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#31
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I agree about the 21.4MHz I.F. in a single conversion scheme. If the
required gain is achievable, this is a very good way to go. I remember one of the Rockwell-Collins ADF receivers that had this setup. As far as a 45MHz 1st I.F.............. these crystal filters seem to be designed with a 455kHz 2nd I.F. in mind because a 910kHz rejection spec is given. I have used that scheme over the years with a diode ring mixer as the 1st mixer. I follow the mixer with a series L, shunt C as the second leg of a diplexer that matches the 50 ohm output of the mixer to the crystal filter............ The first leg being is a parallel LC tuned circuit that is in series with a 50 ohm idler resistor. With a reactance of the LC elements of around 47 ohms, the selectivity at 6 db is approximately 15kHz. The (2 time 2nd I.F. image rejection ) is about 60dB when using four poles at 45MHz. I haven't done any in-band spur calculations with this scheme for the aircraft band but I think that with high-side injection it would be ok. Of course, if a 25kHz tuning step is used I am not sure how it would affect the phase noise of the synthesizer if a Fractional N topology wasn't used. I an NE602 is used as the 2nd mixer and just the I.F./ demodulator sections of a TDA1572 were used, the only adjustment required would be a trimmer adjustment of the 2nd LO in order to have a symmetrical adjacent channel response. Pete wrote in message oups.com... From: "Pete KE9OA" on Fri, Mar 18 2005 6:36 am I would go directly from 45MHz to 455kHz..............this is typical in HF receivers. An NE602 mixer can be used for the 2nd mixer and a 44.545MHz crystal used with the on-board oscillator allows you to derive your 2nd I.F. Alternatively, you can use a TDA1572 as the 2nd mixer/I.F./ demodulator. This is also a good system and it will give you fairly good strong signal handling capability. Pete "Netgeek" wrote in message ... Parts are on the way... First IF = 45 MHz Second IF = 10.7 MHz Any point in going further to 455 KHz for a third IF or just stick with the 10.7 - (MC1350 plus IF transformer) scheme? Bill As a suggestion (too late if parts are incoming), a single conversion to a 21.4 MHz IF is quite suitable. Using a monolithic quartz crystal filter (available from DigiKey for about $15 a pair of two) between the mixer and first IF could eliminate ALL tuned circuits in the IF following the 1st IF amplifier. Those are available in 12.5 KHz bandwidth which will be fine for a VOR signal. [DigiKey has a link to download specs for the ECS-made monolithics] This is now common in mobile radios, both new and retrofit of older ones. If a single-conversion scheme with 21.4 MHz is done, the LO can be 86.6 to 96.6 MHz with an image at 65.2 to 75.2 MHz. That is a low enough frequency to allow a simple L-C "top coupled resonator" fixed bandpass filter for the front end at 108 to 118 MHz (8.9 % bandwidth). Doing double conversion with a first IF of 45 MHz is, by itself, no problem. However the 2nd IF image is a bit too close to the nominal bandwidth of any 45 MHz 1st IF tuning (it's only 0.91 MHz away). With the second's image (on either side depending on 2nd LO above or below 45 MHz), there's still a chance to pick up part of the FM BC band locally. To avoid that, the 2nd LO should be on the high side of 45. Second IF image would then fall into the low end of the 118 to 137 MHz comm band (also AM) and those transmitters are much lower powered ones than BC stations. With a 10.7 MHz 2nd IF, its image would be 21.4 MHz away and rather easy to attenuate in the 45 MHz 1st IF. There's only a slight problem using stock 10.7 MHz IF components: Bandwidth of the whole 2nd IF might be around 160 KHz; less discrimination to nearby VORs and Localizers. A study of FAA sectional charts might be called for to check on potential interfering stations although those are assigned in regards geographic locations to minimize normal interference. |
#32
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And how, pray tell, do you get rid of the image with reasonable Q
components? Jim "Pete KE9OA" wrote in message ... I would go directly from 45MHz to 455kHz.............. |
#33
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On Sat, 19 Mar 2005 08:36:17 -0800, "RST Engineering"
wrote: And how, pray tell, do you get rid of the image with reasonable Q components? like Hi-Q resistors or was it condensers...? --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/91n.htm |
#34
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As a follow-up - I bought and assembled the really simple kit sold
