[hpsdr] hpsdr Digest, Vol 11, Issue 12

[Edson Pereira]

FRANCIS CARCIA wrote:

> I wonder why nobody has thought of using FETs as switches?


Frank,

I have been wandering the same for some time and have been playing with 
some spice simulations of sampling detectors using JFETs for the last 
several weeks. The results are very encouraging. I need to one and 
confirm that it does indeed work as expected. Some caution would be 
needed since the Vgs for JFETs tend to vary from device to device even 
within the same family. For a balanced detector, the devices may needed 
to be closely matched.

I have initially used a square wave LO with 25% duty cycle. As with the 
conventional QSD, the best detection gain is obtained with a 15%-25% 
duty cycle, but later I found out that I could also obtain the same 
performance using a sine wave. The sine LO would simplify high frequency 
designs by eliminating the need for the 4x LO.

Some of my simulations can be found below:

http://ewp.homelinux.net/SamplingDetector_J310_01s.PNG
http://ewp.homelinux.net/SamplingDetector_J310_01w.PNG

http://ewp.homelinux.net/SamplingDetector_BF245B_02s.PNG
http://ewp.homelinux.net/SamplingDetector_BF245B_02w.PNG

I have also done simulations with the 2SK241 and with the BF998 MOSFETs. 
These devices prefer a square wave LO, but all work
very nicely as sampling detectors as well.

73,

-- Edson, pu1jte, n1vtn

 1169037048.0