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There are various ways to detect FM signal. But in the history of the consumer FM tuner, a few circuits have been selected to satisfy the cost-performance. For example, ratio detector, quadrature detector, pulse count detector, PLL detector, etc.

1.Pulse count detector

Pulse count detector is consist of one-shot multi vibrator ( FM to PWM converter ) and low pass filter ( PWM to audio signal converter ). The circuit is similar to 1-bit DA converter.

Because of the upper limit of frequency response of the one-shot multi vibrator, the double conversion circuit was used. 2nd IF used to be selected around 1 to 2MHz.

To gain the S/N ratio, IF doubler circuit ( Pioneer F-120,F120D ) or tripler circuit ( Technics ST-G7 ) was used. In these circuits, the frequency of the 2nd local oscillator must be 20 to 30 MHz. To avoid the drift and the purity of the 2nd local oscillator which cause to lose S/N ratio of audio signal, the crystal oscillator was used as 2nd local oscillator.

Table 1 shows examples of FM tuner using pulse count detector.

Table 1 Pulse count detector

Manu -facturere	Block diagram	Model	THD 1kHz Stereo	THD 1kHz Mono
TRIO Kenwood	1st Mix-->IF Amp(10.7MHz) -->2nd Mix.(1.96MHz)-->Pulse Count Det. | L.O.(8.74MHz Xtal)	600T 700T *1)
TRIO Kenwood	1st Mix-->IF Amp(10.7MHz) -->2nd Mix.(1.96MHz)-->Pulse Count Det. | L.O.(8.74MHz LC)	L-07II L-01 KT-8300 KT-9700 KT-9900 KT-900 KT-990 KT-1000 KT-1100	0.045% 0.03% 0.05% 0.04% 0.04% 0.04% 0.04% 0.04%	0.04% 0.02% 0.04% 0.03% 0.03% 0.03% 0.03% 0.03%
Pioneer	1st Mix-->IF Amp(10.7MHz) -->2nd Mix.(?.??MHz)-->Pulse Count Det. | L.O.(?.??MHz LC)	F-500 F-700 F-780 F-100D	0.04% 0.04% 0.04% 0.05%	0.03% 0.03% 0.03% 0.04%
Pioneer	1st Mix-->IF Amp(10.7MHz)-->doubler(21.4MHz) -->2nd Mix.(1.26MHz)-->Pulse Count Det. | L.O.(20.14MHz Xtal)	<DDD> F-120 <DDD-II> F120D	0.015% 0.009%	0.0095% 0.006%
Technics Panasonic	1st Mix-->IF Amp(10.7MHz)-->tripler(32.1MHz) -->2nd Mix.(0.8MHz)-->Pulse Count Det. | L.O.(31.3MHz Xtal)	ST-G7	0.008%	0.007%

*1) 700T uses ratio detector ( from "Cooltune" on May 16th, 2008 . Thanks for the information )

2.PLL detector

PLL detector is consist of phase comparator, loop filter, VCO and LPF. The phase comparator can handle the 1st IF signal directly. Because the double conversion system is not needed, the cost performance of PLL detector is higher than pulse count detector.

Table 2 shows examples of FM tuner using PLL detector.

Table 2 Phase locked loop detector

Maker	Block diargam of IF and FM detector	Model	THD 1kHz Stereo	THD 1kHz Mono
TRIO Kenwood	1st Mix-->IF Amp(10.7MHz)-->PLL Det.	<DLLD> KT-770 KT-5020 <DLLD +DCC> KT-880 KT-880F KT-1010 KT-1010II KT-1010F KT-1100D KT-2020 KT-3030 D-3300 KT-6040 KT-6050	0.02% 0.02% 0.009% 0.009% 0.0095% 0.0085% 0.0085% 0.008% 0.008% 0.007% 0.007% 0.015% 0.015%	0.0095% 0.015% 0.006% 0.006% 0.006% 0.0055% 0.0055% 0.005% 0.005% 0.004% 0.004% 0.007% 0.007%
Pioneer	1st Mix-->IF Amp(10.7MHz)-->PLL Det.	<DDD-III> F-717 <DDD-IV> F-757 F-777	0.008% 0.008% 0.008%	0.005% 0.003% 0.003%
Onkyo	1st Mix-->IF Amp(10.7MHz)-->PLL Det.	MT-1000X <LTD> T-445XX T-445XG	0.02% 0.008% 0.008%	0.01% 0.0055% 0.0055%