How We Finally Got it Right (and all the secrets too!)

0805 components are supposed to lie flat, but this is one time when "tombstone" position is definitely needed. For those visitors who don't know "0805" means the component is 0.08 inches long by 0.05 inches wide. 0603 size are more difficult to use for hand assembly, but the skill can develop. 0402? ..well, not a good idea for this! 

Here is how we use them to make a termination to the coaxial input that is good to 1.5GHz. The use of two resistors puts their self-inductance components in parallel, the combination being much less than either. The "1000" marking is in fact the 100 ohm resistor. The other resistor is out of sight behind the the incoming centre conductor.

The point here is that the 68 ohm wire-ended resistor seen earlier (closeup view at right) might be OK for frequencies below 100MHz, and has been seen to work effectively beyond 800MHz, it is not the thing to be using if you want it good to beyond 1GHz.

It is tricky to put it all together. First, the coax is rolled into position, tinned centre conductor up against the incoming coax connector centre spill, and gently tacked there with solder. Holding the coax steady with tweezers, another temporary tack at the other end outer to the strip underneath. Once held, the hardline outer can be secured to the strip below with more proper flowed solder joints.

Similar when mounting the resistors. Hold in place, and tack the top end. Then solder the groundplane end. Then return to the tack, suck away solder if necessary, and re-do the top properly. 

So what about those "Plastic Bits"?

Our re-worked mixer is nearly there, but not quite. I try one of those plastic squares - just holding it across the "forks" with a finger. Wow! The effect is dramatic. The trace levels out, and loses most of the wobbles! There is still a slight 2dB droop around 600MHz. The black plastics are slightly curved, as if they need heat or something to flatten out. And they are a kind of brittle!. Ah..Ha - maybe magnetic..YES! By now, one has broken in two, but I confirm the little things stick to a magnet.

They are loaded with ferrite. The big piece stuck under the lid is the same stuff! So how does this work?. Does placing these bits over the structure maybe prevent or absorb stray radiation from the top of the strips? Do they somehow improve the balance? They don't surround any common-mode currents, so I don't know what is going on here. The effect is too good to ignore, yet searching for clear knowledge on this subject yields nothing.

A bit of the microwave "black art"? I hope not! For sure I am going to exploit it, but I should like to know if there is anything about this effect that is quantifiable, calculable, and could be part of a design process instead of taken from the "box of mysterious microwaves tricks". (Of course, it could simply be my ignorance, in which case - somebody do tell!) 

It didn't take long to discover that a old ferrite "binocular" core stuck on temporarily with double-sided sticky tape was easily the best. It will be secured with some tiny spots of epoxy before the lid goes back. The original plastic slabs placed over the input strips yields further improvements.

The trace is looking good, except below 50MHz, where it sags badly. I try replacing the 51 ohms resistor and the 47pF capacitor with a copper link (shown at right). The low frequency gain is now too much, and the trace is not so nice anymore. So I try a 4.7nF capacitor, and take two tries to discover that 12 ohms with it delivers the traces you see next.

It occurs to me that my "repair" ends up looking a bit like the scene I found when I opened it - maybe just a little tidier.

Right about here is where you want to see THE DISPLAY TRACES

You can see some wobbly displays here

That's it folks. I do hope you found this little journey at least a bit informative.
Or - maybe just be entertained that somebody would try something like this.

Graham Seale
Southline Electrolab



Page 1 Special Mixer Mess (description of the problem)

Page 2 - Spectrum Analyser Mixer Dissection - Taking things apart

Page 3 - Spectrum Analyser Mixer Dissection - A closer look

Page 4 - Spectrum Analyser Mixer Dissection - Oh Yes - it is still full of surprises!

Page 5 - Spectrum Analyser Mixer Dissection - How it ended up

How We Finally Got it Right (and all the secrets too!)

The Result - Performance Traces

Some (not so good) Traces


Author : Graham Seale