Home |
Search |
Today's Posts |
#1
|
|||
|
|||
lf to HF preselector design needed
I need an LF to 30 Khz preselector (passive) for a homebrewed receiver
I'm building. I have most of the details worked out, but I am stuck on the preselector design. I want one band to cover 50 Khz to 500 Khz (or 1 Mhz). The second band would be 1 Mhz to 30 Mhz. 50 ohm input with moderate Q (don't want to have to retune it every 100 Khz. The LF unit can be very high Q, peaking it every few Khz is ok as most of the signals in that range are narrowband signals anyway. Input impedance is standard 50 ohms. I'd REALLY like to have one assembly do the entire job, but it appears to be a tall order as that's quite a wide range to cover with one tuned circuit. Google returns to many hits even if I narrow down the search parameters, I tried google. It appears I need to use some sort of permeability tuned variable inductor in conjunction with a variable cap. Have no idea how to go about this or where to buy permeability tuned inductors. Any suggestions? T |
#2
|
|||
|
|||
On Fri, 09 Sep 2005 22:01:27 -0400, TRABEM wrote:
I need an LF to 30 Khz preselector (passive) for a homebrewed receiver I'm building. I have most of the details worked out, but I am stuck on the preselector design. I want one band to cover 50 Khz to 500 Khz (or 1 Mhz). That would be 1:10 (or 1:20) frequency range. The second band would be 1 Mhz to 30 Mhz. That would be 1:30 frequency range and would require a 1:900 reactance range, if only one element (inductance or capacitance) is tuned, which would be completely unrealistic. The alternative would be to tune both the inductance _and_ capacitance in the 1:30 range. However, with a 300 pF variable capacitor, the minimum capacitor would be 10 pF _including_ all stray capacitances, not very realistic. I have no idea what a realistic tuning range would be for a permeability tuned inductor. I have only seen those in FM receivers (87..108 MHz), which only about 20 % tuning range. 50 ohm input with moderate Q (don't want to have to retune it every 100 Khz. The LF unit can be very high Q, peaking it every few Khz is ok as most of the signals in that range are narrowband signals anyway. Input impedance is standard 50 ohms. Maintaining the constant impedance if both inductance and capacitance is tuned would be a challenge. While maintaining the power match makes some sense in HF receivers (and also if you are using some small magnetic loops on LF/MF), but otherwise, I would question the idea to maintain the 50 ohm resistive input impedance on LF (and possibly also MF). On LF, any practical random wire antenna would have a high capacitive reactance. On LF I have simply used a parallel tuned circuit and connected the antenna to the hot end of the circuit with a small (10-100 pF) capacitor. Of course, the tuning scale will not be accurate if you change the antenna, since the antenna stray capacitances will detune the resonant circuit. Use a secondary winding or a source/emitter follower to get the impedance down to feed a 50 ohm receiver input or feed a coaxial cable (if the preselector is just under the antenna). If you intend to use magnetic loop antennas on LF/MF, which have a low efficiency, I would suggest using a (remotely)tunable loop as the preselector, with possibly a preamplifier at the antenna. I would strongly suggest splitting the HF bands and switching in various inductors for various bands and use an ordinary variable capacitor. The situation is quite problematic on LF, since you would need a variable capacitor with a tuning range int several nF to keep the resonant circuit impedances at manageable levels (otherwise, it would be quite "touchy" due to stray capacitances). With practical variable capacitors, you would have to add parallel padding capacitors, with would further reduce the tuning range. Permeability tuning might be an option. You could try making a coil and slide a ferrite bar (from a medium wave antenna) into it, however, I have no experience with this. Paul OH3LWR |
#3
|
|||
|
|||
I realise that you are after a passive solution but I wonder if an
op-amp or two in a Q multiplier configuration *for the first band) might be a better idea? You could then tune the feedback/gain of the device with a R/C combination rather than large values of L. You could of course set the gain to 1 if you wanted. Just an idle thought. Cheers Bob W5/VK2YQA TRABEM wrote: I need an LF to 30 Khz preselector (passive) for a homebrewed receiver I'm building. I have most of the details worked out, but I am stuck on the preselector design. |
#4
|
|||
|
|||
Thanks Paul,
My old Drake C line had a multiple ganged permeability tuned inductor mechanically coupled to a variable cap. One of the assemblies tuned the front end, one peaked the driver stage, etc. It covered from 1 Mhz to 30 Mhz with a half turn on the front panel preselector control. Most likely the variable C and L tuned together maintained a desirable Q across all the bands. I was hoping for something similar, but have no idea where to get permeability tuned inductors today. Regards, T On Sat, 10 Sep 2005 12:16:44 +0300, Paul Keinanen wrote: On Fri, 09 Sep 2005 22:01:27 -0400, TRABEM wrote: I need an LF to 30 Khz preselector (passive) for a homebrewed receiver I'm building. I have most of the details worked out, but I am stuck on the preselector design. I want one band to cover 50 Khz to 500 Khz (or 1 Mhz). That would be 1:10 (or 1:20) frequency range. The second band would be 1 Mhz to 30 Mhz. That would be 1:30 frequency range and would require a 1:900 reactance range, if only one element (inductance or capacitance) is tuned, which would be completely unrealistic. The alternative would be to tune both the inductance _and_ capacitance in the 1:30 range. However, with a 300 pF variable capacitor, the minimum capacitor would be 10 pF _including_ all stray capacitances, not very realistic. I have no idea what a realistic tuning range would be for a permeability tuned inductor. I have only seen those in FM receivers (87..108 MHz), which only about 20 % tuning range. 