2020 Transceiver

January 7, 2023 Update, 2020 Transceiver Version 2.

In early 2022 I came across the Sotabeams DSP audio filter modules and decided to incorporate one of these modules to replace the Rx and Tx audio shaping circuitry in the 2020 Transceiver. I was never really happy with the simple audio filtering for obvious reasons on receive, but also on transmit the variation in passband response meant I was always adjusting transmit drive to keep constant output level for digital modes when moving transmit frequency within the audio passband. These filters provide an extremely flat response between 300 – 2700-Hz, with brick-wall roll off to -70dB on either side.

To make room for the DSP filter module on the SSB Generation, SSB Detection, and Audio Board, I removed the Tx side band / Rx noise crystal filter. Tx side band filtering function was moved to the crystal filter on the 9-MHz IF and Digital AGC Board, and Rx noise filter function following the IF stage was removed.

Relay switching was included on the SSB Generation, SSB Detection, and Audio Board to switch the DSP filter module between the receive and transmit paths.

I also wanted to incorporate GPS correction of the two Si5351s in the transceiver. Accommodation for the GPS receiver and ATmega328 (Adafruit Metro Mini) was already made on the 1st Mixer, Roofing Filter, 2nd Mixer, Post Mixer Amplifier, VFO, and LO Board to correct the Si5351 VFO/LO, however, I still needed to add an ATmega328 to the SSB Generation, SSB Detection, and Audio Board to correct the Si5351 BFO.

The 1pps signal from the GPS receiver on the 1st Mixer, Roofing Filter, 2nd Mixer, Post Mixer Amplifier, VFO, and LO Board used for frequency correction of the Si5351s was brought out to a spare pin on the backplane, making the 1pps signal available for the ATmega328 on the SSB Generation, SSB Detection, and Audio Board for correction of the Si5351 BFO frequency. The technique used to GPS correct the Si5351 frequency is based on work by Gene Markus, W3PM/GM4YRE, An Arduino Controlled GPS Corrected VFO, QEX July/August 2015, pages 3-7.

The ATmega328 on the SSB Generation, SSB Detection, and Audio Board was also used to implement control functions on the DSP filter module, via the transceiver I2C Bus. The DSP filter module has selectable bandwidth for CW or SSB, and selectable gain either x1 or x4; in the transceiver, x1 gain is selected during receive and x4 gain during transmit. Both CW/SSB bandwidth control and x1/x4 gain control functions are accessible via the front panel Function dial.

2020 Transceiver Version 2 block diagram.

These changes required a redesign of the SSB Generation, SSB Detection, and Audio Board, and the 9-MHz IF and Digital AGC Board. Follow the links below for these updates.

SSB Generation, SSB Detection, and Audio Board.

9-MHz IF and Digital AGC Board.

Original Post December 2021.

In the fall of 2020 I decided to learn KiCad. At the same time I was toying with the idea of building a transceiver based on the Express Receiver and Transmitter. The result is the 2020 Transceiver, shown below on the left in receive mode and on the right in transmit mode. The transceiver was first put on the air on October 30, 2021, and has been my main station rig since.

The 2020 Transceiver is an up-converting superheterodyne that covers the 160, 80, 40, 30, 20, 17, 15, 12 & 10M amateur bands. First IF is 69.450-MHz and 2nd IF 9-MHz. Transmit power output is 10W on all bands. Intended for digital modes, the 2020 transceiver does not have provision for a microphone or CW key. A DB25 connector on the rear panel provides connectivity to a Timewave Navigator for audio, PTT, and serial communications with a computer.

The 2020 Transceiver block diagram.

Starting with the Rx path through the transceiver:

• RF from the antenna is routed to relays that perform antenna TR switching and allow selection of a separate Rx antenna connection with an external T/R relay. I use an external T/R relay and Rx antenna connection on the transceiver so the Rx signal bypasses the external power amplifier and power meter.
• The Rx signal is then routed to a 0 – 30dB attenuator with 2-dB steps that is followed by a Mini-Circuits 30-MHZ Low Pass Filter (LPF). The LPF provides additional image rejection for the 1st Mixer which has a 69.450-MHz IF. An RF Pre-amplifier is not currently implemented in the 2020 Transceiver, although there is room for one to be added on the TX PA Driver, RF Attenuator & LPF Board.
• Nine selectable RF Band Pass Filters (BPF) follow the 30-MHz LPF, one for each band providing HF selectivity and additional image rejection ahead of the 1st Mixer.
• The 1st Mixer up converts the RF signals to a 1st IF frequency of 69.450-MHz. A 6-pole, 69.450-MHz Roofing Filter provides 4-KHz selectivity ahead of the 2nd Mixer. The 2nd Mixer down converts the 69.450-MHz 1st IF to the 2nd IF at 9-MHz. The 1st and 2nd Mixers are Analog Devices AD831 monolithic mixers configured to provide a net gain +10dB for each mixer stage.
• A 2N5109 Post Mixer Amplifier follows the 2nd Mixer to provide additional gain and a 6dB pad to provide a broadband 50 Ohm termination for the 2nd IF 9-MHz crystal filter that follows.
• The 1st and 2nd Local Oscillators use a Silicon Devices Si5351a Clock Generator.
• The 9-MHz IF has a single 8-pole 2100-Hz crystal filter ahead of the IF amplifier stages. As the 2020 Transceiver is designed for digital modes, selectable narrower crystal filters have not been included.
• The 9-MHz 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.
• A second 8-pole 2100-Hz crystal filter follows the 9-MHz IF Amplifier stages that functions as a Noise Filter ahead of the Quadrature Sampling Detector. This crystal filter functions primarily as the Sideband Filter in the Tx path.
• The Rx uses a Quadrature Sampling Detector (QSD) based on the QRP Labs Receiver Module. The QSD was selected for its very high dynamic range which is desirable in general, but more so with digital modes such as FT8 where interfering FT8 signals are within the Rx passband and must be handled without overloading the detector stage.
• The QSD uses dual 1:4 multiplexers to generate the 0, 90, 180, and 270 degree phase shifted audio signals. LM4562 High performance audio operational amplifiers are configured as instrumentation amplifiers to combine the four phased-shifted audio signals into IQ audio signals.
• A Si5351a Clock Generator provides the 36-MHz signal to a 74HC74 Dual D-type flip-flop which generates quadrature 9-MHz BFO signals for the QSD dual 1:4 multiplexers.
• As USB selection is done ahead of the QSD using a crystal filter, the IQ audio from the QSD can be used separately to provide Line Out audio to the Timewave Navigator and audio for an external speaker.
• Both IQ audio streams pass through a Maxim DS1807 I2C addressable dual audio potentiometer for speaker audio and Line Out level control.
• Following the DS1807, speaker audio is routed to LM4871 3W Audio Power Amplifier, and Line Out audio is routed to an Audio Isolation Transformer.

