Product Detector, BFO, and Audio Stages

This board contains the Product Detector, BFO, BFO Amplifier, Audio Preamplifier,  Audio Distribution, Audio Output Level Control, and Audio Amplifier functions.

The Product Detector and Audio Preamplifier circuit is based on a classic design from Wes Hayward W7ZOI and John Lawson K5IRK, “A Progressive Communications Receiver,” QSTpp. 11-21, NOV, 1981. The Express Receiver product detector uses a TUF-1, Level 7 Frequency Mixer from Minicircuits. The original Hayward – Lawson design used the SBL-1. The TUF-1 is a smaller device than the SBL-1, and offers slightly higher performance. The combination of C2, R1 and RFC1 form a diplexer network, providing a broadband 50Ω termination for the product detector output, and passing the desired audio frequencies to the preamplifier. The first audio preamplifier transistor, Q1 (2N3565) is in a common base configuration and biased to approximately 0.5mA emitter current to provide a 50Ω termination for the product detector. Q2 (2N3906) is a direct-coupled PNP stage that functions as the second amplifier in the preamplifier stage. This classic design provides a high dynamic range product detector and preamplifier stage that delivers a very clean audio signal to subsequent stages.

The BFO uses the Analog Devices AD9835, 50 MHz Direct Digital Synthesizer (DDS). The AD9835 is programmed by the Right Display and Control Board via the SDATA, SCLK and FSYNC pins to generate the following BFO frequencies:

USB: 8,998,500-Hz

LSB: 9,001,500-Hz

CW: 9,000,000-Hz

FSYNC is a data synchronization signal that informs the AD9835 internal logic that a new word is being loaded into the device. A 16-bit serial data word is applied to the SDATA input, and is clocked into the AD9835 by falling edges on the SCLK input.

Y1 is a 50-MHz clock oscillator that provides the digital clock input (MCLK) for the AD9835.

The AD9835 provides a high impedance current source output, so load resistor R25 is connected from IOUT to ground to provide a voltage drive for the BFO Amplifier stage.

A 78L05 linear voltage regulator is used at U6 to provide regulated 5V for the AD9835 DDS.

Refer to the AD9835 data sheet for additional information.

The BFO Amplifier consists of a 2N2222 in a broadband, common emitter amplifier configuration. The AD9835 generates a square wave signal with the fundamental frequency equal to the programmed frequency. As the TUF-1 is a ring diode mixer, the drive to the LO Port switches the diodes on and off at a rate equal to the dominant fundamental frequency, the level of other harmonics is not sufficient to effect mixer operation. Therefore, filtering of the AD9835 output is not required, and a broadband amplifier was used to eliminate the need for tuning of this stage. The BFO Amplifier amplifies the BFO signal to the +7dBm level required to drive the LO port of the TUF-1. A -3dB pad between the output of the BFO Amplifier and the LO port of the TUF-1 establishes a broadband 50Ω termination for the TUF-1 LO Port.

Product Detector
Product Detector, BFO and Audio Stages, Page 1

The output from the Audio Preamplifier stage is fed to an Audio Distribution stage consisting of two NE5532, dual low-noise high-speed audio operational amplifiers. The operational amplifiers are configured for unity gain, and split the audio into two streams. Both audio streams are routed to the Dallas Semiconductor DS1807 I2C addressable dual audio taper potentiometer, allowing for independent volume control of each stream.

One audio stream from the DS1807 is routed to the Audio Amplifier stage via a standard front panel mounted audio taper potentiometer. In this case the DS1807 is used to implement a mute function.

The second audio stream is routed to an auxiliary audio output for connection to a computer sound card or audio interface device, such as the Timewave Navigator. In this case, the DS1807 is used to set the level of the auxiliary audio output via a function accessible from the Express Receiver front panel controls.

Communication with the DS1807 is via the Express Receiver I2C bus from the Master Display and Control Board.

The Audio Amplifier is a classic LM386 low voltage audio amplifier design. The LM386 delivers approximately 1/4 watt to an 8Ω speaker.

