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AM Stereo Hardware Mods
Uses a surplus ±7.5KHz 6 element ceramic filter from a cordless phone. An
adjustable LC PLL Osc. NRSC De-Emphasis. 10KHz Notch Filter. Low Pass Filter
with a +9dB High Frequency Boost.
Added a window detector on the PLL loop volts for a center
tuning indicator mounted on the dial indicator . Added 10KHz
notch filters and 6dB buffer amp.Added wide and narrow IF
filter selection via the mono/stereo switch for a ±7.5KHz
and ±3KHz selection. Added synchronous detection for L+R
and disabled cosine correction for L-R for true QuAM fixed
at 90°. This can be overridden via a switch on back to
select between QuAM and C-QuAM.
Designed and Built a MC13020P decoder that has a ±7.5KHz
6-element filter with AGC. Pilot filter circuit has a 3rd
order lowpass filter at 40Hz for good co-channel sensing
with good rejection of the regular L-R signal. The pilot
bandpass filter has a Q appraching 10 and a gain appraching
25. Post detection has 10KHz notch filters and 2nd order
lowpass filters providing a boost of 7dB at 8.7KHz to help
compensate for HF attenuation caused RF and IF filter
response. Later I addd a synchronous adapter to it. This
particular unit made it difficult to interface the decoder
since the mono AM signal passed through the FM stereo
decoder chip so I had to use a CMOS Quad Analog DPDT Switch
to overcome this. Iterfaced the mono/stereo button for FM
to also work on the AM stereo decoder. Interfaced the STEREO
ID in the florescent display to the AM stereo decoder.
Basket Woven Ferrite Bar Antenna. This type of winding is said to improve the quality of the inductance.
Built a Homemade Select-A-Tenna using a heavy duty 10"
embroidry hoop, an air core tuning capacitor, #19 enameled
wire, some wood and cardboard for the pointer.
© Copyright 2002 - J. S. Gilstrap