Hi Folks. I have several questions regarding the impedances involved in the K2's I.F., and their impact on the design of the KSB2's filter and matching transformers. I poked around the K2 mail archives and saw a similar question posted some time ago (http://www.ac6rm.net/mailarchive/html/elecraft-list/2003-05/msg01686.html), but found no definitive answer or information.
Several assumptions that I believe are true: * The impedance of the K2's I.F. signal path is 150 ohms. The input impedance of the MC1350 I.F. amplifier chip is around 2000 ohms. This is matched to 150 ohms through T7, a toroidal transformer with a 20:5 turns ratio (16:1 impedance transformation). * The impedance of the crystal filter in the KSB2 is 1500 ohms. T1 on the KSB2 board transforms the 150 ohm I.F. impedance to 1500 ohms (22:7 turns ratio, 10:1 impedance ratio). T1 is terminated either by R11 (150 ohms) on the KSB2 board, or the post mixer I.F. amplifier/noise blanker (150 ohms), depending on whether the transceiver is transmitting or receiving. * T2, on the OTHER side of the crystal filter, has a 22:4 turns ratio, yielding a 30:1 impedance ratio (a 50 ohm impedance, rather than 150). * The KSB2's crystal filter is operated in "opposite directions" in transmit and receive. On transmit, the input is on the side of T2, and on receive, the input is on the side of T1. * On transmit, the balanced modulator provides a proper 1500 ohm source impedance for the crystal filter. R11 provides the proper 150 ohm termination after T1. --- HOWEVER --- * On receive, the output of the KSB2's crystal filter is mismatched by a factor of 3 to 1 (T2's output is 50 ohms, and it is feeding a 150 ohm I.F. amplifier). * The KI6WX +10db modifications appear to treat the output of T2 as being 150 ohms (or at least something higher than 82 ohms, the value of the emitter follower's emitter resistor), rather than 50 ohms, yielding a significant impedance mismatch on transmit. Impact: * Many of the crystal filter response measurements, such as those conducted using programs such as Spectrogram and Baudline, have been performed with the K2 in RECEIVE mode only. Since the filter appears to be terminated differently depending on whether the K2 is in transmit or receive mode, the results may not tell the whole story. Confession: I built my K2 last year and performed the KI6WX KSB2 filter and +10dB gain modifications. The improvement in frequency response on receive was substantial, but I have never been satisfied with my on-air audio quality (despite reports to the contrary). I ran sweeps of the TX chain and found my transmitted frequency response to be around 2 kHz after performing the 2.5 kHz mod. Results are posted at http://www.qsl.net/kd2bd/ksb2.html I put the results of these tests out of my mind for the past 10 months. Yesterday I checked the batch of 82 ohm resistors I used for the +10dB gain modification and found they were all measuring between 120 to 140 ohms! Reflected through T2, this would yield a filter source impedance of about 6000 ohms, a significant mismatch for a 1500 ohm filter. The higher-than-normal source impedance might explain the narrow filter bandwidth I am experiencing on transmit (but not on receive). Before I change the defective resistor, I would like to get a better understanding of the true terminating impedance of the filter. T2's turns ratio might suggest that even an 82 ohm resistor is too high for proper termination. If the KSB2 is truly operating in a 150 ohm I.F. signal path, why aren't T1 and T2 wound with the same turns ratio? Possible solutions: * Decrease the resistance of the 82 ohm resistor to provide a closer match to the 50 ohm impedance provided by T2. * Decrease T2's turns ratio to that of T1 to bring the output impedance of the filter from 50 ohms to 150 ohms for a better match to both the I.F. amplifier on receive, and the KI6WX +10dB gain modification on transmit. Suggestions? Comments? Thanks in advance. 73, de John, KD2BD K2/100 #3563 ===== Visit John on the Web at: http://www.qsl.net/kd2bd/ __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
