• Noisy Regen Test

    agp.cooper01/13/2017 at 06:30 0 comments

    Noisy Regen Test

    Well I assembled and tested the noisy regen today:

    Performance wise with the short antenna it was a little better than the first one. With a long antenna not as good. Selectivity was better (likely due to the higher input impedance as a result of lower collector currents).

    Exciting? No. The reflex circuit was much better (but then I used a proper coil). Would I recommend it? No. Is it all the coil problem? No, I don't think so. To get an idea about how bad this receiver is, without the high gain audio amplifier you would not hear anything. I suspect a Ge diode followed by the same audio amplifier would work better!

    Here is the layout:

    Regards AlanX

  • Noisy Regen Simulation

    agp.cooper01/08/2017 at 02:27 0 comments

    Noisy Regen Simulation

    Alan Yates (http://www.vk2zay.net/article/128) has stripped down the Franklin Audion circuit to its basics and taken the audio off the emitters rather than the tuned circuit. This seems to make more sense to me:

    (Note that the 47n output filter capacitor should be 4.7n!)

    So I stripped my circuit down and was able to get the simulation to honour the expected frequency locking and demodulation:

    The signals (in clockwise order) show the receiver:
    1. on frequency and in lock
    2. off frequency but still in lock
    3. off frequency and out off lock (i.e. beating).

    The green is the input signal, the red is the oscillator and the yellow the output audio.

    Note how the oscillation can be quenched in case 2.


    Now that I have a working simulation I can test component values. Or in this case the component value the 100k emitter resistor. You may have noted I increased the value from 10k R to 100k R in this version. The results indicate that the same operating point can be found with the 10k resistor as the 100k resistor. The main difference is that the oscillation amplitude can be much higher (the full available collector voltage or about 0.7v pp) and consequently the available mixer gain (i.e. higher audio output). This is reflected in actual use. The oscillator frequency also shifts with higher oscillation amplitudes.

    Using the higher (i.e. 100k R) emitter resistance will tame the receiver. Audio gain is best found elsewhere.


    Added the schematic to help show how the oscillator works:

    I have used NPN transistors (as this is what most people are used to) and split the emitter resistor to show the two transistor amplifiers.


    Here is the updates strip-board design:

    If you are interested, the input impedance of the Franklin oscillator in this circuit is about 1.2k (approximately that of the coil impedance tap) and the output is about 10k (approximately the same as the low pass filter).


    Lumped Inductors - How Good?

    Although I have had success with the lumped inductors, I know the Q is quite low.

    But how good or bad are they? That depends on the wire diameter and core material.

    An Internet search suggests that they roughly match the Vishay IRF-24:

    • L = 150uH +/-10%
    • R(dc) = 4.2 vs 4.9 (Vishay)
    • I(dc)= 175 mA vs 150 mA (Vishay)
    • Q = 60 (Vishay)
    • Self resonance = 4.75 MHz

    Now a Q of 60 is far less than 300 to 500 you can get with an air coil.

    For MW (i.e. 1 MHz) a Q of 60 translates to an unloaded bandwidth of 17 kHz.

    Which is why they work in this application although not great when the coil is loaded.


    The alternates are:

    • Air coil - conventional (very good but not suited to a PCB).
    • Air coil - spider (very good but not suited to a PCB).
    • PCB coil - Low Q (~40-70).
    • Ferrite beads (mix 43) - small but low Q (~8!).
    • Iron powder toroid (mix 1 or 2) - high Q but bulky for 200uH.
    • Ferrite toroids (mix 61) - medium Q.
    • Ferrite rods (mix 61) - medium Q but large.

    I have designed and built MW radios using the first four options.

    Using the data from an Amidon datasheet for their mix 61, the maximum Q is about 270 (between 1 MHz and 2 MHz). This is the best ferrite material for MW frequencies.

    Options not tried:

    1. I have a T-200-2 mix 2 iron powder toroid that with 144 turns of 22 or 24 AWG would work well (Q>>200?).
    2. A FT-50-61 with 50 turns of 26 or 28 AWG should work okay (250 uH) and pretty small.
    3. A ferrite rod cut to fit the board width (say 2") and 60 turns of 22 AWG (0.63 mm diameter) wire (4 cm long) would be the same. I have a 15 cm long by 1 cm diameter mix 61 ferrite. The ferrite rod can cut (not sure how yet) and attached to the board with plastic screw-down cable clamps.

