LM567C: Problems when used as a VCO

HSIN-CHE HSIEH

Part Number: LM567C

Tool/software:

I am trying to use LM567 as a VCO, in a way very similar to Figure 10-7 of the datasheet. However, when I tried to apply a DC voltage (0-5 V) to pin 2, output frequency stays exactly the same (~280 kHz) except when pin 2 voltage is close to 0 or 5 V.

My VCC is 5V, R1 = 3.3 k, C1 = 1 nF, RL = 1 k. I did not use C2. I assume pin 4 and pin 7 must be connected to VCC/GND, which are not shown in Figure 10-7.

Any ideas on what I can do to get my circuit to work? Or is it already doing exactly what it is suppose to when used this way? if this is the case, how do I calculate the gain/offset of this VCO?

Please do not try to persuade me to switch to other parts. I just want to get this part to work.

Thank you

11 months ago

0 Kadeem Samuel 10 months ago

TI__Mastermind 22550 points

Hsin-Che,

To be clear, the desired center frequency (given by Equation 2) is 275.482kHz, given by the R1 and C1 selection.

The C2 capacitor is required for setting the loop bandwidth of the PLL. C2 is required for proper operation.

Thanks,

Kadeem

0 HSIN-CHE HSIEH 10 months ago in reply to Kadeem Samuel

Prodigy 140 points

I am not using LM567 as a PLL. Just a VCO. Since in this configuration, Pin 2 is connected to an external DC voltage, why would C2 be required?

0 Kadeem Samuel 10 months ago in reply to HSIN-CHE HSIEH

TI__Mastermind 22550 points

Hsin-Che,

C2 is still required for proper VCO operation. Note that in each of the oscillator use cases, the C2 capacitor is still present:

Thanks,
Kadeem

+1 HSIN-CHE HSIEH 10 months ago in reply to Kadeem Samuel

Prodigy 140 points

I tried to install C2 (tried 1 nF and 10 nF), but this does not really change the behavior of my circuit. I found on some very old, more detailed non-TI datasheets that the valid VCO input voltage range is ~3.7 to 3.9 Volts. This roughly match my experimental results where I can see output frequency controlled by input voltage on pin 2, either with or without C2. I guess now I know what to expect from this circuit.

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LM567C: Problems when used as a VCO