1st & 2nd Mixer and Roofing Filter Board (AD831)

This board contains the 1st and 2nd Mixers, Roofing Filter, and Post Mixer Amplifier.

The 1st Mixer converts RF signals from the nine HF amateur bands to a 1st IF of 70.455-MHz. The 1st IF signal is then applied to a Roofing Filter to provide additional selectivity ahead of the 2nd Mixer. The 2nd Mixer converts the 70.455-MHz 1st IF to the 9-MHz 2nd IF. A broadband Post Mixer Amplifier adds gain and drives a 6dB resistive pad to provide a broadband 50Ω termination for the IF Filter board that follows this stage.

The total gain of this board, RF input to 2nd IF output, is approximately 21dB.

The 1st and 2nd Mixers use the Analog Devices AD831, a low distortion, wide dynamic range monolithic mixer. Each AD831 is configured for single supply operation and powered by individual LM2940-9.0 9V regulators. The gain of each mixer stage is set to 10dB by combination of R1/R2 and R5/R6 in the 1st and 2nd mixers, respectively. Refer to the AD831 data sheet for more information.

Broadband transformer T1 transforms the unbalanced 50Ω RF input impedance to a balanced 1250Ω impedance at the 1st Mixer input. T1 is a conventional broadband transformer consisting of a 35 turn center tapped primary winding, and a 7 turn secondary winding. The transformer design starts with the secondary winding by determining the number of turns required to provide an inductive reatance of at least 4 times the terminating impedance (50Ω) at the lowest operating frequency (1.8-MHz). An inductance of 20µH provides a reatance of 226Ω at 1.8-MHz, requiring seven turns on an FT-37-43 toroid. The number of primary turns is then derived to achieve the desired impedance transformation ratio. In this case, the required impedance transformation ratio is 1250/50 = 25, resulting in a turns ratio of √25 = 5, and primary turns = 5 x 7 = 35 turns. The primary turns are first wound on the toroid using 28 AWG wire, and wound to occupy 330° of the toroid, leaving a 30° spacing between the two ends of the primary. The secondary turns are then wound over the center of the primary winding, in the same direction.

Resistor R13 (51Ω) establishes the correct termination at the output of the 1st Mixer for the Roofing Filter, X1, an International Radio 70.455-MHz 6-pole crystal filter, model no.914.

Broadband transformer T2 provides a proper termination at the output of X1 by transforming the balanced 1100Ω input impedance of the 2nd Mixer AD831 to an unbalanced 50Ω impedance seen by the output of X1. T2 is a conventional broadband transformer consisting of a 19 turn center tapped primary winding, and a 4 turn secondary winding, providing the desired impedance transformation ratio of 22 = (19/4)^2 . The requirement that the smaller winding present an inductive reatance of at least 4 times the terminating impedance is easily met.

Resistor R9 (51Ω) establishes the correct termination at the 2nd Mixer output for the Post Mixer Amplifier.

The AD831 requires an LO drive level of -10dBm, eliminating the need for high power LO drive. This allows the AD831 to be driven directly by the Si570 XO/DSPLL with only a voltage divider to set the correct drive level. Note that maximum LO drive to the AD831 should not exceed 1 Vp-p.

1st Mixer, 1st IF Roofing Filter, and 2nd Mixer

The 2nd IF Post Mixer Amplifier uses a 2N5109 design taken from Wes Hayward W7ZOI and John Lawson K5IRK, “A Progressive Communications Receiver,” QSTpp. 11-21, NOV, 1981.

2nd IF Post Mixer Amplifier

The ExpressPCB file for the board can be provided upon request.

J1, J2, J3 & J5 are female machined pin breakaway headers 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 connector J4.

Ferrite beads FB1 through FB3 are Laird Technologies EMI Filter Beads, Mouser PN 875-HI1806N910R-10 or Digikey PN 240-2541-1-ND in 1806 SMD packages.

All 0.1uF and 0.01uF capacitors and C4, C5, C18 & C19 (68pf) are SMD 1206 style chip capacitors.

C13 & C26 are 33uF 20%, 10V tantalum capacitor in a 1206 SMD package, Mouser PN 80-T491A336K010.

C28 is 10uF 20%, 35V tantalum capacitor in a size D SMD package, Digikey PN 399-3768-1-ND.

Resistors are 1/8W 5% SMD 1206 style chip resistors; except R1, R9 & R13 (51Ω), and R15 (35.7Ω) which are 1% tolerance.

U1 & U2 are LM2940IMP-9.0 low drop out, 1 amp, 9 volt regulators in an SOT223 SMD package, Digikey PN LM2940IMP-9.0/N0PBCT-ND.

U3 & U4 are Analog Devices AD831 low distortion mixers, Digikey PN AD831APZ-ND. The AD831 package is 20-lead PLCC, these are plugged into 3M 20-lead PLCC sockets, Digikey PN 3M2011B1-ND.

Q1 is a 2N5109, Digikey PN 2N5109CS-ND.

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

1st & 2nd Mixer, Roofing Filter, and Post Mixer Amplifier, top view

The RF Input connector, J1, is installed on the bottom side of the board (bottom left corner). The 2nd IF Output connector, J5, is installed on the top of the board (bottom right corner) to facilitate daisy chained connection between the various boards when they are mounted vertically in the Express Receiver case.

1st & 2nd Mixer, Roofing Filter, and Post Mixer Amplifier, bottom view

VFO and LO connections are installed on the bottom side of the board.

© 2014 – 2020 Rod Gatehouse AD5GH

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