Re: PLL Detector alignment

Herb Ward

Mike, Bob, and all interested parties...

One of the eternal questions among the meter readers among us is ...
Are we measuring the DUT or are we measuring our test equipment ?

My recent efforts to build and test prototypes of passive EQ RIAA tube phono preamps for one of my favorite 
designers has given me an enlightened appreciation of the test and measure process. 

One of my early mentors warned me that to confirm a THD measure of .1% THD, ideally the signal source should have 
a residual THD of .01% THD.  And in the days of analog null type THD meters, I suspect that the signal source really needed to
have a signal output with specs having an order of magnitude better than the DUT in terms of THD. These days with many of us using computer based FFT analysis to determine circuit performance, the challenges can be both easier and more difficult.  Knowing the spectral output of your signal source can be a benefit in the determination of DUT performance as the individual THD products measured can be compared from the DUT to the signal source .  

That being said there are two considerations to examine with respect to FM signal generators. What is the quality of the internal
audio generator, and perhaps more important, what is the linearity of the generator's FM modulator ?  You may find for example that the internal audio oscillator is off spec or perhaps just does not offer very good THD specs while the modulator itself is excellent. If an external audio signal source is an option , try a better outboard signal source. ( I am reminded of the Brian Beezley mods to the ST1000a )

BTW a recent measure of a Akitika 1Khz oscillator with my QA401 revealed a THD spec of less than 2ppm. No hum, no noise, and just a tiny bit of 2nd harmonic at 3 volts out. This might be an excellent low buck signal source for all those signal generators with external oscillator inputs. 


On Thu, May 12, 2022 at 10:25 PM Mike Miller <mikeyis43@...> wrote:
On Thu, May 12, 2022 at 08:26 AM, newaag wrote:
This is corrected by new filters at the IF measured, or by moving the IF.
I apologize if you have already answered this, but what do you think of using a detector probe and swept RF input to "steer" the IF frequency (all the L's & C's in the front end actually) thru the middle of the filter bandpass (narrow or wide)?
Swept RF input a few hundred kHz around say 98MHz. I use a -1 to +1V 50ms sawtooth ramp into the Ext FM input of my HP8640B generator, dial set to Peak Dev of 640kHz.(and to Trace 2 on my scope)

Detector probe at the end of the IF chain before the limiter (to Trace 1 on scope). This would be instead of measuring the filter centers and using a counter to set the oscillator?

I just used this to bring in the front of a Pioneer F-99X whose front end for some reason was really out of alignment - as in not functional.
Interestingly, once I got the IF back online, I found the wide and narrow centers are quite a bit different with the stock filters. You probably remember that this tuner has 9 filters in the IF section.
Mike Miller

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