Product Detector, BFO and Audio Stages, Page 2
Product Detector, BFO and Audio Stages, Page 2

In the photos below, note the conductive foam placed over connector J2, the digital interface to the AD9835. J2 brings out the FSYNC, SCLK and SDATA pins directly from the AD9835, and these pins are very susceptible to ESD damage. I destroyed two AD9835 devices before realizing what was happening. As these devices are in a 16-TSSOP SMD package and have 0.5mm pitch pins, they are very difficult to successfully de-solder without damaging the board. I strongly recommend placing conductive foam over J2 any time the cable is disconnected from J2 or the board is being handled outside of the Express Receiver.

The ExpressPCB .pcb file for the board is here: 2016 Prod Det, BFO & Audio Amp.

Product Detector, BFO, and Audio Board Top View
Product Detector, BFO, and Audio Board Top View
Product Detector, BFO and Audio Board Bottom View

J1 is a female machined pin breakaway header from Sparkfun, PN PRT-00743. Refer to the RF Band Pass Filter page for additional information.

Refer to the RF Band Pass Filter page for information on the connectors J2, J3, J4, J5, J6 and J7.

All 0.1uF and 0.01uF capacitors, C18, and C27 are SMD 1206 style chip capacitors.

C33 and C35 are 1.0uF 20%, 25V tantalum capacitor, size A SMD, Digikey PN 493-2387-1-ND.

C13 and C14 are 2.2uF 20%, 16V tantalum capacitors, size A SMD, Digikey PN 399-8278-1-ND.

C20 is 3.3uF 20%, 16V tantalum capacitor, size A SMD, Digikey PN 399-8284-1-ND.

C34 is a 10uF 10%, 10V tantalum capacitor, size A SMD, Mouser PN 80-T491A106K010.

C3, C4, C5, C8, C16, C26, and C32 are 10uF 20%, 35V tantalum capacitors, size D SMD, Digikey PN 399-3768-1-ND.

C9 and C11 are 22uF 20%, 35V tantalum capacitors, size D SMD, Digikey PN 399-3783-1-ND, .

C7 is a 47uF 20%, 16V tantalum capacitor, size D SMD, Digikey PN 399-8392-1-ND.

C19 is 100uF 20%, 16V tantalum capacitor, size D SMD, Digikey PN 495-2256-1-ND.

Resistors are 1/8W 5% SMD 1206 style chip resistors, except R1 (51Ω) which is 1% tolerance.

Q1 is 2N3565, Digikey PN PN3565CS-ND.

Q2 is 2N3906, Digikey PN 2N3906-APCT-ND.

Q3 in 2N2222, Digikey PN 2N2222ACS-ND.

U1 is an Analog Devices AD9835 DDS in a 16-TSSOP SMD package, Digikey PN AD9835BRUZ-ND.

U2 is a Minicircuits TUF-1 Level 7 frequency mixer.

U3 & U4 are TI NE5532 dual low-noise high-speed audio operational amplifiers in 8-SOIC SMD package, Digikey PN 296-13622-1-ND.

U5 is a Dallas Semiconductor DS1807 dual audio taper potentiometer in an 16-SOIC SMD package, Digikey PN DS1807+-ND.

U6 is an NJM78L05UA 5V, 100mA LDO voltage regulator in an SOT-89 package. An equivalent Texas Instruments device such as UA78L05CPK, Digikey PN 296-13531-1-ND can be substituted.

U7 is an LM386 low voltage audio amplifier in an 8 SOIC SMD package, Digikey PN LM386MX-1/NOPBCT-ND.

Y1 is an ESC 50-MHz crystal oscillator, Digikey PN XC281-ND.

The transformers are wound on Amidon toroids with the toroid type, number of turns as noted in the schematics above.

Product Detector, BFO, and Audio Stages Board Installed in the Express Receiver
Product Detector, BFO, and Audio Stages Board Installed in the Express Receiver


© 2014 – 2016 Rod Gatehouse AD5GH

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