FM Ratio Detector
(Note: Aligning this detector can be tricky)

Description: The particular type of detector I have chosen is I think the best non chip detector, it has very good AM rejection so it does not essentially need a limiter [even though I have internally limited mine], and it can be used for AFC that pulls on both sides like a quadrature detector.

The Ratio Detector was once widely used because of it's efficiency, performance, and AFC; It was replaced by the more efficient IC based quadrature detector which only requires one adjustment, reducing alignment time.
If you would like to play with different detectors here: http://www.tpub.com/neets/book12/51c.htm [Hit next / previous for more]; but it's my opinion that the Ratio Detector is easiest to deal with FM detector for transistor based circuits, and the quadrature is best for IC based circuits.

The reason I don't go after the quadrature is the fact that it is harder on the transistor level to build and get to work, and it requires a powerful controlled limiter in order to achieve the necessary AM rejection you want.

Please note: The ratio detector can be very noisy when the bottom transformer is even slightly mis tuned and actually feedback into the IF subsystem.

Minor Notes:

• I have added a 15 kilo ohm potentiometer “Detector Gain” for ease of alignment and problem solving, this can be removed later if you would like.

• I have also added a 15 kilo ohm potentiometer “AFC zero adjust” for possible component tolerance problems with the AFC, this can be fixed as well.

• If you don't build the AFC section, you can put the two 10K resistors off the bottom IF transformer to ground and remove the 15K potentiometer “AFC zero adjust”, 22uf capacitor and 47K resistor.

• The single germanium diode on the middle transistor takes care of some of the more severe AM distortions and general detector squirrelyness by limiting; most novices when they build this detector don't know this simple trick.

Note: Use the 42IF122 www.mouser.com Transformers 123 and 129 I found are less balanced.

Note: The first PNP transistor off the input is a isolator not a amp; without it, putting another coil to ground makes it an oscillator.

Construction and alignment:

1. Build the detector as shown on the right (Note all polarity's), and turn the detector gain adjustment to the middle.

2. Apply a the signal generator to the input and set it at 10.7 MHz.

3. Put a oscilloscope probe on the bottom transformers middle pin (0.001 Volts 0.1uS) and adjust the top transformer for maximum wave.

There are two methods for aligning this detector:

Method A:

4. Adjust the bottom transformer for minim waveform
5 Repeat 3 and 4.

Method B:

4. Move the oscilloscope probe before the 1uf capacitor on the audio preamp output, switch the oscilloscope to DC 0.01 Volts and adjust the AFC zero adjust all the way over to ground. (or until the line is in the middle of the screen)

5. Tune the bottom transformers core until you have found the midpoint between top and bottom of the screen; adjusting the signal generators frequency should now make the line go up and down.

6. Turn the Detector gain all the way to ground.

7. Connect the "To audio preamp" to the audio preamp.

Never do this it isn't doing what you think it is.

Ok, Even though this seems to work, it causes trouble, and I will show you why; this was once done in the early 80's in some commercial equipment.

Description: A one mega ohm resistor is placed to positive, and brings up the bottom static level

What this does is charge one of the diodes and the 10uf capacitor in the detector to get more efficiency out of the detector.

Side effects:

1. AFC appears to function normally, but on slightly weaker stations only one side of the AFC pulls regardless of the tuning on the bottom transformer.

2. A sharp distortion peak is heard on one side of mid / weak stations making it sometimes imposable to tune them in without distortion.

3. Stronger stations are unable to tune to the center without distortion because the 10uf capacitor cannot discharge making it susceptible to AM distortions causing poor sensitivity, making a [tuned][distortion][tuned] effect.

4. By strengthening only one diode you have an AM detector that you probably mis tuned to fit one side of the slope; AM detectors can tune in FM, but the dial or IF must be mis-tuned to one of the sides. (If you want to prove this make a FM transmitter for the 500 – 1600 kHz band and tune it in with an AM Radio.)

Why not save the parts and build this diode detector, it does the same thing, http://www.tpub.com/neets/book12/51c.htm – hit next for more detectors

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