Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" - UK Vintage Radio Repair and Restoration Discussion Forum

doubletriode · 10th Jan 2023, 11:59 am

Hi,

I am currently researching a novel RF attenuator which uses a form of variable inductive coupling.

References to an article from "Electronic Engineering" appeared in the April 1971 Rad_Com "Technical Topics" column describing the attenuator under the heading "Ferrite RF Level Control" and directing readers to the original article written by Laurence Cachia of Australia.

The original and more detailed article appeared in the October 1970 edition of the magazine "Electronic Engineering". Sometimes called a "Cachia control" it was also patented by Philips Industries perhaps around the early 1970's.

Does anyone have a copy of the original article from the October 1970 edition of Electronic Engineering or the Philips patent number please?

Some back issues of Electronic Engineering do appear on the Internet but so far I have not been able to find the October 1970 edition.

Des (M0AYF aka doubletriode)

Ed_Dinning · 10th Jan 2023, 4:21 pm

Hi Des, try the World radio history site. not a complete collection there but quite afew of the later ones. Unfortunately my collection only covers earlier years

Ed

doubletriode · 11th Jan 2023, 1:43 pm

Hi Ed,

Thanks for posting and the pointer to the Word Radio collection.

They do have a few issues of Electronic Engineering but sadly not the one covering the attenuator.

Regards,

doubletriode

John KC0G · 13th Jan 2023, 10:23 pm

Des, here is a scan of the piece in Electronic Engineering. I could not find any details on the patent in either the Australian or European patent databases.

HTH and 73

John KC0G/M0KCY

Mods, if this file is larger than you like, please tell me what to do.

Radio Wrangler · 13th Jan 2023, 10:37 pm

Note that it balances a single-turn-single-turn coupling against a phase-reversed multi-turn coupling. It can be reduced to a bridge circuit with balance/cancellation set by the remote variable resistor. Notice that the remote resistor gets signal RF applied to it, so the coax needs to be short for flatness.

There are other arrangements using DC to bias limbs of a magnetic amplifier sort of transformer to play flux diversion games. DC control makes things simpler.

David

John KC0G · 14th Jan 2023, 3:43 am

David, thanks for your comments

I found an enhanced version by Raymond Haigh in Electronics World, March 2001, p. 222. https://worldradiohistory.com/UK/Wir...1-03-S-OCR.pdf Haigh uses a FET as a variable resistor. The FET is DC controlled, as David suggests. I would be interested to know how what this circuit might look like without the AGC input

73 John

Radio Wrangler · 14th Jan 2023, 5:10 am

As a circuit idea, it's been out there for some time, but never caught on. This begs the question why?

Firstly, it's a brifge nulling scheme, so high attenuation lies at the bottom of a sharp null and that means that the setting to get it is not too stable and adjustment is very sharp, so both where the null centres and how deep it is aren't well controlled. Delay time in the coax gives a phase error between the two paths, limiting null depth.

Secondly, at the time this appeared, PIN diode attenuators were really getting going. Stability with these is a lot better and bandwidth goes up to microwaves. True DC control.

The limitation with PINs is that the diodes have a carrier lifetime effect and set a transition frequency region. Higher freq and they look resistive - lossy and do attenuation. Lower frequency and they start dioding, and go non-linear.

David

doubletriode · 14th Jan 2023, 11:54 am

Hi again,

First of all my thanks to all who posted a response and/or comment on the subject.

Special thanks to John KCOG for taking the time to post the original article from Electronic Engineering plus the bonus article from Electronics world.

My interest in this method of RF attenuation results from problems I encountered about 18 years ago while testing the font end of an old communications receiver. The signal generator I was using at the time was of rather poor quality with the result that at low levels the signal would "leak" making small signal measurements difficult :-( Any meaningful MDS measurements were impossible.

My improvised solution at the time was to use optical coupling with a short length of optic fiber so the signal source and receiver could be better separated. This worked well except the generic LED and photo-diode I used had an upper frequency limitation of around 3 MHz.

The Ferrite Attenuator looked like a straightforward and simple solution to provide some electrical isolation between the signal source and a receiver when working with small RF signals. In any event, I now have a more complete picture of the operation of the Ferrite Attenuator and providing core saturation is avoided and connections are kept short then it could still be a simple and useful method were good issolation between stages is required?

