Superheterodyne Receiver Book

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Hello,

So I was able to take my differential amplifier and use that as a mixer, I also added a colpitts oscillator to the circuit and the mixer works grest with it. For now I'm mixing a 1MHz signal at 500mVrms with a 1.455MHz signal to get 455KHz. So my question is what kind of incoming AM signal should I expect to get like peak to peak voltage? Will it be in the uV?
 

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vk6zgo

Joined Jul 21, 2012
677
Hello,

So I was able to take my differential amplifier and use that as a mixer, I also added a colpitts oscillator to the circuit and the mixer works grest with it. For now I'm mixing a 1MHz signal at 500mVrms with a 1.455MHz signal to get 455KHz. So my question is what kind of incoming AM signal should I expect to get like peak to peak voltage? Will it be in the uV?
My guess is that it will be "deaf as a post",but don't let that deter you.
If this is a real circuit & not just a "pretend" one done in simulation,if you connect a fairly long length of wire to where you are injecting the 1MHz now,& adjust the LO frequency,you should be able to find an MF Broadcast station.

It's not a marvellous mixer circuit,though,& you would be better off building something from an existing design.
You are more likely to become discouraged & give up,rather than learn much,if you insist on re-inventing the wheel all the time.
 

vk6zgo

Joined Jul 21, 2012
677
I suppose,looking at it,that if fulfills most of the requirements of a balanced mixer,in the way you've fed the two inputs to it,etc,but it still looks like what it is,an audio differential amplifier,fiddled to become a mixer.

By the way,what is D1 for?

You have obtained a 455kHz output with an input of 1MHz,& a local oscillator at 1.455MHz.
Just for laughs,without changing anything else,change the input frequency to 1.910 MHz.

What happens?
 

vk6zgo

Joined Jul 21, 2012
677
Cool!
You've learnt one of the limitations of simple Superhets.

This phenomenon is known as "image interference",& manifests itself as two stations appearing at the same tuning setting,or the same one appearing at two different settings.(Commonly known as "Double-Spotting").

There are a few classic ways around this:-
(1)
Provide some selectivity prior to the mixer with a variable tuned circuit.
In your situation,that circuit would be peaked on 1MHz,& have quite substantial rejection of 1.910MHz.

(2)
Use a fairly high IF frequency,along with a bandpass filter at the input of the mixer,so that the "image frequency" is a long way out of
the required tuning range of the receiver.
The downside of this,is it is hard to get good selectivity with a high IF frequency in a simple Superhet.

More complex units often use what is termed "Double Conversion",where a high frequency "1st IF" is used to minimise "Image Interference",then fed to another mixer which converts the signal to a lower frequency "2nd IF",to get better selectivity.

(3) A combination of (1) & (2),with the variable tuned circuit taking the place of the LPF.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Ok I do understand what your getting to. One of my major problems in RF are inductors/transformers, I see them get use to couple say an rf amplifier to a mixer. Do you or anybody have an example of how to calculate the values to a transformer to couple one to another.

Thanks
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
So I need advice regarding the circuit I've attached below. I have the oscillator, mixer and rf amplifier, I still need the IF amplifier, demodulator and audio amplifier.

I have physicaly tested the mixer, oscillator and rf amplifier and they do work well together my main concern though is I have an antenna that I stole from my sound system and I'm wondering if the circuit will work if I just add the antenna directly to the circuit?

I did check my antenna with a spectrum analyzer and then used the rf amplifier i made and I could see spikes every 50 khz or so which I believe are my AM radio channels?

I'm worried Its not going to work properly I need opinions on my circuit.

Thanks
 

vk6zgo

Joined Jul 21, 2012
677
You will still have problems with Image Interference,as your RF amp is a wide band RC coupled circuit.

As you have a fair bit of gain & no selectivity before the mixer,you may also
suffer from "front-end overload" which causes intermodulation between the incoming signals prior to mixing.

It is a Medium Frequency AM band Radio,so you don't have to worry too much about how good the antenna is---just connect it up,& have a listen!
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Ok but I seem to have a hard time designing an antenna that i can tune to a certain frequency I see alot of ferrit antenna with a variable cap but cant grasp how to make one. An how could i listen if I dont have a demodulator?

Thanks again
 

vk6zgo

Joined Jul 21, 2012
677
Lots of radios use a ferrite rod with a variable capacitor,but it is still just an LC tuned circuit,which also functions as a compact antenna.

You can make an ordinary coil up on a small former (try about 50 turns),& combine that with a variable capacitor at the input of the RF amplifier.
You can couple the antenna (a random piece of wire) by connecting it to the top of the coil for now.

There isn't much point in making a Superheterodyne if you don't have selectivity at the IF frequency,so you could put a similar LC circuit after the mixer,but tuned to 455kHz.

For a demodulator,I would suggest you Google for something like "Simple detector" or "Crystal Set".

You are really doing this the hard way,as there is a lot of information about Superhets in books at the local Library & elsewhere.
Here is a good link:http://www.users.cloud9.net/~stark/ct9.pdf

Don't kid yourself that you will learn more with your approach--you won't!
 
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Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Alright so I'm starting right from the beginning now starting with a tuned rf amp. Heres what I have but what I would like to know and learn is to couple one stage to another using a transformer.

Here's my circuit and a drawing of what I'd like to do to couple them. So I guess my main question would be how do you calculate the value of the secondary winding.
 

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Audioguru

Joined Dec 20, 2007
11,249
The "next stage" is the input of a transistor that has a certain fairly low impedance. You can calculate the impedance then select a transformer turns ratio so that the primary tuned circuit has a high enough Q for good selectivity and its Q is not reduced too much by the load of the "next stage".

In your first schematic each transistor needs a coupling capacitor at the output of each secondary winding to keep the winding from shorting the base bias voltage to ground.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Ok so if I were to use the exact same stage as the first one I calculated the input impedance of my transistor being 27.5KΩ

Rin = Vb/Ib
= 7.7/30.42uA
≈ 253KΩ
Rintotal = 253KΩ][80KΩ][50KΩ
≈ 27.5KΩ

Now this is pretty high now how do I take this value to calculate my primary/secondary winding and calculate Q?

Thanks
 

Audioguru

Joined Dec 20, 2007
11,249
Your calculation of the input impedance of the transistor is WRONG.
The input impedance is the beta of the transistor times the internal emitter resistance of the transistor, in parallel with the voltage divider resistors.
 

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