Frequency Demodulation  assisted by Synchronous oscillator 

 

                                    

 

 

Circuit of Synchronous Oscillator  with an FM demodulator 

I have been meaning to try this  for quite some time ,The  idea is to use the S.O  as the " amplifier "   for FM modulated signals and to demodulate the Oscillator  output to audio .  I Built up the  circuit above   but added a quadrature demodulator based on a Gilbert cell mixer  ie NE612 . The quad demod unit was driven from the S.O output . I had a simple circuit I drew up as  above  and etched up a double sided Printed Circuit and assembled it , it took me some time to get it to perform properly as I found i had made an error in the  circuit diagram.  I discovered  more to  my good  fortune, more  than design, an improved  bias configuration that relates to the RF  osc and the lower switching  transistor  . II managed to get the circuit performing very well  ie lock at -100 dbm  , When I measured the resistors id used I found I had a ridiculously low value   (317 ohms ) biasing the RF  and an equally high  value ( 420K ohms) biasing the lower switching transistor .( cant read metal oxide resistors!)  But it worked  very well ! I connected the input of the  S.O to the   N9310A Signal generator set to 455 Khz and I took a 10 pf capacitive tap from the output link coupling that feeds the FM Quad  demod  circuit ,  and fed that to the input of the ESA4407  spectrum analyser set   at 455 Khz and 200 Khz  span . The  Audio output of the quad  demodulator was plugged into some amplified computer speakers so I could monitor the input  signal . I set the Signal generator to 2.5 KHz deviation with an audio tone of 1.5 Khz

I measured the voltages around the circuit with a 220 Kohm resistor in series with the positive  probe  of the DVM to minimise the loading effect and provide some RF  isolation .( this gives a slightly lower  real voltage that normal my real rail voltage was 12.04 v .
The Original S.O circuit was  based on a  capacitive  tap on the tuned circuit  for feedback . I used a 455 Khz interstage transformer  and I decided I would use the RF out of the centre tap of the tuned  circuit  to provide a feedback connection in the OSC  circuit  , Since I was having problems getting the circuit to function I rehashed the tap system to a capacitive version in case that was responsible  for the malfunction ????  ( still not sure if it actually was)  ,  I took a similar 455 IF  can to bits removed the  capacitor  and measured it , approx 125 Pf  so that means I require  two 250 Pf capacitors in series, for a start ,  to be equivalent of this capacitance .

When the circuit was functioning I took some measurements of the performance . I fed  signals at various levels into the synchronous oscillator  and measured the S.O   RF  output  on the spectrum analyser .  Here are the results :

  S.O out Level in dBm (modulated)                 Sig Gen level  dBm                   lock  range Khz ( no mod  )                       S.O out RF Level in dBm

- 50.7                                                                    -100                        454.2---456.5                                                        -48.9
-48.06                                                                   -90                           451.5--459.6                                                         -42.84
-48.01                                                                     -80                        443.3---469.2                                                        -42.79
--47.92                                                                    -70                        420.8---506.2                                                         -42.72
-47.72                                                                    -60                        380.2---606.2                                                        -42.49
-47.12                                                                    -50                        321.0---541.0                                                        -41.88
-43.23                                                                    -40                        no wider but inter mod seen                                 -38.05
-40.24                                                                    -30                                    ---------                                                        -35.09
-35.65                                                                    -20                                    ---------                                                        -30.56
-33.13                                                                    -10                                    ------------                                                    -28.02

THE ABOVE WAS MEASURED   WITH A 10 pF TAPPING  CAPACITOR TO THE LINK  COUPLING OUTPUT the dBm levels are NOT absolute but just a reading .

 

Lock range was achieved by sweeping the signal generator to extremes where the spectrum analyser showed   loss of  lock  and  the single carrier  broke up and noise was heard from the  FM demodulator .

 Note the S.O  RF output level at -100  dBm excitation ,     The amplification of the SO is best seen and heard with  an stimulus FM signal , as if you use  just straight carrier ,all you will see is the S.O  carrier lock solidly to the exciting carrier .  Hence I placed the FM Demodulator after it . It can be seen that the RF output of the  S.O increases with the increase in excitation from the signal generator , this could be the summation of the  S.O RF  output and the stimulating  input signal ,  or it could be the wider bandwidth low level FM sidebands being amplified as well . 
 

