6018	2011-04-04 03:48:13	Ashhar Farhan	wide range double tuned circuit
Message	Date	From	Subject
hi, i am in the process of brewing up a front-end for a general coverage transceiver. rather than use the traditional broad band up-conversion, i am sticking to a narrow-band design with manually tuned DTC band-pass filters. The IF is at 12 MHz, and the RF preselector will tune in two ranges of 3.5 - 11 MHz and 13 MHz to 30 MHz. This will be tuned using 365 pf, dual ganged variable capacitors. I will use either tapped inductors or secondary winding to ensure that the my conversion to and from 50 ohms is proper. However, my confusion is over how do i couple to two tanks with each other? using a top-coupling with a small capacitor will not work very well over the entire tuning range. as the coupling co-efficient will change for the capacitor with frequency. is there any other method that will allow a better form of coupling? - farhan
6019	2011-04-04 04:28:00	joop_l	Re: wide range double tuned circuit
Another low-side tap on your inductors that you tie together? So instead of the capacitor coupling. You might still notice some change over the tuning range, but the effect could be smaller than with the capacitive coupling. Joop
6020	2011-04-04 04:36:24	joop_l	Re: wide range double tuned circuit
Here is an interesting circuit (figure 3-22): http://www.radiomuseum.org/forum/tuned_bandpass_filters.html Perhaps you should put all bandpass filter flavours into SPICE to see which one suits your purpose best. Joop
6021	2011-04-04 04:45:05	g3oth	Re: wide range double tuned circuit
Farhan Have you considered this as an alternative? http://yu1lm.qrpradio.com/BP%20FILTER%20HF-YU1LM.pdf 73 Charles G3OTH
6022	2011-04-04 04:45:29	Roelof Bakker	Re: wide range double tuned circuit
Hello Farhan, You might also look into the peaked low pass filter design as described by Wes in 1985, I believe in Ham Radio Magazine. This filter tunes over a 1:4 frequency range and has the beauty of an almost constant insertion loss over the full tuning range. I have the article somewhere, but unfortunately I don't have a scanner. Maybe someone else can provide a copy. I have used it to good effect in a home made DC receiver. 73, Roelof, pa0rdt
6024	2011-04-04 07:03:59	Ashhar Farhan	Re: wide range double tuned circuit
joop, i saw http://www.radiomuseum.org/forum/tuned_bandpass_filters.html at a cursory look, it seems to be two low-pass pi-networks coupled back-to-back. i suspect that it may not have sufficient band-rejection. though, it is only a hunch. i am not much of an LT-Spice jock, prefering to do things the good ol way just counting the poles and zeros. i must get around to learning to use spice, soon now. however, i will build it and run it through my sig generator and report. - farhan
6025	2011-04-04 07:05:58	Chris Trask	Re: wide range double tuned circuit
> > i am in the process of brewing up a front-end for a general > coverage transceiver. > rather than use the traditional broad band up-conversion, i > am sticking to a narrow-band design with manually tuned DTC > band-pass filters. > > The IF is at 12 MHz, and the RF preselector will tune in two > ranges of 3.5 - 11 MHz and 13 MHz to 30 MHz. This will be > tuned using 365 pf, dual ganged variable capacitors. > > I will use either tapped inductors or secondary winding to > ensure that the my conversion to and from 50 ohms is proper. > However, my confusion is over how do i couple to two tanks > with each other? using a top-coupling with a small capacitor > will not work very well over the entire tuning range. as the > coupling co-efficient will change for the capacitor with > frequency. is there any other method that will allow a > better form of coupling? > I ran into this dilemna many years ago and resolved it by using a preamplifier with a single section tank before and after it, creating a preamplifier/preselector. A small trimmer added to one of the sections helps to resolve any difference between the two. Using a coupled resonator approach as you described is usually relegated to designs where the preselector is fixed (not variable) for a predetermined band. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
6026	2011-04-04 07:13:03	Ashhar Farhan	Re: wide range double tuned circuit
