Intermediate Projects

 

The Brue CW TCVR

This is a builder friendly design with several advantages over earlier versions of direct conversion projects for those wanting to work CW. The parts are now laid out with more space making it much easier to build. It is a single band rig, normally for 80m (see below), which uses a ceramic resonator for the VFO and gives coverage of most of the CW section of that band. It has a direct conversion RX with narrow 750 Hz bandpass audio filtering as well as a humped 750 Hz low pass filter. The output can drive 8R loudspeakers or 'Walkman' type 32R headphones. It has a small PCB front panel with full sized potentiometers for the AFG and Fine/RIT controls, the main PolyVaricon tuning capacitor, a toggle switch for the RIT, and sockets for the LS and key. The Fine tuning control doubles as the receiver incremental tuning which is automatically removed during transmission, or by the RIT switch. The keying operation is semi-break in, with relay change over of the aerial circuits. The rig includes receiver muting during transmission and a sidetone oscillator, with a separate preset for its level in the LS. There are double tuned receiver band pass filters and double half wave transmitter output low pass filter. Transmitter output is 1.5W using a 13.8 volt supply. It can be easily mounted in your own case if you prefer. There are many options for experiment and it can also drive the 10W Linear RF amp. The main PCB is doubled sided about 100 x 100 mm. It includes protection for reversed supplies which should be between 9 and 16 volts.  Price is 44.

For bands other than 80m, up to 20m, you can fit a suitable QRP calling frequency crystal instead of the ceramic resonator but the tuning range will only be a few KHz. 14.06 and 7.03 MHz crystals are 2 each. For experienced constructors, the VFO can be derived from the Mini Mixer kit but this is a complication that is really beyond this simple rig so it is better to consider the Berrow instead.


Brendon Phone TCVR

This is a phone transceiver project aimed at builders with limited experience where ease of assembly is important; the PCB has been laid out with plenty of space and it includes a small front panel.  It is normally supplied for 80m using a ceramic resonator VFO giving coverage of the lower half of the phone section of the band. (Please consult me if considering bands other than 80m - it has the same restrictions as the Brue above!) Tuning is by a PolyVaricon variable capacitor. The double tuned receiver band pass filters feed the single mixer chip, which acts as a product detector for the direct conversion receiver. The audio amplifier has a bandwidth of 3 KHz suitable for voice signals, with an AFG pot and is able to drive a loud speaker. The transmitter uses double sideband modulation, with a suppressed carrier - this is fully compatible with other stations using single sideband! During transmission, the mixer chip is used as a balanced modulator. The speech amplifier, with a mike gain preset, is designed for low impedance dynamic microphones. The robust IRF510 RF output transistor is mounted on a heatsink to make it more tolerant of abuse, and produces 1.5W of RF on a nominal 12 volt supply. The transmitter RF amplifier has its own RF drive control preset to ensure that the output stage does not limit on speech peaks. A double half wave filter removes the unwanted transmitter harmonics. The rig’s TR relay is controlled from the mike’s PTT switch and mutes the RX during transmission. There are many options for experiment and it can also drive the 10W Linear RF amp described in the Accessories section. The main PCB is double sided 100 x 100 mm. It needs an 9 to 16 volt supply. Price is 49.

 


Marsh and Mells

The Marsh is a 3 band direct conversion receiver for phone or CW, normally for 20, 30 and 40m. It uses a crystal mixing scheme for the local oscillator, to cover any 3 bands of the group 20 - 80m, and freedom form chirp when driving a transmitter! The band switch uses diodes to select the appropriate crystal in a digital oscillator whose output is mixed with the 2 MHz VFO to provide excellent stability; the VFO has main tuning by a PolyVaricon with Fine or Incremental tuning using a pot and varactor diode. This scheme provides the same incremental tuning for all bands. The wanted LO mix is selected by double tuned LO filters using a combination of TOKO coils and trimmers under relay control from the band switch. After squaring up in a digital gate, the LO signal drives a strong switching product detector using 74HC4066 switches. The detector feeds into a conventional audio chain comprising a grounded base audio pre-amp,  main amp and switch selected CW filter in a dual low noise op-amp, followed by a LM380-8 output stage for phones or LS. Ahead of the product detector there is a MOSFET RF amp which much reduces unwanted radiation of the LO signal during reception. This RF amp can be turned off for lower gain by a front panel switch, or turned off completely when transmitting to protect the RX. The strong detector, when coupled with a decent AMU, means that the complications of switching a set of RF band filters can be avoided! The normal kit covers 20, 30 and 40m but 80m is available on request, the cost is 60.

The local oscillator of the Marsh can also drive the Mells 3 band CW transmitter giving it full band frequency freedom! With this arrangement, the Marsh's Fine control becomes an RIT control for reception on either sideband of the other station with the offset being automatically removed when you press the key. You do not need any band crystals for the transmitter as it is totally driven by the RX. While single band operation is possible like this, it will usually be three band so you need the Mells optional extra two band kit. With the control of the RX's RF amp as above, together the two kits provide a pretty useful full break in three band transceiver. This is what the Marsh and Mells combination looks like. The price is discounted to 90 for the a three band Marsh and Mells combination!

 


I am working on new simple superhet RX called the Rode for any single band band 20, 40 or 80m, which is planned to work with the matching simple phone SSB Rudge transmitter. This transmitter might also be able to do stand alone DSB phone for use with the Yeo etc.- watch this space!


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