73

doubletriode (aka M0AYF)

10th Jan 2023, 11:59 am	#1
doubletriode Diode Join Date: Aug 2021 Location: Lincoln, Lincolnshire, UK. Posts: 8	Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" Hi, I am currently researching a novel RF attenuator which uses a form of variable inductive coupling. References to an article from "Electronic Engineering" appeared in the April 1971 Rad_Com "Technical Topics" column describing the attenuator under the heading "Ferrite RF Level Control" and directing readers to the original article written by Laurence Cachia of Australia. The original and more detailed article appeared in the October 1970 edition of the magazine "Electronic Engineering". Sometimes called a "Cachia control" it was also patented by Philips Industries perhaps around the early 1970's. Does anyone have a copy of the original article from the October 1970 edition of Electronic Engineering or the Philips patent number please? Some back issues of Electronic Engineering do appear on the Internet but so far I have not been able to find the October 1970 edition. Des (M0AYF aka doubletriode)

10th Jan 2023, 4:21 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" Hi Des, try the World radio history site. not a complete collection there but quite afew of the later ones. Unfortunately my collection only covers earlier years Ed

11th Jan 2023, 1:43 pm	#3
doubletriode Diode Join Date: Aug 2021 Location: Lincoln, Lincolnshire, UK. Posts: 8	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" Hi Ed, Thanks for posting and the pointer to the Word Radio collection. They do have a few issues of Electronic Engineering but sadly not the one covering the attenuator. Regards, doubletriode

13th Jan 2023, 10:37 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" Note that it balances a single-turn-single-turn coupling against a phase-reversed multi-turn coupling. It can be reduced to a bridge circuit with balance/cancellation set by the remote variable resistor. Notice that the remote resistor gets signal RF applied to it, so the coax needs to be short for flatness. There are other arrangements using DC to bias limbs of a magnetic amplifier sort of transformer to play flux diversion games. DC control makes things simpler. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Jan 2023, 3:43 am	#6
John KC0G Hexode Join Date: Mar 2009 Location: Minneapolis, Minnesota, USA. Posts: 278	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" David, thanks for your comments I found an enhanced version by Raymond Haigh in Electronics World, March 2001, p. 222. https://worldradiohistory.com/UK/Wir...1-03-S-OCR.pdf Haigh uses a FET as a variable resistor. The FET is DC controlled, as David suggests. I would be interested to know how what this circuit might look like without the AGC input 73 John

14th Jan 2023, 5:10 am	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" As a circuit idea, it's been out there for some time, but never caught on. This begs the question why? Firstly, it's a brifge nulling scheme, so high attenuation lies at the bottom of a sharp null and that means that the setting to get it is not too stable and adjustment is very sharp, so both where the null centres and how deep it is aren't well controlled. Delay time in the coax gives a phase error between the two paths, limiting null depth. Secondly, at the time this appeared, PIN diode attenuators were really getting going. Stability with these is a lot better and bandwidth goes up to microwaves. True DC control. The limitation with PINs is that the diodes have a carrier lifetime effect and set a transition frequency region. Higher freq and they look resistive - lossy and do attenuation. Lower frequency and they start dioding, and go non-linear. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Jan 2023, 11:54 am	#8
doubletriode Diode Join Date: Aug 2021 Location: Lincoln, Lincolnshire, UK. Posts: 8	Re: Ferrite RF Level Control, October-1970 edition of "Electronic Engineering" Hi again, First of all my thanks to all who posted a response and/or comment on the subject. Special thanks to John KCOG for taking the time to post the original article from Electronic Engineering plus the bonus article from Electronics world. My interest in this method of RF attenuation results from problems I encountered about 18 years ago while testing the font end of an old communications receiver. The signal generator I was using at the time was of rather poor quality with the result that at low levels the signal would "leak" making small signal measurements difficult :-( Any meaningful MDS measurements were impossible. My improvised solution at the time was to use optical coupling with a short length of optic fiber so the signal source and receiver could be better separated. This worked well except the generic LED and photo-diode I used had an upper frequency limitation of around 3 MHz. The Ferrite Attenuator looked like a straightforward and simple solution to provide some electrical isolation between the signal source and a receiver when working with small RF signals. In any event, I now have a more complete picture of the operation of the Ferrite Attenuator and providing core saturation is avoided and connections are kept short then it could still be a simple and useful method were good issolation between stages is required? 73 doubletriode (aka M0AYF)