by Ramsey - and described in detail he www.sentex.ca/~mec1995/aviarx/avrxkitc.html I wasn't expecting much - but was really surprised! It works really well and has provided a platform that I can hack to death without feeling bad about destroying it if it comes to that... I'm about 8 miles from Washington National (DCA) with alot of "stuff" in between. With a lousy 21" wire, I have no problems picking up both Potomac Departure and Potomac Approach (with the first I heard being Air Force One coming in to Andrews AFB). The only modifications I've made are to increase the supply to 12V (from 9V - per Jim Weir's comments) and to replace the worthless 10k varactor tuning pot with a 3-turn widget salvaged from some old Tektronix gear. The thing works great! Now I'm looking for suggestions on how to "improve" it. My list includes: Replace NE 602 with SA612A mixer and add "real" tuning via National PLL and Minicircuits VCO. Replace 10.7 Mhz. ceramic filter with 4-pole Xtal-based filter. Replace MC1350 with MC1349 (per Len Anderson) and rework AGC. Add serious LP audio filter to output of AM detector diode. Throw away or rework "squelch" circuit - as it is makes lots of noise and is annoying (loud pops when switching in and out)... Get a decent (e.g. dipole 1/2 wave antenna). It seems to me that after attempting to improve the RF section the biggest gains are to be had in tweaking the audio stages - so that's what I'm focusing on. Just for fun - alot of filtering, noise gate, compression, limiting, etc. seem to be areas where I can really play with the thing and improve the performance. I've been looking at both Analog Devices and THAT components as a step short of going to DSP. I think some real improvements can be made. The basic signal is there (certainly at levels which are "good enough") even with such a simple receiver. I'd like to see what can actually be done (keeping in mind that this is a "learning exercise"..).... Any suggestions appreciated! (And, yes, there are plans to tweak the IF per previous suggestions!)... Thanks, Bill wrote in message oups.com... From: "Pete KE9OA" on Fri, Mar 18 2005 6:36 am I would go directly from 45MHz to 455kHz..............this is typical in HF receivers. An NE602 mixer can be used for the 2nd mixer and a 44.545MHz crystal used with the on-board oscillator allows you to derive your 2nd I.F. Alternatively, you can use a TDA1572 as the 2nd mixer/I.F./ demodulator. This is also a good system and it will give you fairly good strong signal handling capability. Pete "Netgeek" wrote in message ... Parts are on the way... First IF = 45 MHz Second IF = 10.7 MHz Any point in going further to 455 KHz for a third IF or just stick with the 10.7 - (MC1350 plus IF transformer) scheme? Bill As a suggestion (too late if parts are incoming), a single conversion to a 21.4 MHz IF is quite suitable. Using a monolithic quartz crystal filter (available from DigiKey for about $15 a pair of two) between the mixer and first IF could eliminate ALL tuned circuits in the IF following the 1st IF amplifier. Those are available in 12.5 KHz bandwidth which will be fine for a VOR signal. [DigiKey has a link to download specs for the ECS-made monolithics] This is now common in mobile radios, both new and retrofit of older ones. If a single-conversion scheme with 21.4 MHz is done, the LO can be 86.6 to 96.6 MHz with an image at 65.2 to 75.2 MHz. That is a low enough frequency to allow a simple L-C "top coupled resonator" fixed bandpass filter for the front end at 108 to 118 MHz (8.9 % bandwidth). Doing double conversion with a first IF of 45 MHz is, by itself, no problem. However the 2nd IF image is a bit too close to the nominal bandwidth of any 45 MHz 1st IF tuning (it's only 0.91 MHz away). With the second's image (on either side depending on 2nd LO above or below 45 MHz), there's still a chance to pick up part of the FM BC band locally. To avoid that, the 2nd LO should be on the high side of 45. Second IF image would then fall into the low end of the 118 to 137 MHz comm band (also AM) and those transmitters are much lower powered ones than BC stations. With a 10.7 MHz 2nd IF, its image would be 21.4 MHz away and rather easy to attenuate in the 45 MHz 1st IF. There's only a slight problem using stock 10.7 MHz IF components: Bandwidth of the whole 2nd IF might be around 160 KHz; less discrimination to nearby VORs and Localizers. A study of FAA sectional charts might be called for to check on potential interfering stations although those are assigned in regards geographic locations to minimize normal interference. |
#35
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I can comment on the filter change.............the 10.7MHz ceramic filters
that are being used right now have an IN/OUT Z of 330 ohms.................a crystal filter will have an IN/OUT Z of 3000 to 6000 ohms. It will work, but you will need to match the impedances. If the crystal filter is not properly matched, you will experience a deep nose in the response in the center of the passband, and your insertion loss could rise sharply. If you are interested, e-mail me directly and I can send you an Excel spreadsheet that will calculate the values needed for a series L, shunt C type of matching network. Pete "Netgeek" wrote in message ... As a follow-up - I bought and assembled the really simple kit sold by Ramsey - and described in detail he www.sentex.ca/~mec1995/aviarx/avrxkitc.html I wasn't expecting much - but was really surprised! It