View Single Post
Old 27th Jul 2019, 11:20 pm   #19
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Eddystone 750 digital readout

Back in the late 1990's I investigated the problem of how to extract the L/O from radios without disturbance to the frequency when the external counter may or may not be connected.

The results were surprising, you would think for example with a 5 stage fet input buffer, that would do nicely, or perhaps mutual coupling as loose as two coils one inch apart. (its the mutual part that gives the game away, its a two way street)

The problem is, no matter how much isolation you think your buffer amp is providing, if you load its output (shorting it is the acid test) the changes in impedance are reflected back to the input. Its exactly the same with with any form of capacitive or inductive coupling too.

In the end to only way to get what I called perfect isolation was optical, I made the device in the photo to couple the L/O out of one of my EC-10's.

However, I did note the other way to do this with valve radios; with a result easily as good as a buffer amp (better in some cases), is by using a small (1pF) coupling capacitance and a shunt to ground a much larger capacitance (either that of some coax or a fixed capacitor).

That way at the operating frequencies even if the shunt capacitance is loaded, or shorted or whatever, it makes little difference to the fixed frequency offset of the small capacitor. This method avoids having to add buffer circuitry inside your valve radio. However, since it significantly reduces the signal amplitude, it requires that the sensitivity of the counter be increased to a few mV, I simply added a broadband monolithic amplifier inside my counter.

I think this idea was just simply overlooked and most designers felt the way to get good isolation was to use a small coupling capacitance into a high impedance buffer amplifier.
Attached Thumbnails
Click image for larger version

Name:	buff1.jpg
Views:	108
Size:	67.4 KB
ID:	187453   Click image for larger version

Name:	BUFF2.jpg
Views:	93
Size:	58.7 KB
ID:	187454   Click image for larger version

Name:	BUFF3.jpg
Views:	97
Size:	77.8 KB
ID:	187455  

Last edited by Argus25; 27th Jul 2019 at 11:36 pm.
Argus25 is offline