# VCO SN74LS624 - understanding pin out / how to read datasheet?

I have a SN74LS624 chip and I am trying to get it working on a simple breadboard with an oscilloscope.

I can not understand a few things from the data sheet:

1) The intro paragraph mentions you need an external capacitor. Where does this capacitor go exactly and how do you wire it up after determining the value for C

2) What are the pins CX1 and CX2 for? How should you connect them?

3) How to use the RNG and FREQ COUNT pins?

Can somebody please explain how I am supposed to determine the answers to these questions simply from reading the datasheet.

It appears to me that not enough information is in the datasheet to use this chip successfully by the enduser.

CX1 and CX2 can be used to connect to an external capacitor or crystal to set the frequency. Figures 3 contain the information needed to select the value for the 'LS624:

If you want a 100KHz center frequency, you are half way between 10^-8 and 10^-9 F (non-linear scale, so 3.3nF is probably the right value.

If you are building this on a breadboard, you will want to stay with lower frequencies (100KHz, or 1MHz if you are bold).

(V)RNG determines the sensitivity to the FC input (slope) for the '624. Figure 1 contains the information you need to choose this value:

For maximum sensitivity (maximum frequency swing), use a lower voltage. If you don't need the range or you have stability problems, increase the voltage on this pin.

I think RC = RNG, the datasheet does appear a little ambiguous on this.

(credit) All pictures taken from the referenced TI datasheet.

• OK, are you suggesting just tieing CX1 to CX2 with a bipolar capacitor as determined by Figure 3? CX must be an outdated term for Capacitor? Commented Aug 30, 2019 at 19:45
• C for capacitor, X for crystal, you can use either. Yes, use a high quality C0G 3.3nF (3300pF) capacitor, actually given that chip and bread boarding, use any ceramic 3.3nF cap. Commented Aug 30, 2019 at 21:07
• XTAL means crystal in EE just like X-mas means christmas. somebody must have thought to themselves, hey, let's just abbreviate the acronym and mix it with a second one, CX, should be crystal-clear. Commented Aug 31, 2019 at 7:27

That is a really poor datasheet. So yes, indeed, information is lacking.

For what it's worth, your first and second questions answer each other: the frequency-setting capacitor goes to the CX1 and CX2 pins (or more properly -- nothing makes sense unless that's the case).

I'm guessing that the RNG and FRQ CONT ("CONT", not "COUNT") pins control coarse and fine frequency control, but I'm not sure. I gather from comments that they can be connected together. I'd just put a pot on each one and twiddle, to see what they did.

And a final note: everything about this part screams "obsolete!" Take that into account with whatever you're doing.

• The voltage on pin 2 (RNG) set the frequency range (F_min and Fmax). And the voltage on pin 13 (FRQ CONT) regulates the output frequency.
– G36
Commented Aug 30, 2019 at 19:05
• Is CX an outdated term for capacitor? Commented Aug 30, 2019 at 19:46
• I don't know where the 'X' comes from. They mention using a capacitor or crystal on the oscillator, though -- 'X' means "crystal", so it could mean "cap or crystal". Commented Aug 30, 2019 at 20:05