The Tx path through the transceiver is as follows:

• Line In from the Timewave Navigator is routed to an Audio Isolation Transformer followed by LM4562 audio pre-amplifier and 12-KHz Low Pass Filter stages.
• A TUF-1 Diode Ring Mixer is used in the Tx path for generation of a 9-MHz Double Sideband (DSB) signal.
• A Si5351a Clock Generator provides the 9-MHz Carrier Oscillator signal to a 2N3904 amplifier which provides +7dBm drive to the TUF-1 local oscillator port.
• The 9-MHz DSB signal is routed to the Sideband Crystal Filter which removes the Lower Sideband (LSB) leaving a 9-MHz Upper Sideband (USB) signal. The Sideband Crystal Filter doubles as a 2nd IF Noise Filter in the Rx path.
• The 9-MHz SSB signal is routed to the 1st Mixer where it is converted to the 1st IF frequency of 69.450-MHz, routed through the 1st IF Roofing Filter, and then converted to one of nine HF band frequencies by the 2nd Mixer. To facilitate the different operation of the 1st and 2nd Mixers in Tx mode, the LO and VFO frequencies are swapped between the 1st and 2nd Mixers compared to Rx mode. Swapping of LO and VFO frequencies between the two mixer stages is achieved by reprogramming the respective Si5351a clocks whenever a T/R changeover occurs.
• The 2N5109 Post Mixer Amplifier provides ~20dB gain and drives a 6 dB pad to provide a broadband 50 Ohm termination for the HF BPFs that follow.
• The HP BPFs attenuate unwanted mixer products ahead of the Tx Power Amplifier (PA) Driver stage.
• The Tx PA Driver stage consists of an Analog Devices HMC311ST89 15dB gain block followed by a 2N5109 broadband amplifier providing 50 Ohm input and output terminations with +12dB gain, delivering approximately 22dBm/160mW drive to the PA stage.
• The PA stage consists of a Pennywhistle power amplifier purchased as a kit from TAPR. Although it is no longer available as a kit, it would be fairly straightforward to reproduce.
• Following the PA stage there are six selectable Tx Low Pass Filters followed by antenna connection and T/R relays.

All display, control, and serial communications functions in the 2020 Transceiver are handled by an Arduino Mega. All RF switching is done using relays.

The 2020 Transceiver is built into the chassis of a HP436A Power Meter. Enclosure, Front & Rear Panels

The AF/RF Gain rotary encoder controls all variable gain functions in the transceiver, including, AF gain, RF Attenuator, IF Gain, and Line Out level. The Function rotary encoder provides access to all other functions, including, band selection, AGC, mode, tuning rate, functional presets, and calibration. The Tuning knob is in the middle, with red LED “On Air” indicator and power switch to the right.

The Rear Panel is shown below with the Pennywhistle PA heatsink on the left.

To achieve the level of integration required, and to allow for testing, experimentation, and maintenance, a backplane approach was taken that allows for seven 3.5″ high by 7.0″ wide PCBs as shown below.

The seven PCBs, from right to left in the above photos breakdown the transceiver functions as follows:

• Arduino & I2C Relay Control Board
• SSB Generation, SSB Detection, and Audio Board
• 9-MHz IF and Digital AGC Board
• 1st Mixer, Roofing Filter, 2nd Mixer, Post Mixer Amplifier, VFO, and LO Board
• RF Band Pass Filter Board
• Transmitter PA Driver and Receiver RF Attenuator Board
• Transmitter Low Pass Filter and Antenna Switching Board

In addition to these seven PCBs, the following PCBs were built:

• Backplane Board
• Front Panel Board

The 10W PA is a TAPR Pennywhistle amplifier that mounts to the Rear Panel as shown below on the left. The view on the right is looking behind the Front Panel.

© 2021 Rod Gatehouse AD5GH