EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: jj5 on January 02, 2025, 04:53:39 am
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I was reading the manual for my new Peak Atlas LCR45 (https://www.peakelec.co.uk/acatalog/lcr45-lcr-impedance-meter.html) and it says the frequency 14.9254 kHz is abbreviated as 15 kHz. But it doesn't say what's so special about 14.9254 kHz. Is there an important reason why that very specific frequency is used? The full set of test frequencies supported by the device are: DC, 1kHz, "15kHz", and 200kHz.
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1 MHz / 67 = 14.9254 kHz
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The microcontroller does divide the clock and that was the closest to 15kHz achievable with given clock frequency and divider resolution, thats my guess.
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I don't remember exactly, but was not this originated from the horizontal scan rate of early B/W CRT monitors? (Hercules or something like that)
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Why would Hercules be relevant for a LCR meter?
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1 MHz / 67 = 14.9254 kHz
Are 1 MHz crystals (or some multiple) cheap?
I know about the 14.31818 MHz crystals that were very cheap because they were used in NTSC TVs.
The Apple II's and C64's 1.023 MHz was 1/14 of that and the IBM PCs 4.77 MHz was 1/3. The Atari 400's 1.79 MHz was 1/8 -- probably the TRS80 also, which was advertised as 1.78 MHz. The TRS80 CoCo's 0.895 MHz is 1/16.
NTSC divided by 955 would be 14993 Hz, which is a lot closer to 15000 than 14925 is.
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1 MHz / 67 = 14.9254 kHz
Are 1 MHz crystals (or some multiple) cheap?
Internal oscillator of the PIC16F1788
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Why would Hercules be relevant for a LCR meter?
Hercules line rate was 18425/second, which is 1/777 of the NTSC 14.31818 crystal frequency. That would make sense for a 720 pixels wide monitor using the NTSC crystal for the pixel rate, with 57 pixels of time for flyback.
As mentioned in the previous message, NTSC/955 would give 14993 Hz which is actually closer to 15000 than the figure given.
In short, related frequencies in seemingly unrelated products could be the result of both being derived from some particularly common and therefore cheap crystal.
This however does not turn out to work for the 14.9254 kHz given by OP.
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I don't remember exactly, but was not this originated from the horizontal scan rate of early B/W CRT monitors? (Hercules or something like that)
MDA/Hercules was 16.257 MHz / 882 = 18.432 kHz.
Most composite video is 315 MHz / 88 / 227.5 = 15.734 kHz
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1 MHz / 67 = 14.9254 kHz
Are 1 MHz crystals (or some multiple) cheap?
Internal oscillator of the PIC16F1788
A lot of µC internal oscillators have a nominal 1 MHz frequency (ATTiny85 too, for example), with an option to multiply them up using a PLL, but they are not very exact!
It's NOT a crystal.