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RFLEGO Schottky Diode Ring Mixer#

YooFab Diode Ring Mixer M0OOZ - YooFab / CC BY NC ND

Main characteristics#

The RFLEGO diode ring mixer, once correctly built, will provide a highly sensitive three port frequency mixer module that operates across all HF bands.

  • This module does not need power connectors, as it is a passive circuit.

  • The circuit is a hybrid mixer design which owes some characteristics to a single balanced mixer while other characteristics are derived from a typical double balanced mixer topology.

  • The circuit functions as a double balanced mixer.

  • The style of PCB supplied is popular with RF design engineers.

Component List#

bom M0OOZ - YooFab / CC BY NC ND

The color / style of some hardware components like pins or connectors may vary.

Warning: on the picture, the PCB hasn't been etched yet.

Name Value Quantity Notes
C1 C2 10pF 2 disk ceramic capacitor
C3, C5, C6 100nF 3 disk ceramic capacitor
C4 10nF 1 disk ceramic capacitor
D1, D2, D3, D4 BAT42 4 Schottky diodes
4T:10T Impedance Transformer FT37-43 1
TRIMMER Potentiometer 200Ω 1 POT
Copper wire 33SWG 40cm in length
Printed Circuit Board 1 Vinyl protected copper
35 mm stand-off 4 Plástico negro
M3 pernos 4 Black plastic
RCA (Phono) connector 3 Gold Plated
Sticker 1 To put on the PCB
Polystyrene piece
1 To organize your components

Circuit Diagram#

circuit diagram of the YooFab Diode Ring Mixer M0OOZ - YooFab / CC BY NC ND

Circuit Operation#

Help provided by Onno

Onno - PA2OHH kindly carried-out extensive experimentation while barefoot during the ice-cold winter. His quest was to find the best ratio for the mixer-transformer. His endeavour included: 1) swapping round the transformer and 2) making a 1:1 ratio. The conclusion of this exercise was that on receive either swapping round the transformer or making a 1:1 version, marginally increases the base band output. However, when using this mixer to transmit DSB-RC, the present design is better, as otherwise a feeble modulated output is obtained. He has also suggested different values for some of the coupling capacitors, which results in a far better output on receive. His wisdom and modifications are gracefully accepted and have now been incorporated into the RFLEGO mixer kit.

This circuit takes the shape of a ring of Schottky diodes; hence the 'name' Diode Ring Mixer. The circuit is drawn similarly to a bridge rectifier, except that instead of the diodes being alternately connected cathode to cathode then anode to anode, the diodes are all connected anode to cathode.

In this design, the Local Oscillator (LO) is fed into a 200 Ω (200R) Potentiometer (POT). This is a preset type POT as it only needs setting once. The LO signal, which after passing through the 100nF capacitor, alternates between positive and negative current. This causes one pair of diodes to conduct, while the other pair are reverse-biased.

The two ends of the POT are decoupled with 10pF capacitors and the upper and lower current paths are connected to the 4 turn end of the impedance transformer. Therefore, the diodes see a DC path on both the upper and lower sides of the diode ring, which is fundamental to this circuit's operation. Note that the LO port is not intended to be bidirectional.

The Audio in/ out port is the junction of the 100nF capacitor and the 10nF capacitor that goes to ground from the diode ring. When used within a receiver configuration, audio appears at this port, which may be fed into the YooFab headphone amplifier or other sensitive amplifier.

Three Ports definition#

  1. Local oscillator: the signal level of the LO may be anywhere between 447mV (p-p) and 1.736 V (p-p). When used for radio reception, the voltage level at the LO port does provide scope for adjustment in the reception of weak and strong signals. A good starting place is 1V (p-p). Note that this mixer is very robust and will not be damaged when fed with anything up to 20 Volts! Having said this, it will not function correctly at higher voltages; indeed the mixer functions best at the lowest voltage that will assure the diode are in commutation.

  2. RF in/ out (bidirectional): The blue coloured glass Schottky diodes used, permit this mixer to detect low signals when used as a receiver. It is possible to connect an antenna directly to the RF in/ out port. Using an audio amplifier such as the YooFab Headphone amp, SSB, Morse and AM may be heard at the Audio in/ out port.

  3. Audio in/ out (bidirectional): This port of the mixer allows input or output of audio frequency waves. When used in a receiver, this port will output Audio from SSB CW or AM signals appearing at the RF in/ out port. When Audio is injected into this port, together with a LO frequency at the Local Oscillator port, then the RF in/ out port produces a Double Sideband Reduced carrier (DSB-RC). Through this port, it is possible to listen to SSB, Morse and AM using an audio amplifier like the YooFab ™ headphone amplifier.

The Local Oscillator (LO) peak to peak (p-p) voltage range needed by the RFLEGO diode ring mixer.

Volts LO pp M0OOZ - YooFab / CC BY NC ND

Signals at the RF port#

Here we look at signals that may be produced at the RF output port of this mixer.

In both pictures below, the Orange coloured trace is the signal at the RF in/ out port, whereas the Green coloured trace is the signal at the Audio in/ out port.

Both the Audio and the RF ports are bidirectional, such that signals may be produced or injected in or out of these ports.

The DSB-RC waveform#

  • The output has a characteristic slanted modulated waveform.

  • The waveform shows that both the negative and positive going parts of the Audio input appear as separate modulation envelopes.

Double Side band Reduced Carrier DSB-RC M0OOZ - YooFab / CC BY NC ND

The AM waveform#

  • This mixer may also be used to produce a simple amplitude modulation (AM) waveform. The picture below shows this.
  • It is achieved by unbalancing the mixer such that the Potentiometer is moved toward one or other extreme of its travel.
  • Compare the above DSB-RC waveform with the picture below, where we see that negative going Audio produces a 'narrowing' of the amplitude, whereas positive going Audio produces a 'widening' of the amplitude.
  • Note that the use of a diode ring mixer, is certainly not the most elegant nor the least problematic method of Amplitude Modulation. (AM)

The diode rigng Mixer configured to produce AM M0OOZ - YooFab / CC BY NC ND

Instructions and more#

Click HERE for assembly instructions in SPANISH.

Click HERE for assembly instructions in ENGLISH.

Here you can follow the construction sequence:

M0OOZ of YooFab / CC BY-SA


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