Richard Prosser gmail.com> writes: > Magnetic amplifiers are in still in widespread use. True, and likely the physically nearest one to almost each and every one of us, is a CFL fluorescent light, whose electronic circuit uses a saturating toroid. I have yet to see one which does not use such, more than 12 years after the article linked to below was published. The original 'public' schematic of the 'standard' CFL lamp, which seems to have been created by a Philips engineer and placed on the web circa 1998, and likely copied by *everyone* with small variations, still makes for interesting reading, and is here: http://www.nxp.com/acrobat_download/applicationnotes/AN00048_1.pdf The interesting part is that such a toroid is designed to saturate at about 0.1 Ampere-turns (judged by the number of turns on it and the known currents in CFL circuits), which makes it interesting for flux gate magnetometer applications without the chore of complex windings on toroids imho, and I will likely test this real soon now [tm]. Obtaining the toroid core from a defunct CFL lamp should provide the experimenter (me) with the necessary 'pre-approved' part. Usually attempts to buy a small saturable toroid from the usual outlets is met by blank stares. Anyway the person who seems to have developed the first usable CFL circuit was an American GE engineer, but he was too far ahead of his time and the idea did not catch on for complexity and cost reasons, during the 1970s energy crisis. Back to VCOs and VXOs, another method to make a VXO with wide pull range is to use a non-overtone (!) higher frequency VXO and mix it with a suitable fixed XO to obtain the actual desired output prequency. F.ex. I am pretty sure that mixing a 433MHz SAW filter stabilized XO with another identical one tuned by a varicap should yield a pull range close to 2MHz which may go down to 0Hz (subject to low phase noise oscillators and mixers). Filtering out the high carriers is very easy on account of the enormous delta-f. These SAW filters are mass produced for the keyfob remote market and are cheap. Using thermally coupled identical SAW filters in identical oscillators and beating them against each other should yield covariant drift and thus cancel the drift almost completely, leaving only the varicap voltage and temperature dependence to disturb the balance. It should even be possible to build the oscillators strictly identical and change the tuning voltage differentially. One problem which needs to be addressed is 'lock in' where the two oscillators running at frequencies very close to each other will tend to lock each other onto the same frequency. Using good shielding and directional couplers should fix this problem, and selecting a SAW filter with a steep slope should help to get the phase noise if not under control then at least as low as possible. Similarly any beat (difference) frequency can be selected as needed by selecting the second (XO) crystal. For example to span the 3.5-3.8 MHz ham band a 20MHz VXO (likely multiple crystals in parallel or crystal ladder filter based) could be used and beat with the third harmonic of a 5.5xx MHz oscillator. Again, because these carriers are very far away from the received frequency they are very easy to filter out. Finally most VXO schemes whose output is destined to drive digital circuits use an integrated VXO oscillator, like this one (pull range 380ppm!): http://petermanntechnik.org/products/pdf/pl/PLL502-51.PDF Designing your own can be tricky (note the phase noise specifications of the part linked above). Beating two of the above oscillators against each other should yield any beat frequency range span of about 19kHz to 38kHz (from 380ppm up to 380ppm*2 when pulling both) between 0 and 30MHz. This can include 0Hz (!), and span the audio range, and with the phase noise specs of the part quoted above it should be more than very usable. There are of course other parts to choose for this role. (not an expert but I did put in dozens of lab hours looking for solutions in the XO VXO VCO domain) -- Peter PS: Sorry for the ragged formatting of my message, I cannot help it, the web client I am using for posting does it. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist