Board 2

Board 2 || Second Mixer || Crystal Oscillator/Amplifier || IF Strip || AGC || S-Meter || BFO/Amplifier || Product Detector || Audio Amplifiers || Board 1

Second Mixer to Audio





The picture is a top view of Board 2. This board contains the tail end of the receiver: the Crystal Oscillator and Amplifier, Second Mixer, 455KHz IF Strip, Product Detector, BFO, BFO Amplifier, Pre-audio Amplifier, and Audio Amplifier.

At the upper left is the Second Mixer. This Mixer is fed RF from Board 1 at the box labeled "Xtal Filter In". This signal is mixed with the crystal oscillator located at the far upper right.

The Crystal Oscillator is amplified by a MOSFET amplifier.

Unmarked holes are at the output of the crystal amplifier to place a one crystal filter. The filter will eliminate the second harmonic of the oscillator and decrease phase noise. But careful tuning of the oscillator frequency is needed to minimize the losses through the filter. An oscilloscope or an RF measuring device is needed to properly set up the one crystal filter.

The output of the Second Mixer goes to the 455kHz IF amplifier chain. The first two 455kHz IF amplifiers are used to amplify the signal for delivery to the Product Detector. The third amplifier is an AGC amplifier, which sends its signal to a 1N270, a germanium diode with low forward voltage drop.

The resulting current from the 1N270 is sent to the AGC circuit, which is similar to the one in the Progressive Communication Receiver. The output from the AGC regulates the voltage on Gate 2 of the MOSFETs which controls the gain of the IF strip.

Two jumper wires are used on this board that connect the bold lined boxes named "AGC". These wires connect the AGC voltage to all the IF amplifiers.

Since the LEDs are in the drain/source circuit of the MOSFETs, they show the variation of the AGC voltage. This makes for a 'flashing display' which follows CW notes and the fading of SSB signals.

After the second IF amplifier, a signal line runs down to the product detector through a 455kHz IF filter. The filter is very broad (6kHz) and is used to block noise from the IF strip. There are footprints for two 455KHz filters, but only one is used. The two footprints cover 99% of the 455kHz filters made, so any available filter may be used.

The product detector uses Schottky diodes providing a very low loss product detector (part number HP5082-2877). The difference between the Schottky diodes and 1N914s was detectable in the audio at the speaker.

On the bottom left of the board is the BFO followed by a variable gain MOSFET amplifier. This amplifier helped considerably in reducing noise.

Since there is abundant amplification, the 455kHz IF can in the output of the amplifier is detuned by running the slug all the way to the top, and the gain adjust of the amplifier is adjusted for the audio output desired by the operator. If the LED at the BFO amplifier is just barely on, it will provide the cleanest output at the speaker.

On the right of the product detector is the audio pre-amplifier. After some complaints that the B&E receiver didn't have enough audio output for some people, and is a problem noted in a lot of articles about receivers, the receiver was designed to have plenty of audio gain.

The "Gain Adjust" capacitor in the Pre-audio Amplifier is used if more audio output is needed. Recent tests have shown that a 2.2mfd capacitor is the best value to use.

A 12" woofer speaker can be driven with this capacitor installed. For those who like a lot of audio, add the "Gain Adjust" capacitor.

The audio amplifier is the TDA2002, a very common audio amplifier used in commercial radios. This amplifier has a maximum output of 8 watts into a 4 ohm load. A 4 ohm speaker should be used with this amplifier, though a 8 ohm will work, with slightly reduced volume.

The Audio Amplifier likes a high wattage 4 ohm speaker. For those who wear headphones, a low impedance set will plug right in. Just be careful with the audio control!

The 455 kHz IF amplifier strip.

Breaking the Board Apart

This board may be broken into three pieces, isolating each section into shielded boxes, if desired. Two rows of soldering pads are provided for easy cutting of the board into the three pieces.

If the boards are not broken, shields may be soldered along the soldering pads. The most important shielding is at the Second Mixer. If you live right under a broadcast tower, shielding the second mixer might be a necessity.

Holes are provided around the mixer for complete shielding. The second mixer is very sensitive and can pick up local high wattage commercial broadcast stations adding birdies to the receiver.

A shield between the IF strip and the BFO/Audio board is also useful to keep the BFO signal out of the IF strip.

Very short shields, about 1/2" to 1" high of PCB material or tin strips will be adequate. Copper or tin strips, 2" by 6", can be purchased from a hardware store.

The BFO was placed in the lower left hand side of the board away from the IF strip. This placement was determined when the BFO signal was found in the IF strip on the early prototype boards. Moving the BFO circuit cured the problem.

One of the reasons for the spaced out layout of the board was that it helped considerably in the isolation of all the different RF signals on the board.

A picture of the top board highlighted by the LEDs on the first board. Several mounting holes are provided to either stack the boards, or break them apart and mount individually.

Board 2 || Second Mixer || Crystal Oscillator/Amplifier || IF Strip || AGC || S-Meter || BFO/Amplifier || Product Detector || Audio Amplifiers || Board 1

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Last Update: 8/25/06
Web Author: David White, WN5Y