The RF  bias resistor was changed to 19 K ohms  and the modulated RF  out put was measured    from -80 dBm up to -10 dbm    input from the signal generator at 455 Khz  2.5 Khz deviation with 1.5 Khz modulation tone  
  -80 dBm    gave  -24.12 dBm  out  and linearly to -25.6 dBm  S.O. output at -10dBm  stimulating signal  input . 70 db range and approximately the same RF output level , I have yet to measure the more important  parameter  signal to noise ratio at -80 dBm .
The measured RF  output of the Synchronous Oscillator  is only an indication, not a true power measurement  .  The IF can in the Oscillator was a 500 ohm output impedance one and the connection to the link coupling of 10 PF  was just to monitor the changes  while varying the input stimulus .  this providing a large mismatch .
 The S.O seems to be able to provide amplification particularly  with frequency modulated signals . I have yet to try it with real live 455 Khz NBFM  signals to hear  what the demodulated audio sounds like on weak signals  .I am keen to see if the claim to digging signals from the noise  has any truth   .. will it improve readibility ??

The Tuned circuit in the S.O used a split capacitor  network with the feedback tap of the junction of  the two 180 pf  caps . I actually pulled a 455 Khz if can to pieces and extracted it with care ,, measured the capacitance in the  agilent hand held capacitance meter  and it was  approx 120 Pf  ,  if you try this  circuit at 455Khz  just crush the little dogbone capacitor under the tuned circuit with a sharp tool (as they are so fiddly to remove with out breaking the fine winding wires on the transformer )  .I did actually try  connecting the feedback to the centre tap of the 455 IF can to see if the S. O would still work as well and  for some reason  not .  I also tried the centre tap as the  decoupled  plus rail connection but i couldnt get  S.O action  perhaps in these cases if I played with the bias ??? 

 

                                   

Deviation synchronisation observation

Circuit has been  modified again to see how I can screw as much performance as possible from it . The RF  Feedback capacitor was increased to 1800pf , the V+ had a 1 K ohm resistor between osc and quad  section ,  1k bias resistor to RF transistor  and a variable bias  divider to feed bias to the lower  injection transistor ..

  

Well  have done some more experiments with this circuit , and its hard to say if it does any better to enhance detection so far . Comparing the Aor2001 scanner demodulating received signals , I took a 455 Khz tap off the low level IF  before demodulation to effect  a comparison  . but ill have to use the sig/noise meter to quantify the results .
However I did evaluate the Synch L.O as a simple FM detector IF as its only two transistors . I fed a signal into the circuit and looked at what can out of the Synch Osc.


 
                      Output of signal generator   feeding Synch  Osc                                             output of  Synch osc  feeding  quad demodulator  and Spec an                    

  

                -40 dBm input   to Synch Osc                                                                                                                                       -10dBm  input

The Signal generator was set to 2.5 Khz deviation  , with a modulation frequency of 1035 Hz . at 455 Khz , This showed the classic FM carrier null  which I could use to see if there was any distortion  of  the locked frequency produced by the  Synch Osc  recovery, over a range  from -83 to -10 dBm and by listening to the tone from the quad output ,  . There was no audible distortion or changes in its timbre , so it seemed to be consistant at most input levels  . I listened to the a faithful copy of the input  ,  the only aspect I'm  not sure of the amount of signal feed through to the quadrature demodulator from the stimulating source at  input high input levels as there is some "amplification "  occurring   note there is  an output level change of -24 dBm output    through to -17 dBm dBm output  from  -90 dBm in to -10 dBm in  .  it is well  into limiting at -10dBm input ..there must be some feedthrough ???

Recovered Audio Observation

with the oscilloscope I measured the deviation sensitivity  ,which is really a function of the quadrature detector  , There is no loading resistor across the 455 Khz IF  interstage coil so it recovers good audio on low deviation  , 455 Khz @  2.5  Khz  deviation input   yields around 56 mV of recovered  audio  , at 1 Khz modulation tone ... note there is no de-emphisis .

