D-trigger as heterodine mixer: aliasing

Posted: 3/21/2017 4:15:25 AM
Buggins

From: Theremin Motherland

Joined: 3/16/2017

Hello,

 

I'm trying to find out suitable osciller and heterodine schematics for digital theremin.

Yesterday I was experimenting with d-trigger in mixer.

Setup:

  • Variable pitch oscillator on unbuffered hex invertor (mostly like in Open.Theremin V3) with separate voltage regulator for power supply. Frequency ~ 500kHz.
  • 10mm copper tube as antenna.
  • Reference pitch - generated by MCU - as 60Mhz / K.
  • Mixer: D-trigger, powered with separate voltage regulator (just in case).
  • Mixer output frequency - somewhere in range 1kHz..10Khz
  • Measurement of frequency - using Teensy 3.6 (ARM based MCU board) - counting number of 60MHz pulses during measured signal interval.

Results were strange. Measured values were jumping in strange way. Some values appeared often, others - never appeared.

Making histogram INTERVAL / COUNT - measured number of 60MHz pulses / count of measurements gives results like


interval count
14879    1
14880    8
14881    25
14999    20
15000    79
15001    2
15119    6
Best: 15000    79
interval count
15001    2
15119    3
15120    102
15121    17
15239    15
Best: 15120    102

There are peaks near some value N and N+1 or N-1 e.g. 15120, 15119 and 15121 (this is expected). But as well there are side peaks - e.g. 15001, 15239. I've modified my program to show new found peaks in histogram while hand is approaching to antenna.

 

Freq T T(i)-T(i-1)
1605,73 37366 119
1610,86 37247 119
1616,03 37128 119
1621,22 37009 119
1626,45 36890 119
1631,72 36771 119
1637,01 36652 119
1642,35 36533 119
1647,71 36414 119
1653,12 36295 119
1658,55 36176 119
1664,03 36057 119
1669,54 35938 119
1675,08 35819 119

Peaks are distributed equally in distance 119. But what is 119? 60000000/119=504201 - very similar to reference (fixed pitch) clock.

So with D-trigger as a mixer, there is alias - dependence from reference clock frequency. Aliased positions are located in 1-3cm intervals when hand is near antenna. Although it's possible to do some averaging to get measure distributed between aliasing peaks, but it looks ugly.

Why people ever using D-triggers for heterodine mixer? As I see, OpenTheremin.V3 and UNO are using D trigger. OpenTheremin.RC uses XOR mixer.

What is best mixer for digital theremins?

Next time I'll try XOR with 3 stage lowpass filter...

 

Best regards,

     Vadim

 

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