EMRFD Message Archive 10119
Message Date From Subject 10119 2014-07-01 01:50:21 ad5rj_slash_7 Crystals for S7C Simple 7 MHz Superhet I have not built it yet, but I want to. The article describing the S7C, at the end, mentions a variation of this radio built by AA3SJ. It used an 11 MHz LO with a 4 MHz IF. I have found some crystals that should work in this scheme. The LO is at 11.232 MHz (crystals available at http://www.ebay.com/itm/221122217185). The IF is at 4.194304 MHz (crystals available anywhere). By my math, this should tune about 7038 kHz.
I have some old crystals in HC-6/U size holders at 4.194304 MHz. The measured parameters for my best two crystals are as follows:
Motional Inductance: 121.4 mH and 121.9 mH
Q (thousands): 190 and 180
Holder Capacitance 4.2 pF
For these two crystals, the double tuned circuit design is simple. Just add a 180 pF coupling capacitor and terminate each end in 200 ohms. Computed BW is 375 Hz at -3 dB.