Frequency Accuracy

I have a task I need to do for calibrating a radio. In order to adjust the internally generated local oscillator to the right frequency, I bought a 10 MHz reference generator, based on a TCXO probably pulled from a cellular station. More than you probably want to know…

Anyway, I need to measure the audio output frequency of the detected tone when the radio is tuned to the reference standard.

Here’s my question: How accurate is a QA40x with regard to measuring the frequency of an audio tone?

Thanks! I know that this is off the beaten path for this forum, but I just want to get an idea how accurately my QA401 can measure the frequency of an audio tone.

Yes, very accurate.
Here I’m aligning a HP3325A Synthesizer/Function Generator analog phase interpolation board.
Object is to reduce a 3 kHz spur. The spur is clearly shown at 3 kHz.
I’m using a scope probe.
10 MHz GPSDO from a old TrueTime receiver supplants the on board oscillators on the HP equipment.
Basically the QA403 is taking the place of the HP service manuals suggested HP3580A/HP3585A low frequency spectrum analyzer.
For higher frequencies past the QA403 Nyquist limit I use a TinySa Ultra spectrum analyzer.
The QA403 is very versatile, good stuff.

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Thanks very much. So, within a couple Hertz or so? better?

No, because it’s extremely difficult to have a low noise environment outside a lab.
This particular signal is varying appx + or - 5 Hz with no averaging.
Bump up the averaging and the result is a measurement of exactly 3.000 KHz.
Max out the sample rate, FFT, and do about 50 averages, and the results will be very close.
How close + and - I don’t know.
Nothing else on the market can touch the QA403 at it’s price point.

I forgot to use the Hann window here, was using Flat Top. Better peak with Hann.
Getting very good results reducing spur frequency by looking at amplitude with 7 sample average.
Probably bump up the other settings for a frequency measurement.
The signal resolution of the QA403 is much greater than the trimmers in the HP. One barely perceptible movement of a trimmer makes a dig difference of amplitude on the screen.

Going back through the HP 3325A alignment a second time and getting better results using the QA403 as a low frequency spectrum analyzer by ditching the oscilloscope probe and simply using a 10k resistor as HP recommends when using the HP 3580A or HP 3585A spectrum analyzers.
This works amazingly well.

That’s an old QA-400 I can see up on the shelf too!

Yup, and still works great.
After a long trial with various sound cards, computers, and REW, that little QA400 was a true revelation.
Here’s a thumbs up to QuantAsylum.

Glad to see the B&K Isolation xformer- I have one of those from my TV repair days. Have not needed it for audio work, though. You have a very nice bench!

Thanks! I’m a hobbyist with no electronics background. It’s a good hobby, especially during the winter months.

Back to the original topic, in case anybody is watching…

What I ended up doing was to check the OCXO reference against WWV’s 10 MHz transmission. I measured the detected audio tone output of the receiver, which was tuned to 9999 KHz in Upper Sideband mode (1000 Hz or so) and then adjusted the OCXO to be within a couple tenths of a Hz of WWV. The absolute value of the audio tone didn’t matter - I just needed to match OCXO to WWV.

Then, I used a digital divider to take the 10 MHz output of the OCXO down to 1 KHz. Since that is dividing the signal by 10000, any error at 10 MHz is divided by that same factor of 10000. So, even 1 Hz off at 10 MHz becomes irrelevant by the time you get to 1 KHz.

Inserting that divided signal into the QA401 gave me a frequency readout of 1000 Hz; maybe a few hundredths of a Hz off. Close enough! So, I’d say that the QA401 is pretty good in frequency accuracy. At least my sample is.

Dividing a 10 MHz reference oscillator seems like a decent way to generate a high quality 1 KHz tone. Of course, you’d need a pretty good low pass filter to get rid of the harmonics, which are mostly odd order. I need to explore that some more.

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