I have been using the AOR2001 Scanning receiver to tap of 455 Khz I.F  as a signal source   and by placing lots of attenuation in the antenna lead and listing to NBFM voice communications, have  found that the Synch osc as a demodulator  when compared to the MC3357 based IF  strip, does have a slight edge on recovery of weak signals   with poor  S/N  ,  You cant quite hear what they are saying out of the scanner  speaker   but you  can understand voice   on the Synch Osc ,  its still very noisy  but readable , but the tuning of the S.O is very fiddly to enable this  and I certainly would say its only slightly better .  perhaps de emphasis application would help more  to improve intelligibility ,more  work needs to be done  here   , but its a simple IF strip to play with ,  I will try a simple single Fet transistor Quad demodulator circuit to see if I can simplify things more ..

Sweep Observation

The Spectrum analyser was set up for sweep operation using the internal tracking generator  . The tracking generator output was attenuated through a 50 dB 50 ohm pad  to bring the tracking output down  to the range  -116 dBm to -50 dBm   .Incremental  low levels  of swept signal where passed thru the Synch Osc and the lock frequency width was observed
   

    -110dBm    input      almost 1.6 Khz lock range                                                                                                        -100dBm  input   3.7 Khz lock


 
     

                            - 80 dBm in and 35 Khz lock range                                                                                     -60  dBm    and 500 Khz lock range

    

                    -50 dBm input   and  at maximum tracking range 

Synchronous  Osc  and  Amplitude  modulation

I have tried the stimulation of the synchronous oscillator with an amplitude  modulated signal  90% at 455 Khz  and noticed that when it is tuned exactly to the carrier frequency it ignores all modulation and one hears no  detectable demodulated audio from the  quadrature detector . This is a consistent  observation within  a range of input amplitudes from -100 dbm through to -40 dBm where some AM  modulation appears to get through into the audio output ,  I suspect this may be direct feed through of the AM stimulating  signal from the signal generator .
.  However  for this phenomenon to show , the S.O must be locked to the carrier of the AM signal and  not any of the sidebands .This is not a simple operation and it is easy for the S.O to drift and lock on an  AM  sideband . The signal generator is a single tone AM source  and I have not tried it on true audio modulated eg speech carriers to see if this  AM sideband suppression phenomena holds in that situation .
 

Addendum  I have now tried the  introduction of AM  voice at 455 Khz into the Synch..Osc  , I tuned to our local Airport automatic information service on 128.800 Mhz  and listened to the output of the sync Osc - Quadrature demodulator  and there is a solid lock of the system to the A.M carrier  and the modulation is stripped from the regenerated carrier  , I was quite surprised at how stable and "locked"  it was , I could tune  the L.O coil at least half a turn  and the system remains in lock.. I am sure a lot of this AM rejection was  contributed to the S.O action  as seen by the spectrum analyser and not the  nature of the Quadrature demodulator .

      

       Output of signal generator 80% mod  -80 dBm                                                                  Output of synch Osc  locked to AM carrier  80%  modulation

Synchronous oscillator locked to a Submultiple of the output frequency

I can  not see why a  UHF Synchronous oscillator cannot be  locked  to the  oscillation of  a stable crystal controlled controlled Fundamental source ie a simple multiplier  situation . . This may be an easy and simple way of generating a quartz crystal stable VHF-UHF oscillator without having to use the complications of frequency multiplier stages ??
 I have tried this experiment with the 455 KHz Synch Osc I built and there was no problem locking it to the second , third and fifth subharmonic of 455 KHz . however  I did notice the spectrum analyser display showed an awful  lot of phase noise around the locked carrier ,  Perhaps I have to change the transistor bias configuration of both transistors  to improve the subharmonic locking operation . 

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Vasil Uzunoglu designs and develops circuits and systems using synchronous oscillators for his company, Synchtrack (Gaithersburg, MD). He has an MSEE from the University of MissourióRolla and is a member of the IEEE. He has published numerous papers on synchronous oscillators and related circuits. Tech memo CL-38-81, Comsat Laboratories, Clarksburg, MD.

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