NE567 Tone Decoder as AM FM   demodulator at 455 KHz                                                     back to home page

The Ne567  tone decoder IC is a phase lock loop designed for detection of audio tones , it is very useful and has been around almost as long as the NE555 .  in order to detect tones within its adjustable capture range ,it uses a synchronous demodulation technique where the VCO output is fed to the second  on board  balanced mixer in quadrature   ( actually 80 degrees ) but that minor error doesnt prevent it from performing its desired function very well . Thus it has a Synchronous detector for AM !
The ic is rated up to 500 Khz . and being a synchronous demodulator  and a PLL  there is no reason why it  shouldn't demodulate  Frequency modulated  signals  and  Amplitude modulated  signals .

In order to work at 455 Khz, the Correct resistor and capacitor need to be chosen , 470 pf and a 4.7 K ohm variable  enable tuning either side of the wanted VCO frequency , the only two other variables to choose are the filter capacitors for the FM loop and the  output of the synchronous  demodulator  . Because we are not interested in the Lock  function of the ic at the moment,  the values will be somewhat different than those suggested by the I.C designer .   For a dedicated  FM only demodulator , there is no reason why the correct lock range capacitor be used on the synchronous detector  to enable an Audio   mute function  simultaneously to Frequency demodulation . I built up a simple double sided etch PC board and made an operational circuit , the FM loop capacitor  was a 470 pf  bypass and the  AM detector was .01 uf   , the v+ rail was 6 volts via a 3 terminal regulator .  The VCO was set to 455 Khz and the fm audio output was connected to my usual  amplified computer speakers  , these are not the most sensitive  audio amp  but 10's of millivolts of audio input yield  audio loud enough to hear on the work bench .
I connected the 50 ohm source from the signal generator  directly to the capacitor input of the NE567 , this is a rather large mismatch as the input impedance of the 567 is 25 k ohms  ( more about this later)  I connected the audio amp input to the Demodulated fm output pin 1 , and punched up 455 Khz , FM , 2.5Khz Dev at 1 Khz tone , and then ramped the RF output amplitude up from -100 dbm  until I heard sound from the speakers  ..


I tested the PLL at 5 KHz  and 2.5 KHz  deviation, and measured the audio output level with the oscilliscope in high impedence (~1 meg )   mode  . The NE567 requires a minimum of -35 dBm across 50 ohms to achieve reliable lock .. I noticed the value of the filter capacitor has an influence on the broadband noise appearing from that pin (2) ,in this test I had a 470 Pf ,  I noticed the rise in apparent  level  on the oscilloscope, with increases in the input level  for  FM it should remain the same , but I suspect not having an adequate low pass audio filter network , allowed the broadband noise to increase .  subjectively I did not detect and increase in the loudness of the demodulated signal as the input level stepped up ..

Input Level            5.0  Khz                                2.5 Khz         

 -35 dBm                58 mV                                   29 mV

-30                             59                                        31

-25                             62                                        32

-20                             64                                        35

-15                             66                                        36

-10                             67                                        37

-5                               72                                        40


As a matter of interest I connected the input to the NE567 pll to the tracking output of the spectrum Analyser  ( at -20 dbm) and connected  to the input  of the analyser a capacity probe placed  very close to the VCO components to look at the  VCO frequency , I manually adjusted the VCO pot ( 4K7) for maximum frequency and looked at the flat plateau ( lock range) on the screen this NE567 at +6v rail would go to a centre frequency of 1200Khz and sweep +/- 350 Khz . considerably higher than the 500 Khz advertised in the data sheet !

The next test was  455 Khz ,1 Khz tone ,  AM 80% modulation over  a range of input levels and look at the audio output level from the synchronous demodulator (pin 1 )

            -40  dBm              44 mV

            -35                        77        Clean sounding and looks correct on scope

            -30                      130

            -25                      233

            -20                      320

            -15                      350   1 Khz tone starting to sound  slightly distorted

            -10                      305    sine wave shows distortion  and  sounds it

            -5                        195   sine wave shows more distortion  and  sounds worse  

There is quite a wide dynamic range over which the synch demodulation sounds very clean   and in a normal radio receiver the levels would be chosen to be held by the AGC  in the receiver   

There is a large mismatch between the 50 ohm signal source at 455 KHz amd the 25Kohm input impedance of the ne567 , I placed an interstage transformer in the circuit with the low impedance ends connected  and the top of the tuned  circuit to the capacitively coupled  pin 3 , this enables a more sensitive PLL and it will lock at lower  levels due to the increase in voltage derived via the 455 Khz tuned circuit input . However when connected to the 455 Khz IF tap out of the scanner , I found I required more voltage gain than the tuned interstage coupling transformer could  provide, so a sinlgle transistor bc547, class A stage was lashed up and connected to the transformer input .  .This enabled good reliable clear  demodulation of both  modes at the -70 dBm scanner 455 KHz output .



Listening  test ;

The 455 Khz of the AOR2001 scanner was connected to the PLL demod and  both forms of modulation listened to  off air from air traffic control (AM)  and local police ( nbfm ) the demodulated audio of the weather broadcast on 128.800MHz  AM was clear and clean  sounding , the nbfm was also clear and clean

The  NE567 as a tone decoder is  a more versatile device than first meets the eye  . I have not had a look at the   xr2211  PLL chip but I gather that chip is obsolete  ..

Hope my findings are of use to someone or promotes some new idea  


cheers  Mike  ZL1BTB


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