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Radio Receiver Circuit Questions

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Sukoshi

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So I'm planning out a shortwave radio receiver design, and I've been reading up on theory, and looking at other designs and such, and I had a few questions.

1. A lot of ham designs use RF transformers from the antenna input into the rest of the circuit. Up 'till now I've assumed that the transformer has been for impedance matching. If I've been assuming correctly, then what makes the RF transformer method so much more desirable than, say, an emitter follower (other than the fact that an emitter follower needs to be biased correctly) ?

2. In my design, I'm contemplating on using a doubly-balanced mixer IC SA602AN . I've seen the two output nodes of transformers placed across the two inputs, and the two outputs of the mixer I've seen joined by a capacitor, and then using one node of the capacitor as the actual output. What's the point of this capacitor across both output nodes, and having an inductor across both input nodes? Does this have something to do with the "double-balanced" concept, that I don't really understand?

3. 1/8" Headphone Jacks have 3 slots in them -- I understand that the two inputs on the sides are the two channels, and the middle input is the ground?

4. I've read that up 'till VHF, radio circuits can be built on perfboard. Is this possible with a shortwave receiver?
 

MikeMl

Well-Known Member
Most Helpful Member
So I'm planning out a shortwave radio receiver design, and I've been reading up on theory, and looking at other designs and such, and I had a few questions.

1. A lot of ham designs use RF transformers from the antenna input into the rest of the circuit. Up 'till now I've assumed that the transformer has been for impedance matching. If I've been assuming correctly, then what makes the RF transformer method so much more desirable than, say, an emitter follower (other than the fact that an emitter follower needs to be biased correctly) ?
Impedance matching, and lightning protection (static, corona, induced surge.) Antenna is at DC ground. Antenna lead touches no solid-state device directly.

2. In my design, I'm contemplating on using a doubly-balanced mixer IC SA602AN . I've seen the two output nodes of transformers placed across the two inputs, and the two outputs of the mixer I've seen joined by a capacitor, and then using one node of the capacitor as the actual output. What's the point of this capacitor across both output nodes, and having an inductor across both input nodes? Does this have something to do with the "double-balanced" concept, that I don't really understand?
Your verbage is hard to follow. Post a diagram (jpg)

3. 1/8" Headphone Jacks have 3 slots in them -- I understand that the two inputs on the sides are the two channels, and the middle input is the ground?
Tip is left, Ring is right, sleeve is common (or so I've been told)

4. I've read that up 'till VHF, radio circuits can be built on perfboard. Is this possible with a shortwave receiver?
No. You need a ground plane, shielding, lots of bypassing, and mechanical stability. Un-etched CopperClad P.C. Board is a good substrate. Just mount transistors & ICs upside down with leads sticking up right on the copper. You can make inter-stage shields by soldering 2cm high strips of the same PC material on edge.
 
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Sukoshi

New Member
Hey, thanks for the answers.

I've attached a pic of two pins of the IC joined by a capacitor, and using one pin as output.
 

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MikeMl

Well-Known Member
Most Helpful Member
Post the circuit of the mixer section. Your drawing was not very enlightening ;)
 

Sukoshi

New Member
Haha, sure, I'll do that. Also another question. What should I be looking for in an RF transformer that connects to the antenna input? Any specific criteria other than impedance matching?
 

Sukoshi

New Member
I'm actually getting design ideas from the Ramsey HR-30 30m kit http://www.electro-tech-online.com/custompdfs/2009/06/HR30.pdf. I'm looking at the PCB diagram and tracing it, to see how it looks. It looks to me like a very standard design -- antenna hooked into an RF transformer, that's hooked into the mixer ... the output is taken from two pins of the mixer, joined with a capacitor, and that's fed into the LM386 which takes the output into a 1/8" speaker jack.
 

flat5

Member
You want selectivity at the front of the receiver. Usually the transformer is part of that network. Your choice of mixer is a simple one but often has overload problems.
Do read the ARRL handbook for receiver design information.
 

Sukoshi

New Member
I guess a better design to go off of now is the Neophyte Receiver (http://www.arrl.org/tis/info/pdf/28814.pdf) which seems to be really famous.

In the Neophyte, what's the point of C1 and C13? What's the point of C14, and R6, and R4? Also is there any rule of thumb for getting the oscillator on these chips to oscillate? Any simulation model, or equation to ensure that oscillation occurs? (Reading some docs about NE/SA chips, it says a large enough resistor shunted to ground should get the circuit to oscillate...) Any pointers?

And thanks for all the help so far!
 

JimB

Super Moderator
Most Helpful Member
In the Neophyte, what's the point of C1 and C13?
C1 in conjunction with C4(A and B) tune the input circuit T1 to the required frequency.
C13 decouples RF from the AF output to the audio amplifier.

What's the point of C14, and R6, and R4?
C14 and C16 are coupling capacitors which take the audio from the output of the mixer to the AF amplifier.
Without these capacitors the DC conditions of the mixer and the AF amp would affect each other.
R4 and R6 provide a reference to 0v for the inputs of the AF amp.


Also is there any rule of thumb for getting the oscillator on these chips to oscillate? Any simulation model, or equation to ensure that oscillation occurs?
Have a look here for design information on colpitts oscillators:
colpitts oscillators
You dont need a computer game (simulator), just do some calculations and get out the soldering iron!

(Reading some docs about NE/SA chips, it says a large enough resistor shunted to ground should get the circuit to oscillate...) Any pointers?
Uh?? Sound like rubbish to me.

JimB
 

JimB

Super Moderator
Most Helpful Member
http://www.electro-tech-online.com/custompdfs/2009/06/an1982-1.pdf was the link I pulled up about oscillator configurations. The docs suggest adding a 22K shunt resistor to get an LC tank to oscillate.
OK, I found it.
They are just suggesting to add the resistor when the Q of the tuned circuit is low.
Adding the resistor will increase the standing current and hence the gain of the oscillator transistor. This willallow the oscillator to start more easily.

JimB
 
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