charles, the YU1LM design needs an gang capacitor that is isolated from the ground. this makes fabricating it a bit messy. i also suspect that it cannot, on it's own be a good choice for a front end. it is essentially a T-network with just one pole. whereas i am looking for something that will provide 80db or more of stop-band rejection. at this point, i am also thinking of using an active element for coupling (an amplifier) between the two tanks. this will probably sort out the issue with insertion loss as well. i hope to put together a few tunable band-pass filters and run them through the sig - gen and do a manual 'sweep' . - farhan
6027	2011-04-04 07:18:09	Ashhar Farhan	Re: wide range double tuned circuit
chris, those are my very thoughts at the moment. i suppose the active element can also provide a reasonable termination / source for the tanks. how does the selectivity and stop-band performance of a tuned Rf amplifier (with a tank in the input and output) compare to a top-coupled dtc? - farhan
6028	2011-04-04 07:28:46	ehydra	Re: wide range double tuned circuit
The theory behind is that of the chaotic double-penduluum. - Henry -- ehydra.dyndns.info Ashhar Farhan schrieb: > joop, > > i saw http://www.radiomuseum.org/forum/tuned_bandpass_filters.html > > at a cursory look, it seems to be two low-pass pi-networks coupled > back-to-back. i suspect that it may not have sufficient > band-rejection. though, it is only a hunch. i am not much of an > LT-Spice jock, prefering to do things the good ol way just counting > the poles and zeros. i must get around to learning to use spice, soon > now. > > however, i will build it and run it through my sig generator and report. > > - farhan
6029	2011-04-04 07:34:33	Chris Trask	Re: wide range double tuned circuit
> > chris, > > those are my very thoughts at the moment. i suppose the > active element can also provide a reasonable termination > / source for the tanks. > > how does the selectivity and stop-band performance of a > tuned Rf amplifier (with a tank in the input and output) > compare to a top-coupled dtc? > You can do all sorts of interesting things with coupled resonators. If the coupling is loose (small coupling capacitor) you can get fairly sharp selectivity and a low-ripple passband. If the coupling is tight (large coupling capacitor) you end up with the same selectivity as with a single-section resonator. The overall Q, passband ripple, and bandwidth is dependent on the Q of the individual resonators and the coupling capacitor(s). Coupled resonator bandpass filters are not symmetrical as there are more zeroes than poles in the total response. You can remedy this in a 3-pole filter by replacing one of the coupling capacitors with an inductor. There's no math available for a closed-form solution for this, you have to use a circuit simulator that has an optimizer. It's far easier to make a highly selective (meaning narrow bandwidth) preselector by using individual one-pole sections separated by amplifiers. I have a vacuum tube preselector/preamplifier that I built over 40 years ago that has three tuning sections, a regenerative first amplifier (6AK5), and a variable gain second amplifier (6BA6). It's a bit of a monster, but I snagged some nice QSL cards using it with a Heathkit GR-64. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
6030	2011-04-04 08:36:40	Leon Heller	Re: wide range double tuned circuit
A Cohn filter is often used in that situation, IIRC. 73, Leon -- Leon Heller G1HSM
6032	2011-04-04 12:41:31	victor	Re: wide range double tuned circuit
I have developed such a filter that tunes 1:4 frequency range with proportional constant bandwidth and constant inserti
6033	2011-04-04 13:03:05	Chris Trask	Re: wide range double tuned circuit
> > I have developed such a filter that tunes 1:4 frequency range with > proportional constant bandwidth and constant insertion loss. > It was published in the magazine "Applied Microwave and Wireless" > at September 2000. > You can see more about such a filter at DG5MK site: > "http://dg5mk.de/CMS/wb/pages/artikel/abstimmbarer-hf-bandpass.php" > It is in German but you can use Google to translate it. > I remember that one, and I probably have around here someplace. Too bad AM&W went bust. You can thank Randy Rhea for that. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/