50 ohm input with moderate Q (don't want to have to retune it every 100 Khz. The LF unit can be very high Q, peaking it every few Khz is ok as most of the signals in that range are narrowband signals anyway. Input impedance is standard 50 ohms. Maintaining the constant impedance if both inductance and capacitance is tuned would be a challenge. While maintaining the power match makes some sense in HF receivers (and also if you are using some small magnetic loops on LF/MF), but otherwise, I would question the idea to maintain the 50 ohm resistive input impedance on LF (and possibly also MF). On LF, any practical random wire antenna would have a high capacitive reactance. On LF I have simply used a parallel tuned circuit and connected the antenna to the hot end of the circuit with a small (10-100 pF) capacitor. Of course, the tuning scale will not be accurate if you change the antenna, since the antenna stray capacitances will detune the resonant circuit. Use a secondary winding or a source/emitter follower to get the impedance down to feed a 50 ohm receiver input or feed a coaxial cable (if the preselector is just under the antenna). If you intend to use magnetic loop antennas on LF/MF, which have a low efficiency, I would suggest using a (remotely)tunable loop as the preselector, with possibly a preamplifier at the antenna. I would strongly suggest splitting the HF bands and switching in various inductors for various bands and use an ordinary variable capacitor. The situation is quite problematic on LF, since you would need a variable capacitor with a tuning range int several nF to keep the resonant circuit impedances at manageable levels (otherwise, it would be quite "touchy" due to stray capacitances). With practical variable capacitors, you would have to add parallel padding capacitors, with would further reduce the tuning range. Permeability tuning might be an option. You could try making a coil and slide a ferrite bar (from a medium wave antenna) into it, however, I have no experience with this. Paul OH3LWR |
#5
|
|||
|
|||
TRABEM wrote:
Thanks Paul, My old Drake C line had a multiple ganged permeability tuned inductor mechanically coupled to a variable cap. One of the assemblies tuned the front end, one peaked the driver stage, etc. It covered from 1 Mhz to 30 Mhz with a half turn on the front panel preselector control. Most likely the variable C and L tuned together maintained a desirable Q across all the bands. I was hoping for something similar, but have no idea where to get permeability tuned inductors today. Regards, T Fair Radio used to have some in their catalog, and they were used in all of the old Delco (and some other brands) car radios of the '60s and '70s. -- ? Michael A. Terrell Central Florida |
#6
|
|||
|
|||
Michael A. Terrell wrote:
Fair Radio used to have some in their catalog, and they were used in all of the old Delco (and some other brands) car radios of the '60s and '70s. Just about all the radios I've seen with mechanical pushbuttons for station presets used permeability tuned inductors -- the pushbuttons each just positioned a ferrite core to a preset depth in a coil. It wouldn't be difficult to construct one, unless you're mechanically declined like I am. Roy Lewallen, W7EL |
#7
|
|||
|
|||
Roy Lewallen wrote:
Michael A. Terrell wrote: Fair Radio used to have some in their catalog, and they were used in all of the old Delco (and some other brands) car radios of the '60s and '70s. Just about all the radios I've seen with mechanical pushbuttons for station presets used permeability tuned inductors All of the american made radios were, but some cheap imports weren't. -- the pushbuttons each just positioned a ferrite core to a preset depth in a coil. It wouldn't be difficult to construct one, unless you're mechanically declined like I am. Roy Lewallen, W7EL I once told one of the Vice president of marketing at United Video Cablevision that he was mechanically declined and my boss freaked out. ;-) The VP yelled at him and told him to shut up because it was true, and he was happy that whenever he had to launch a new channel in the Cincinnati system that everything was set up and ready for him when he arrived. Of course, you could make them as individually tuned coils with a stepper motor or servo per coil. -- ? Michael A. Terrell Central Florida |
#8
|
|||
|
|||
Thanks to all that commented. Although I don't have an answer, the
content of the discussion was helpful. I'm not exactly fond of electronic switching, but it's probably adequate since atmospheric noise dominates in the vlf and lf range anyway. I'm trying to understand the butterworth filter now, with the hope of designing a bandpass filter over a wide range of frequencies rather than a variable unit. Regards, T |
#9
|
|||
|
|||
This site might have some suggestions.
http://www.geocities.com/amdxlog/index.html and maybe http://www.qsl.net/wa1ion/ Cheers Richard |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
60 meter mod info needed... | Equipment | |||
Voiceover talent now needed! | Broadcasting | |||
PRO 23 INFO NEEDED | Scanner | |||
Icom R8500 : help needed | General |