How does it Work?
(2) IF amp and BFO
(4) Audio Amp
(5) Power Supply
When we attach a wire antenna to our Conar, we provide a path for every signal in the universe to come into our receiver. The first thing we must do is filter out just the signals we want. We do this in the "Tuned Input Filter" stage. Notice that there are three coils and capacitor sections connected to the switch. In this manner, we can select signals from the bands (80, 40, and 20) we are interested in. The arrows above this stage tells us that only one group of signals can go through unstopped. Other signals that are outside the "bandwidth" of the filter are attenuated (reduced in strength). However, strong nearby signals can still get by our filter. Also notice that one of the arrows has a head going the opposite direction. This is meant to show that this filter is a 2-way street and signals from the receiver can go through to the antenna. One through the filter, the signals are injected through pin 7 into the grid of our 6BE6 mixer tube. Notice the trimmer capacitor C 9? Thats our antenna "peaker". It changes the overall tuning of the filter circuit and maximizes signal coupling into the tube grid.
The each circuit is tuned through a single "coil slug" for each band. You can adjust the slug using a special plastic coil "tweaker" tool (not a metal allen wrench!) By nature, this filter has to be somewhat broad and tuning it is non-critical.
Our next circuit is the local oscillator. This circuit is different in that it is small "transmitter" and actually generates a signal. Again, you will notice the bandswitch, the coils and the capcitors for each band. How do you adjust the frequency of this little transmitter? Right! By adjusting the coil slug with you magic tweaking tool. You can also "fine tune" (change) the frequency using the variable capacitor C11. This is our "Main Tuning Control" and it's purpose is to tune the signal. The generated signal is then coupled through C12 and injected into the grid of the 6BE6 through pin 1.
Now you have 2 signals in our mixer tube. These two signals are "mixed" (Hetrodyned) and a third signal is produced. This signal has a fixed frequency of 455 kilocycles (Just below 550 in the AM broadcast band). The arrows above show that other signals can be produced but only the 455 Kc signal is allowed through the "IF" transformer. This 455 Kc signal is called "the intermediate frequency" or "IF" for short.
Show hows does mixing really work?
Supposed the signal you want to listen to is on 7.045 megacycles.
You must set the frequency in your local oscillator to ( 7.045
Megacycles + .455 Megacycles (455
kilocycles) = ) 7.500
Now remember when we said the tuned input filter was a two way street? The local oscillator signals are strong enough that they couple through the grids of the tube, go right through the filter and on to the antenna. If you take a shortwave receiver tuned to 7.5 Megacycles, you can actually hear the local oscillator!
Here is another way of producing .455 Megacycles IF signal. ( 7.045 Megacycles - .455 Megacycles (455 Kilocycles) = ) 6.590 Megacycles. Again, tuning your shortwave receiver to 6.590 "Megs" will allow you to hear it.
Either method works fine. One method is
called "Hi side injection" and the other is called "LO side
injection" for obvious
AS if that's not complicated enough, lets look at things the other way around! You have your little "local oscillator" tuned to 7.500 Megacycles. and you have a signal on 7.045 Megs. ( 7.5 - 7.045 = .455 ) now you start picking up another signal from 7.955 megs!
How is that possible? Well we have ( 7.5 - 7.045 = .455 ), but we also have ( 7.955 - 7.5 = .455 )!
Now you see the inportance of the Tuned Input filter! 7.955 is pretty far off from 7.045 and if the signal is not too strong it should not cause us any problems ( It's in the noise or at least very faint ).
If we went with "LO side injection" then it would be ( 7.045 - 6.590 = .455 ) and we would expect to hear the "Voice of America" or the "BBC" as well on 6.135 Megs because it just so happens ( 6.590 - 6.135 = .455 ).
Now this IS BAD!!!
Turns out 6.135 Megs falls right in the middle of the shortwave broadcast bands which is just chock full of megawatt transmitters. While our Tuned Input filter is tuned (peaked) for 7.045, the shortwave broadcast signals are so strong, they barge through the filter and get into the mixing tube.
Use LO side injection on 40 meters and you'll be getting all sorts of interference!!! Not GOOD!
OK this was more than you wanted or needed to know. But it's important. The Mixer is the "heart" of a receiver. Expensive rigs have more stages "in front" of the mixer such as an RF pre-amp and more filtering stages which takes care of the two signal interference problems just discussed.
Poor CONAR! Only has the one mixer tube against the entire Universe! Perhaps an unfair match. Still, it isn't much of a problem if you know how it all works.
(2) IF amp and BFO
(4) Audio Amp
(5) Power Supply
Complete Schematic for the Conar Model 500 Receiver
Conar Transmitter Manual - and - Conar Receiver Manual