The Express Receiver is a homebrew, high performance HF amateur band receiver covering the 160, 80, 40, 30, 20, 17, 15, 12 and 10m bands. You can see the receiver in operation here.
The Receiver is constructed entirely using 2.5″ x 3.8″ ExpressPCB MiniBoards. The following boards comprise the Express Receiver.
- RF Attenuator, LPF and RF Preamplifier Board
- Bandpass Filter Board
- 1st and 2nd Mixers and Roofing Filter Board
- VFO and LO Board
- IF Filter Board
- IF Amplifier and Digital AGC Board
- Product Detector, BFO and AF Amplifier Board
- Display and Control Board
- I2C Relay Control Board
- Keypad Board
- Power Distribution Board
The Express Receiver has been fitted with front and rear panels from Front Panel Express.
RF from the antenna is routed to a 0 to -30dB RF attenuator that is followed by a 30-MHz low pass filter. The low pass filter provides image rejection for the 1st Mixer. Nine selectable RF band pass filters follow the 30-MHz low pass filter, one for each HF band, providing additional image rejection and HF selectivity ahead of the 1st mixer. An RF preamplifier follows the RF band pass filters, providing a switchable +10dB gain block.
The 1st mixer up converts the RF signals to a 1st IF frequency of 70.455-MHz. A 6 pole, 70.455-MHz roofing filter provides close in selectivity ahead of the 2nd mixer. The 2nd mixer down converts the 70.455-MHz 1st IF to the 2nd IF at 9-MHz.
The 1st and 2nd mixers use Analog Devices AD831; a low distortion, wide dynamic range, monolithic mixer with a +24 dBm third order intercept (IP3) and a +10dBm 1dB compression point, with an LO drive of -10dBm. The AD831 are configured to provide a net gain of +10dB for each mixer stage. A 2N5109 post mixer amplifier follows the 2nd mixer to provide additional gain and a solid 50Ω termination via a 6dB pad for the 2nd IF crystal filters that follow.
The VFO and LO use Silicon Devices Si570 I2C programmable crystal oscillator / DSPLLs.
The 2nd IF has selectable 8 pole 2100-, 1800-, 500- and 250-Hz filters ahead of the IF amplifier stages.
The net gain from the antenna input through to the output of the 2nd IF filters is ~32 dB on 160m through 12m, and ~31 dB on 10m (2100- and 1800-Hz filters). Gain compression for all bands starts at an antenna input level of -20 dBm (>S9 + 50dB).
The 9-MHz 2nd IF amplifier is based on the classic hybrid cascode IF amplifier by Wes Hayward W7ZOI and Jeff Damm WA7MLH, “The Hybrid Cascode — A General Purpose IF Amplifier,” QST, pp. 30-33, DEC, 2007. The AGC detector in the original hybrid cascode design is replaced with an Analog Devices AD8307 logarithmic amplifier. An on-board Microchip dsPIC33F microcontroller replaces the remainder of the classic hybrid cascode AGC circuitry, providing digital AGC control and signal level information for the S meter.
I have included a 2100-Hz noise filter following the broadband 9-MHz 2nd IF amplifier. This is optional.
The product detector uses an Mini Circuits TUF-1 diode ring mixer, the BFO uses an Analog Devices AD9835 DDS. The product detector is followed by an audio preamplifier which drives an audio distribution circuit consisting of NE5532 operational amplifiers. The audio distribution circuit provides two audio streams, both audio streams are routed to a Maxim DS1807 addressable dual taper audio potentiometer. Following the DS1807, one audio stream is routed to a LM386 audio amplifier that drives a speaker, the second audio stream provides an auxiliary audio output to drive a computer sound card / interface. The DS1807 allows independent volume control of the two audio streams.
The upper deck contains the Power Distribution Board and the bulk of the digital circuitry; including, three Display and Control Boards, two I2C Relay Control Boards, and the VFO and LO Board.
The bottom deck contains the analog boards:
The RF chain on the right hand side – from chassis back to front, RF Attenuator Board, three Receiver Bandpass Filter Boards, and RF Preamplifier Board.
The IF chain in the front left – from right to left, 1st and 2nd Mixers and Roofing Filter Board, two IF Filter Boards, and the 2nd IF Amplifier and Digital AGC Board.
The Product Detector, BFO and AF Amplifier Board in the rear left of the chassis.
© 2014 – 2020 Rod Gatehouse AD5GH