages!

You're being very argumentative and make it sound as if I have a hidden agenda here. I have no dog in this. Look at the block diagram, read the explanation (it's nearly word for word from the service manual) and try to open mindedly understand that FG Servo reactance to feedback involves multiple stages - some of which may, or may not exist in different implementations. ...and BTW, I did not say feedback is DC voltage - I said it's voltage. You can clearly see the feedback's AC waveform in the NEC μPC1003C block diagram.

 

[Wolfie62]

One thing I’ll say: I love the results of the QL and FG Servo tables I have. To get those features today, you pay high dollar. They work very well to keep speed accurate and stable.

 

[ripblade]

I'm happy with Tom's explanation. And FWIW, the FG servos I've had didn't hold speed as well the Quartz lock. I'm not talking wild drift here, just not as good.