Recently I released a design for what I hope is a Precision Diode Wideband AC-DC converter. It got me to thinking If I coupled this to a linear sweep generator and a wide rang VCO 100Hz-200 KHz With an oscope I might have the basis for a network analyzer. The key is the VCO. My first thought is to make Schmitt trigger with variable set points similar to how a 555 does it and make a hysteric Oscillator ( I'll draw and post the concept later but it occurs to me that someone has a better idea, So here I am hat in hand asking for ideas, and no, I have not Googled it yet.

 

My favorite has always been the AD654 for LF work. V/F devices are designed for high linearity. This will definitely interest you: www.analog.com/en/products/clock-and-timing/direct-digital-synthesis/ad5933.html
If you want to use a 555, charge the timing cap with a voltage controlled current source. eg https://www.electroschematics.com/7170/wide-frequency-range-555-vco/

 

charge the timing cap with a voltage controlled current source

That is a common approach used in commercial VCOs. But a frequency range of 2000:1 is a big ask, especially if linearity and stability are important.

 

My first thought is to make Schmitt trigger with variable set points similar to how a 555 does it and make a hysteric Oscillator ( I'll draw and post the concept later but it occurs to me that someone has a better idea, So here I am hat in hand asking for ideas, and no, I have not Googled it yet.

A while back I built a voltage-controlled oscillator using a That Corporation 2180 VCA as a voltage-controlled current source, a FET op amp with low input bias current, and a CMOS 555 as the hysteresis element (NOTE: the control voltage is applied to the EC+ pin of the THAT2180, pin 2):

It worked very well over a 1 Hz - 20 kHz range, with a good, clean triangle wave on the op amp output and a square wave on pin 3 of the LMC555; I imagine that with a faster op amp it would go higher in frequency, but I didn't check.

 

Anyhow- back to your idea Wendy, if indeed that is your real name!
Can we see your wide band detector?

 

Sure my name is Wendy Jean Marsden wasn't always, but it has been legally changed and everything. I'll modify my 1st post in this thread to include the link.

Wendy's Index

 

VCO technology was a great way of doing sweeps for response, today
using DDS chips give orders of magnitude greater sweep range, and
the ability to control phase as well.

VCO's, Pease articles on VCOs (and other stuff) - https://s1904481191.t.eloqua.com/e/f2

Jim Williams, some VCO considerations -

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0ahUKEwjRmrOYiafbAhXE0VMKHUImD5sQFghCMAQ&url=http://www.introni.it/pdf/Williams%2006%20-%201995%20-%20EDN%20-%20A%20designer%27s%20guide%20to%20Innovative%20Linear%20Circuits.pdf&usg=AOvVaw11IYkU-pYq5j6EIK-TWGRe

A 7 decade VCO design -

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0ahUKEwjRmrOYiafbAhXE0VMKHUImD5sQFghHMAU&url=http://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf&usg=AOvVaw194BpNDI-S507MPZxGBOsM


DDS - https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj_j4fciKfbAhXP21MKHX3bAXkQFggnMAA&url=http://www.analog.com/media/en/training-seminars/tutorials/MT-085.pdf&usg=AOvVaw03gYL5hoaxwRRt4N1DHzoB

Regards, Dana.