July 2016 - Faraday's Biscuit Tin

To use the TV dongle and zener noise source to characterise filters at VHF I needed to do something about the FM Broadcast interference that tended to swamp any readings. Searching around the web I found some hints at reducing USB lead pick up.

The toroids were from the junk box. So I've no idea what material they are made of. Initially I made each coil with two cores and the TV dongle just didn't work on SDR#.

So too much inductance is clearly a bad thing. Going to one core did the job.

Well that helped - as did clamping choke ferrites on the USB lead, but it was clear that it all needed to be shielded. So my wife and I embarked on a shopping trip to find a metal biscuit tin. We discovered that they are mostly plastic nowadays!

Eventually we blew £9 on a tin of some very nice shortbread from Marks & Spencers. This is where people with more money than sense in the UK buy their groceries. Good quality, but very pricy.

I built the filter into the tin with the TV dongle sitting in a section of plastic trunking to keep it from shorting out.

Then I tried an experiment which you can see in this video.

The video initially shows you that there is no broadcast pickup from the dongle USB lead - the filter cured that.

Then I attached a little jig that I made to test filters - and the interference signal comes way up. (These TV dongles are quite sensitive) .

Lastly, you see me laying the jig and cables inside the tin - and it all goes quiet again – even without the lid on.

I built a N0SS (SK) zener noise generator to go into the tin.

The board was cut using my new milling capability. It's a mix of surface mount and conventional components. I used standard capacitors because I have a huge bag of them!

Tom N0SS comments that you have to adjust the zener load to hit the sweet spot for noise output. Hence the blue trim pot that you see in the photos above and below.
In some of his schematics he also shows an output level potentiometer. Below you will see that I added one.

In the hope that copper slug tape will have the same effect on interference as it does on the vermin that eats my vegetable crop, I have screened everything inside.
You can solder to that tape so I will add some straps to electrically connect the tape to the tin.

The test results are both encouraging and somewhat a disappointment.

Encouraging, as there is no sign of broadcast band interference. The tin is working.
Disappointing in that the noise generator falls off quicker than I had hoped. It works brilliantly in the HF region. I just hoped for a bit more "wellie" at VHF. I will need to look into improving VHF performance.

January 2017. The Comb Generator may be the way forward.

March 2017. Very interesting video here where the use of filters can improve the (otherwise easily overloaded) front end of the dongle with filters.



Articles (2016)

November 2016
A Bench Power Supply
More test gear.

September 2016
Return Loss Bridge
Simple to build.

July 2016
Faraday's Biscuit Tin
Screening a TV Dongle

June 2016
A super fun project.

June 2016
Milling PCB's
Cross Clamp Vice

May 2016
Etching PCB's
Using the Toner Transfer Method