Friday, October 5, 2018

RF hook-up faults

In the old days we made radios on plywood and masonite  even old coffee cans. Let's take a quick look at then and now.
This is a good example. A 1" by 4" board and some screws and finish washer. Nothing simpler.
Hear is the board populated with parts. Today we have so many rules that seem to be violated here.
Here is an excerpt from an IC hookup.
Another excerpt. Watch the grounding?
Here we have one ground by the circuits are separated.

In this example we need to understand the lead is an inductor. Making this hookup puts an inductor between the two resistors and ground.
Back to the Ap Note, separate pins 4 and 8 or cause a feedback which can be fatal to your project.
 Tag board circuit was common in the past.


 This worked great so what changed?
I have a box of pins and some masonite.
I put the IC on some double sided tape.
Dill holes and insert pins.
These are nice pins with a shoulder and knurled section so the can be soldered to on either side of the board.
ready to do some hookup right? Well look back at the rules.
It seems I have violated all the rules before I start hookup. If I use clip leads to connect the input and output I will be cross feeding and the circuit will not work. What if the input is rated at 3 microvolts and the generator is feeding .3 volts RF. I'm expecting a direct conversion radio but the generator signal radiates into the signal port and overloads it so I can hear no signal. To make this circuit function as a direct conversion receiver I need shielded cables feeding the LO and RF signals. (Remember to separate the shields too) Back to my original circuit. How could we get away with that?
It was a TRF receiver. The amp is AF and as such is not going to cross talk with the antenna signal. The Rf is not being amplified so is not as easy to overload.

Another quick comment on this grounding. Note the signal flows from left to right so any inductance in the ground path will tend to 'buck' the input feeding into pins 5 and 6. If you cascade these amps you are to tie the input of the next stage to the point where 7 and 8 are grounded and separate 5 and 6 of the next IC.

With a SA602 you would apply this and see pin 1 and 2 are the input 3 is ground 4 and 5 are the output. So feed the end with pin 1 and take the output from the pin 4 end. ground is in between. 6 and 7 are the oscillator connection and are as far from the input as possible. Remember NEVER feed a trace under the chip from input or output or you just killed the isolation.
It's not rocket science or magic . We just need to remember to separate the input and output and keep those leads short.
Now what to do with that board I made for show and tell? Oh I will use a LM386 audio amp so I can get away with some longer leads and such.
 



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