works really well and has provided a platform that I can hack to death without feeling bad about destroying it if it comes to that... I'm about 8 miles from Washington National (DCA) with alot of "stuff" in between. With a lousy 21" wire, I have no problems picking up both Potomac Departure and Potomac Approach (with the first I heard being Air Force One coming in to Andrews AFB). The only modifications I've made are to increase the supply to 12V (from 9V - per Jim Weir's comments) and to replace the worthless 10k varactor tuning pot with a 3-turn widget salvaged from some old Tektronix gear. The thing works great! Now I'm looking for suggestions on how to "improve" it. My list includes: Replace NE 602 with SA612A mixer and add "real" tuning via National PLL and Minicircuits VCO. Replace 10.7 Mhz. ceramic filter with 4-pole Xtal-based filter. Replace MC1350 with MC1349 (per Len Anderson) and rework AGC. Add serious LP audio filter to output of AM detector diode. Throw away or rework "squelch" circuit - as it is makes lots of noise and is annoying (loud pops when switching in and out)... Get a decent (e.g. dipole 1/2 wave antenna). It seems to me that after attempting to improve the RF section the biggest gains are to be had in tweaking the audio stages - so that's what I'm focusing on. Just for fun - alot of filtering, noise gate, compression, limiting, etc. seem to be areas where I can really play with the thing and improve the performance. I've been looking at both Analog Devices and THAT components as a step short of going to DSP. I think some real improvements can be made. The basic signal is there (certainly at levels which are "good enough") even with such a simple receiver. I'd like to see what can actually be done (keeping in mind that this is a "learning exercise"..).... Any suggestions appreciated! (And, yes, there are plans to tweak the IF per previous suggestions!)... Thanks, Bill wrote in message oups.com... From: "Pete KE9OA" on Fri, Mar 18 2005 6:36 am I would go directly from 45MHz to 455kHz..............this is typical in HF receivers. An NE602 mixer can be used for the 2nd mixer and a 44.545MHz crystal used with the on-board oscillator allows you to derive your 2nd I.F. Alternatively, you can use a TDA1572 as the 2nd mixer/I.F./ demodulator. This is also a good system and it will give you fairly good strong signal handling capability. Pete "Netgeek" wrote in message ... Parts are on the way... First IF = 45 MHz Second IF = 10.7 MHz Any point in going further to 455 KHz for a third IF or just stick with the 10.7 - (MC1350 plus IF transformer) scheme? Bill As a suggestion (too late if parts are incoming), a single conversion to a 21.4 MHz IF is quite suitable. Using a monolithic quartz crystal filter (available from DigiKey for about $15 a pair of two) between the mixer and first IF could eliminate ALL tuned circuits in the IF following the 1st IF amplifier. Those are available in 12.5 KHz bandwidth which will be fine for a VOR signal. [DigiKey has a link to download specs for the ECS-made monolithics] This is now common in mobile radios, both new and retrofit of older ones. If a single-conversion scheme with 21.4 MHz is done, the LO can be 86.6 to 96.6 MHz with an image at 65.2 to 75.2 MHz. That is a low enough frequency to allow a simple L-C "top coupled resonator" fixed bandpass filter for the front end at 108 to 118 MHz (8.9 % bandwidth). Doing double conversion with a first IF of 45 MHz is, by itself, no problem. However the 2nd IF image is a bit too close to the nominal bandwidth of any 45 MHz 1st IF tuning (it's only 0.91 MHz away). With the second's image (on either side depending on 2nd LO above or below 45 MHz), there's still a chance to pick up part of the FM BC band locally. To avoid that, the 2nd LO should be on the high side of 45. Second IF image would then fall into the low end of the 118 to 137 MHz comm band (also AM) and those transmitters are much lower powered ones than BC stations. With a 10.7 MHz 2nd IF, its image would be 21.4 MHz away and rather easy to attenuate in the 45 MHz 1st IF. There's only a slight problem using stock 10.7 MHz IF components: Bandwidth of the whole 2nd IF might be around 160 KHz; less discrimination to nearby VORs and Localizers. A study of FAA sectional charts might be called for to check on potential interfering stations although those are assigned in regards geographic locations to minimize normal interference. |
#36
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I guess it would have been handy if I'd posted the *correct* link
to the kit and schematic...8-) The kit was from Ramsey and it is identical to the design and schematic posted he www.uoguelph.ca/~antoon/circ/aviarx/aviarx.html As you can see it's surprisingly simple (and surprising how well it actually works). Thanks again for all suggestions! Bill "Netgeek" wrote in message As a follow-up - I bought and assembled the really simple kit sold by Ramsey - and described in detail he www.sentex.ca/~mec1995/aviarx/avrxkitc.html |
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