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