This AM detector comes from the article "Recent developments in circuits and techniques for high-frequency communication receivers", by Ulrich L. Rohde, KA2WEU/DJ2LR, Ham Radio, April 1980, page 20.
Under the subtitle "linear detectors", Mr. Rohde explains the AM detector:
"AM detectors are frequently required to have very low distortion and because of this, the IF level must be kept at a very high level. Because of gain distribution, this is not necessarily very desirable, and a feedback am detector as shown in fig. 8 is ideal."
My note: This is the case with this receiver, the IF level going into the product detector is very low, -60dBm to -40dBm. To raise the level, the output from the 455 KHz filter is routed to the input of the BFO MOSFET amplifier. The 3.3pf cap is removed at the input to the BFO amplifier. Then the output of the BFO amplifier, at the 47pf cap, which is also removed, is routed to the input of this detector coupled with a .01 capacitor.
Continuing with Mr. Rohde:
"The output distortion is substantially less than one per cent up to a very high modulation percentages; it should be driven from an impedance of less than 200 ohms which can easily be provided by using an emitter follower."
The following picture shows the implementation of the AM Detector on Board 2 including switching so that either AM or CW/SSB (product detector) can be selected.
There is also a modification to the AGC circuit, adding the AGC amplifier to the AGC circuit. AGC Amplifier modificationPlease note that this picture does not show the 10mfd capacitor at the output. It is needed and shown in a picture below.
Shown below is the "dead bug" style of construction to build the AM Detector with a 1" by 1" PCB board.
|1||Single sided PCB
1-1/8" by 1-1/8"
|Dead Bug const|
|1||7 small PCB pads||glued with Super Glue|
Small pads are glued with Super Glue onto the board with the following layout. The Manhattan Layout Mirrored can be copied and printed to use for Blue Stick type of construction.
Resize mirrored layout Width 1" by Height 1.19" after loading into a graphic program (i.e., IrfanView, free graphic viewer on the Internet), print and stick on the pads, place glue on pads, turn over and stick on PCB.
The parts are soldered to the pads as shown below.
Note the Ground wires soldered at the top left and right side middle that go through holes in the PCB and are soldered to the ground plane on the bottom side of the PCB.
The 10 mfd capacitor noted in the schematic is needed at the ouput. A .047 mfd capacitor will also work if you ar cramped for space.
The AM Detector board is laid on Board 2 as shown below with ground wires as shown to hold it to the PCB and provide ground for the circuit. Note the 10mfd capacitor at the output.
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Adding the switching for 12V to the board includes a couple of steps.
First, a trace is cut beneath the board underneath the 12 Volt box located right below the text "BFO Amplifier" as shown above.
You have the choice of a couple of types of switches. They are both DPDT types that will deliver 12V to either the BFO or the AM Detector.
The small one mounts in the 12 Volt box with the right lead bent over so access is available on top of the Board.
The larger DPDT switch can be mounted underneath the Ground box at the bottom of the picture with the two holes on either side providing a place for a plastic tie or a couple of screws and a short metal strip to hold the switch against the PCB.
Make sure it is mounted forward enough not to short against the signal trace right next to the ground holes (LED, 100 ohm, .01 trace) or use a piece of black tape (doubled to make sure) on the switch to prevent shorting to the trace.
Another option is to drill out the right hand hole to 1/4" and mount the switch in it. Make sure there is room, I did not test this mounting position.
After a switch is mounted, attach three wires to the 12V, In, and a 10mfd capacitor (at the Out) to the places on the AM Detector board and wire them to the appropriate places as shown below. Remember, a 10mfd capacitor is placed between the output and to the audio input - not shown below - see picture.
____In wire, 2-1/2" long goes to the right side of the 1N270 diode.
Also note that the wires from the audio pot to the Audio Control box have been changed from what is shown in the picture. Number 1 from the audio pot goes to 2 in the Audio Control box, and Number 2 from the pot goes to 1 in the Audio Control box. See Wiring the Potentiometer.Back to Building Board 2, Section 3
AGC needs to be added to the AGC amplifier. It was noted that if the amplifier ran wide open that it would overload the AM Detector and distort the audio.
The AGC still works fine with both CW/SSB and AM.