Re: Re. problem with crystal oscillators
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Arv Evans
开云体育Rahul VU3WJMThat is the beauty of the BITX design.? It has so much potential that it gets one to thinking about all the interesting possibilities.? Here are some more "ideas" that relate to the BITX design: ? (1) Since the BFO is a 10 MHz crystal oscillator, one might be able to divide that oscillator down to become the reference clock for a digital dial (frequency counter).? It seems this would require that it be right on 10.000,000 MHz, so this would require that the ladder filter pass band be positioned properly relative to the BFO frequency.? Or...maybe the accuracy of the 10.000,000 MHz is not that critical.? Someone needs to do the math to see how far "off frequency" the counter reference oscillator can be and still provide reliable 100 Hz resolution for a digital dial.? Remember that for an accurate 100 HZ counter the reference oscillator will probably be divided down to 50 Hz to drive the count gate circuit. ? (2) What about building multiple "front-ends" on different bands for the BITX, but re-using the IF, BFO/detector and audio sections.? This could provide a low-cost way to obtain multiple band coverage.? Something like tunable transverters for the front end, interfacing with a 10 MHZ fixed frequency back end module is what comes to mind. ? (3)? As Rahul suggested, the IF does not have to be on 10.0 MHz.? However, deviating from the original design will require that you pay particular attention to harmonics and resultant image frequencies to avoid unwanted signals in the passband.? I have been considering an LF version of the BITX on 160-190 KHz ( the US unlicensed experimenter band ).? Whatever might be done there should be applicable to areas of the world where you are allowed amateur operations on 76 KHz or 136 KHz.? Alas, our Federal Communications Commission (FCC) in the US does not yet allow licensed amateur radio operation on any LF band so we are stuck with the 1 Watt and 50-foot maximum radiator restrictions of our unlicensed LF band. ?????? Developing an LF version of the BITX (BITX-LF...?) provides a temptation to modify the basic design by customizing it for the lower frequencies, but I think that finding a set of VFO, BFO, & IF frequencies that will let one adhere closely to the original design, except for the frequency coverage, is more interesting.? ? (4)? Rahul's thoughts on using the BITX-xx as a front-end for SDR (Software Defined Radio) is an easy way to enter this fascinating aspect of our hobby.? All that is required for single-band operation is to couple the RX audio from the top of your Volume control to the sound card input on your computer ( so you can adjust the speaker volume without disturbing the sound card audio level ), and to bring the audio output from your computer sound card through a suitable attenuator and into the microphone input of the BITX.? Then with any of the appropriate FREEWARE or SHAREWARE programs that are available via the Internet, you can be on the air with computerized CW, PSK-31, MSK-xxx, RTTY, and so on.? This would be a manually-tuned version of SDR, but if you add a DDS VFO to your BITX, then the frequency can also be controlled via computer.?? Most of the available programs provide PTT (Push To Talk) keying via the DTR lead on your serial port, but it should also be possible to incorporate a simple VOX circuit that keys the transmit function whenever the computer begins sending any type of tone from the sound card.? This way you will not be tying up your serial port just to key the transceiver. Maybe we will have to start an "IDEAS" public file on the BITX20 forum as well as one for? MODIFICATIONS.? People could "check out" an IDEA and develop it until it becomes a MODIFICATION. Keep thinking!? It is fun, and it may help keep your brain alive and active well into old age. Arv _._ On Sun, 2004-07-18 at 12:20, Rahul Srivastava wrote: Hi! Good thought re. using the 10 MHZ for quad-phase detector and also dividing it down to a lower IF frequency for the BFO function.? Let us all know how it works. I wonder if anybody is thinking about a SSB phasing-mode version of the BITX with phase-synchronous detection of SSB signals ( Rahul's idea )?? That might let us get by with a minimal IF filter ( just enough filtering to avoid desensitization by other strong in-band signals and to avoid out-of-band image reception ). ? |