Minima

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Minima1lores.jpg

The Minima is a simple and easy to operate transceiver for all the HF bands with a crisp receiver, a clean output and component cost of less than $100.

To join the Minima mailing list you can go here: http://www.freelists.org/list/minima

Two prototypes of the Minima have been built. Both work well, and show that the design is easily replicated.

So, what's this Minima anyway?

  • Superhet design. The IF is at 20 MHz.
  • No RF amp before the crystal filter. As a result, receiver has very low distortion and yet, it’s strong enough to deal with the loud signals in today’s band conditions.
  • Easy to use. A single knob tunes the entire HF band. Sideband selection is automatic.
  • One Button. The single F-button that toggles the RIT and swaps the two VFOs.
  • 1 watt output. Add your own power amplifier to take it to your prefered power level.
  • No tune up. Just set the BFO frequencies, notch the low pass filters and you’re done.
  • Easy to build. The component count is not much greater than that of the BITX.
  • Arduino based. The transceiver is controlled with the simple, universally available microcontroller.
  • Hackable. It is easy to reprogram the Minima, change the circuit and try out new things.

Engineering is all about making choices and deciding what to leave out. So, what's not included in the Minima?

  • No AGC. A good AGC to match the Minima will add complexity to the transceiver. Preference was given to undistorted audio from an unencumbered signal chain.
  • 1 Watt output. You’ll want to add a linear to boost the power level.
  • 20 Mhz notch. The transceiver has a deep notch around 20 Mhz. So, the transceiver works everywhere in the HF bands, except at 20 Mhz.

How the Minima works.

Arduino’s code for the Minima is at [github|https://github.com/afarhan/radiono] Here’s the entire circuit:

Minimacktlores.png

The Minima architecture is slightly different from that of a traditional superhet transceiver.

Choice of 20 MHz Intermediate Frequency

For a general coverage receiver, we’ll want to use the highest possible IF so that our image frequency is as away from the receiver passband as is possible. We choose 20 Mhz IF, as it happens to be the highest frequency for which fundamental-mode microprocessor crystals are easily available. The advantage of such a high IF is that the image response of the transceiver is now from 40 to 70 Mhz - safely away from the 0-30 MHz operating range of the Minima.

Front-end filtering

The front-end uses a pair of low pass filters. The first one covers from 0 to 15 MHz and the second covers 15 MHz to 30 MHz. They perform three functions :

  • Operating between 0 and 15 MHz, the local oscillator tunes between 20 and 35 MHz. Though the mixer balance nulls out the local oscillator at the RF port, the first low pass filter with the 15 MHz cut-off is needed for sufficient isolation of the local oscillator from the RF port when operating below 15 MHz.
  • Operating between 15 to 30 MHz, the second low pass filter with 30 MHz cutoff is used. Additionally, the second filter should not allow signals at 20 MHz to pass in either direction. To achieve this, three traps are placed inside the second low pass filter. These are notched to tune out 20 MHz signals.
  • When the transceiver tunes from 0 to 30 Mhz, the the local oscillator tunes from 20 to 50 Mhz, producing the image frequencies from 40 to 70 Mhz. Both the low pass filters suppress these images sufficiently.

The KISS Mixer

The KISS mixer is a little unusual. If you carefully look, the gates of the two JFETs are being driven by the VFO and not the signal! The two transistors Q1 and Q2 are alternatively turned on and off by the VFO signal at their gate. These transistors connect either of T1’s primary ends to the ground (Actually to the common bypass capacitor). So, the RF signal appears chopped up at the local oscillator frequency. This is what leads to switching mixer action. Like most switching mixers, the KISS mixerworks in the reverse direction as well.

This is such a simple design that Chris Trask (N7SWY) has called it the “Keep IT Simple, Stupid!’ configuration in his paper.

The KISS mixer does two amazing things :

  • Unlike a diode mixer, it doesn’t behave too badly without a diplexer or a post-mix amp.
  • Unlike other passive mixers, the KISS mixers losses are quite low.

These features allow us to directly connect the mixer to the crystal filter. This architecture the reception sparklingly clear and it also eliminates any front-end switching when going from receive to transmit and vice versa!

Oscillator

The Minima uses an Si570 chip from Silicon Labs as the local oscillator. Unlike DDS frequency generators, this chip has remarkably clean phase noise and virtually no spurs. It is an 8 pin package that is easily soldered. The prototypes were built without PCBs over a copper clad board with a regular 25 watts iron using a flat tip.

Crystal Filter

The crystal filter at 20 Mhz is an important part of the Minima. We use slightly larger than normal bandwidth for a much better receiver experience. The filter uses two parallel crystals in the end-points. This helps the filter have very low ripple. The crystal filter is cut for 50 ohms termination.

IF amp

During reception, the signals suffer 10 db of losses in the LPF, the mixer and the crystal filter before entering the RX IF amp. It must, therefore, have a very low noise figure to preserve the receiver sensitivity. A common-base amplifier of Q3 with 0.5mA emitter current has low noise and 50 ohms input impedance.

The Q4 and Q5 transform the 470 ohms collector load into a low impedance drive for the modulator. A requirement for the crystal filter to exhibit low ripple in the passband.

The transmit IF amplifier uses a standard feedback amplifier with higher current as it needs to handle the stronger signals from the modulator side. Both these amplifier are derived from Kopski and Hayward’s work on BITX amplifiers.