    Option 2 seems to be the best for a PCB mount but the toroids are a few weeks away (just orded them on ebay).


  • Test Fire

    agp.cooper01/07/2017 at 14:45 0 comments

    Test Fire

    Using a 4 foot wire aerial the radio worked but not very loud. Regeneration was smooth and sound quality was good. The circuit Q is pretty poor and regeneration only improves the Q to a limited extent. I was not that impressed! Here it is:

    Hooked up a 14 m aerial (no earth) and the volume was quite loud. Clearly, the "detector" is not that sensitive (need a strong signal to perform). There is a design which has AGC, now I know why:

    I was thinking it may be better to take the audio off the emitters rather than from the coil tap. I found an example at http://www.vk2zay.net/article/128:

    Worth a try.


  • Lump Inductors

    agp.cooper01/07/2017 at 05:36 0 comments

    Lumped Inductors

    Often radio circuits have coil taps and/or coil couplings. These be simplified as lumped inductors.

    For example a 50% tapped 200 uH inductor is the same as two 100 uH inductors in series with regard to LC oscillation frequency and impedance transformation. The main down side with lumped inductors is that for similar coil construction the total resistance will be higher, that is the Q will be lower (i.e. a 200 uH inductor needs only 41% more turns than a 100 uH inductor).

    So my tapped 320 uH coils consists of a 100 uH and 220 uH inductors in series. The simulation suggests a frequency range (using a BB212 varicap diode not the 1SV149 diode) of 1.27 MHz (for an 8 volt control voltage) and 564 kHz (for 1 volt control voltage. Note the series resonance response and "roll-up"!:

    This is half way between the 25% and 50% taps shown the the schematic:

    Modelling a coupled coil has no series resonance response and a normal "roll-down":

    Changing the coupling produces a normal response:

    History Repeats

    I was rummaging through my junk box looking for some inductors and came accross an old super-regenative tube radio I build in the mid-nineties. I used the lumped inductors then and this receiver worked okay:

    Here is the schematic:


  • The Varicap Diode

    agp.cooper01/07/2017 at 03:44 0 comments

    The Varicap Diode

    I have not used one before but my reading suggests that they work pretty well as a "single" in receiver projects (refer to "B") but for (high powered) oscillators a "twin serial" set up (refer to "C") is recommended:

    Image result for varicap diode circuit

    (source: www.jestineyong.com/wp-content/uploads/2014/09/varactor-diode-checking.jpg)

    In each case value of "R" is not critical but 1M ohm is common. Usually a capacitor at "Vc" is added to minimise unwanted RF in the control voltage (Vc) circuity as shown in the following:

    Image result for varicap diode circuit

    (source: www.oddmix.com/tech/px/cr_varactor_crystalset_f1.gif)

    The 1SV149 (=1/2 BB212) Varicap

    I purchased a couple of these some time ago off ebay. There is the datasheet (http://www.qsl.net/df7tv/datasheets/1SV149.pdf). At 1 volt the capacitance is about 485 pf and at 8 volts the capacitance is about 25 pf.

    Using a 320uH inductor and a 1000 pf series capacitor the tuning range should be about 500 kHz to 1.8 MHz.

    Regards AlanX

  • The Franklin Oscillator

    agp.cooper01/07/2017 at 03:22 0 comments

    The Franklin Oscillator

    The Franklin oscillator is a little strange and not immediately obvious how it works:

    The following helps a lot:

    (source: www.radiosparks.com/images_d/Franklin Oscillator.png)

    So positive feedback at parallel resonance (0 degrees phase shift).

    The next image is a transistor version:

    Image result for franklin oscillator

    (Source: www.uploadarchief.net/files/download/franklinoscillator.gif)

    Here is a rationalised (long tail pair) FET version:

    Image result for franklin oscillator

    (source: www.uploadarchief.net/files/download/osci.png)

    Still it takes a leap of faith to get to here:

    Image result for franklin oscillator

    (Source: i178.photobucket.com/albums/w269/notxyl97/DL7MWNQ-Multiplier.png)

    But at low voltages most of the biasing components can go!