VCO design

Joachim Verghese jocke at netcontrol.fi
Fri Jun 2 08:22:09 CEST 1995


On Wed, 31 May 1995, Rick Jansen wrote:

> The discharge time is kept very short here by using a
> HC4066 analog switch instead of a discrete (MOS)FET. FETs
> with a low Rds,on are hard to find, while the switches in
> the common HC4066 have an Rds,on of only 25 ohm! Normal
> MOSFETs have a Rds,on of ca 100 ohm. On the downside: a 5V
> supply is needed now.

The IRFD120 HexFET has Rds(on) = 0.27 ohms, max continuous
Id = 1.3A. It is made by at least IR and Harris, comes in a
nice DIP4 package, and costs around $1 in small qtys.

> The discharge time of the capacitor introduces an error in
> the sawtooth. As the discharge time is always the same it
> gets increasingly important at higher frequencies. This
> can be compensated for by decreasing the treshold voltage
> at higher frequencies, so the charge time is a bit
> shorter, and thus linearity is ensured. The cost is that
> sawtooth amplitude gets a little bit lower at higher f.

Are you familiar with the Chroma VCO design? Quite interestingly, it's
based on the RC4151 V-to-F converter which allows for good linearity
characteristics. A minor drawback with this design is the relatively
long retrace time (20 us), during which the output is forced to gnd.
This slightly alters the saw shape (-> harmonic structure) at high
frequencies. I've used this design for my VCOs, and it works well.

The 4151 is made by Raytheon and Exar, and costs around $2. The 4152
is an enhanced version with even better linearity, but at much higher
cost. National has an equivalent, the LM331.

-joachim




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