Crystal oscillator, (de)modulator

The crystal oscillator uses a single crystal in a VXO. An Arduino controlled relay switches either of the two trimmers that pull the carrier frequency for lower or upper side-band.

Diode modulators usually have a carrier null adjustment. This adjustment is often critical and insufficient. The Minima dispenses with the that adjustment by using pre-matched diodes. The diode matching scheme is shown at the left-bottom of the Minima’s circuit diagram. Using that circuit, test out a couple of diodes and select a pair with best matched forward voltage drop across them.

The overall carrier suppression can easily exceed -50 dbc with carefully selected diodes, tightly wound T3 and the steep slope of the crystal filter.

Receiver audio chain

Up to the audio chain, the receiver gain has been carefully kept down just enough to preserve sufficient noise figure up to the detector. As a result, almost all of the Minima’s gain is at audio frequencies. This makes building Minima non-critical.

The audio amplification chain is entirely built with discrete transistors to provide high fidelity and low distortion in keeping with the rest of the receiver’s performance.

Transmission

During voice transmission, the following happens:

  1. The PTT is pressed. It energizes the T/R relay, powering up the mic amplifier and the TX IF amp while powering down the RX IF amp.
    1. The Arduino detects the PTT line going down and switches to the TX frequent (if the RIT is on).
    2. The other pole of the T/R relay mutes the audio by disconnecting the RX audio pre-amp from the audio power amp.

CW is generated by Injecting an audio tone of 700 Hz into the modulator. During the CW transmission the following happens:

  1. The key press is detected by the Arduino, it pulls the PTT line down and switches the T/R relay on. It also shifts the TX frequency if # required (only on RIT)
    1. The T/R relay powers up the TX IF amp, powers down the RX IF amp and shifts the audio amplifier input from the RX audio pre-amp to the tone generator.
      1. The tone generator is keyed by the Arduino to modulate the carrier as well as provide the side-tone to on the audio amplifier.
        1. The Arduino moves back to receive mode after a timeout on the key line.

Audio Power Amplifier

The output RF power has been kept down to 1 watt. This allows it to drive a higher power amplifier of builder's choice. The power chain has extensive feedback in all stages to keep the gain within the range across the HF spectrum. It uses the inexpensive 2N2219s.

Arduino

[Arduino|http://arduino.cc] is a very popular and easy to use microcontroller system. It is based on a specially programmed ATMEGA328P chip and PC based development environment. The PC based environment allows one to program it in simple C language (instead of assembly language). It is downloaded to the chip through a serial port without requiring programmer.

The Arduino handles a number of functions:

  • Controls the Si570’s frequency with the I2C interface and Wire library.
  • Interfaces to the 16x2 LCD display with a 5 wire interface.
  • Tuning with RIT and two VFOs.
  • Automatic switching of low-pass filters, sidebands.
  • Transmit/Receive switching.
  • CW keying.
  • The Arduino's code are called 'sketches'. The Minima sketch is available for download from https://github.com/afarhan/radiono

Building and Operating the Minima

Now, remember :

  • Use the only the CMOS version of Si570.
  • I knew you’d forget it, so I am telling you again, only the CMOS version of the Si570 works.
  • There is no carrier null control.The modulator diodes are matched for equal forward voltage drop.

Minima is very easy to build. Considerable effort of a few months has gone into refining the Minima to be a very easy to duplicate transceiver. The prototypes have been made on copper sheets and unetched PCBs. One could even make it on a perf board!

  • Only three trifilar transformers are used.
  • No front-end tuning required. The 20 Mhz notch is easily tuned by tuning to 20 Mhz and setting the trimmers to minimum noise.
  • Frequency stability is easily achieved with the Si570 and Arduino.

Apart from the Si570, no special parts are required. There are many ways to build a transceiver. We recommend building the separate modules of Minima on small copper clad boards and interconnecting them through shielded cables.

We have built two prototypes. Pictures of both are at the end of this article.

The User Interface of the Minima reflects a personal preference for clean, simple controls. It doesn't have 5000 memories, and variable bandwidth settings etc.

  • To transmit voice, press the PTT.
  • Disconnect the mic and press the key to transmit CW.
  • The Minima uses shuttle tuning. Keep the tuning knob at 12 o’clock position at all times. Slight clockwise rotation starts tuning up the band slowly.
  • Anti-clockwise starts tuning down the band.
  • Rotating the knob to the end position changes the frequency at 100 Khz per step. There is no band switch. Just tune over to any frequency in the 30 Mhz spectrum.
  • Tapping the F-button toggles the RIT control.
  • Minima has two VFOs. Double tap the F-button to swap the VFOs
  • Push and hold the F-button to set both the VFOs to the current frequency.
  • While scanning the band (with the knob away from the center position), pressing the F-button stops the knob. Use this to stop at an interesting signal.
  • The appropriate sideband is automatically selected.

Conclusion

The Minima is one of the simplest transceivers to make. It can be constructed in almost any way. Nothing is very critical. It also lends itself to variations. One can easily add a linrad backend, provide separate band-pass filters for each of the ham bands under Arduino control, etc. You can hack it in various ways.

This is an immensely enjoyable rig. It has a great tuning system that takes a little getting used to. Over the last few months it has been a travel companion, a bench receiver, and the evening ragchewer. In all these ‘modes’ it performs very well.