Install Filter inside amp?? - HomeBrew Forum at eHam.net

eHam.net Forum : HomeBrew : Install Filter inside amp??

Forum Help

1-5 of 5 messages

Page 1 of 1

Install Filter inside amp??	Reply
by K9FV on April 22, 2002	Mail this to a friend!
I’m in the process of building a solid-state amp using the 2SC2879 RF transistors. These amps are usually built on a Printed Circuit Board that is mounted directly to the heatsink with the heatsink becoming the top of the cabinet. There are a couple of inches of open space between the PCB and components and the bottom of my cabinet. If I mount a Chebyshev filter in this space, will there be too much coupling between the RF components on the PCB? If this will work, it sure would be nice to have the filters inside the amp cabinet with the switch on the front panel. If there is to much coupling, would installing an Aluminum plate between the components help? Installing the filter inside an aluminum box inside the amp? Thanks for any Comments? Ken H> K9FV

RE: Install Filter inside amp??	Reply
by WB2WIK on April 22, 2002	Mail this to a friend!
It sounds like you're speaking of "one" Chebychev filter; I don't see how that will do the job for covering multiple bands. Then, you also speak of a "bandswitch," which is more along the lines of what I've done. I've built high powered solid-state amps into small aluminum enclosures, using the Motorola Applications Notes and Engineering Bulletins such as the semi-kits provided by C.C.I., and installed bandswitching filters in the same box. There is no coupling to the amplifier circuitry at all, since the amplifiers have no tuned circuits -- everything's broadband using toroids which contain their magnetic field to a very small area. To minimize critical wiring and high current handling, I used PIN diodes to switch the filters, and the bandswitch was only switching 12Vdc at small current. Although this raises cost a bit (the diodes, for high RF power, are not cheap), it makes wiring so much neater I thought the tradeoff was worth it. WB2WIK/6

RE: Install Filter inside amp??	Reply
by K9FV on April 22, 2002	Mail this to a friend!
Thanks for the input Steve. Actually I'm speaking of a filter for each band - hence the bandswitch.... I should have been more clear on that item. You say there is no coupling inside with the use of toroids - is this also true with air coils (magnet wire wound on a wooden dowel)? I would love to have the auto switching using pin diodes, but do not have the knowledge (YET!!) to do that. I think I can do the normal Chebychev filter. I've built a 5-band, 7-pole Chebychev filter in the past and was planning to redo that inside the case of my amp. And yes, there bandswitch is a small cheap unit that only switches with NO load. Thanks for your help, I knew you'd come thru for me again:) Steve (and anyone else with comments). Since this type of amp is using a push-pull setup which mostly cancels the second harmonic, would using 3 filters work? The 2nd harmonic for 160 is at aprox 3.6mhz and way down. The 3rd (and above) would be at approx 5.5mhz and would be knocked out by the 80 mtr filter. The 2nd harmonic for 7mhz is 14 and should be waay down due to the push-pull setup. The 3rd harmonic would be about 21mhz and would be knocked out by the 20mtr filter. Then a 10 mhz filter with a cutoff about 30mhz. Is the 2nd harmonic really that knocked out enought by the push-pull configuration? Thanks for the help. Ken H> K9FV

RE: Install Filter inside amp??	Reply
by WB2WIK on April 22, 2002	Mail this to a friend!
Air wound coils tuned to the same or similar frequencies and aligned in the same plane (parallel to each other) can indeed couple. Aligning them in perpindicular planes reduces coupling a great deal. Or, if you're laying out a PCB to build a filter, anyway, why not place adjacent coils on opposite sides of the board, and use a full ground plane on the board as a shield? (I've done this, it works fine.) It's not tricky to use PIN diodes, they are just electronic switches, nothing more, nothing less. The problem is that for a 5-band switched network, it takes 10 diodes that might cost as much as $15 each, making the PIN approach a bit pricey. Regarding the "push-pull" amplifier, I must say I'm surprised! Most solid-state, higher powered amplifiers are fed via combinations of splitters and combiners, and the stages are in phase, not 180 degrees out of phase as in PP. I think making a PP amplifier actually balance over four octaves of range as is the case with an HF amp is quite difficult, and I would not rely on that critical balance to create a lot of even-order harmonic cancellation. WB2WIK/6

RE: Install Filter inside amp??	Reply
by K9FV on April 23, 2002	Mail this to a friend!
Thanks Steve, I now realize where I got the idea of the push-pull - in the MOtorola AR-313 and EB63 papers they are showing the 2nd harmonic being down way down. IN AR-313 there is a chart showing the even order harmonics being down -40 to -50 db below the signal over the freq range of 15 to 50mhz while the odd order harmonics are only down -10 to -20 db. In EB63 they show a 30mhz signal 2nd harmonic -50 db down with the 3rd harmonic being only -25db down. Now I see these are single module amps without the splitter and combiner circuits you mention. For some reason I was thinking the PP design was what knocked those even harmonics down instead of something else. Hey, it was a great idea anyway allowing the use of smaller filter inside the amp. Thanks for the Help Steve, Ken H>

Page 1 of 1

Next Topic: Researching for a Book
Previous Topic: FE 5650A Rubidium oscillator??

Email Subscription
You are not subscribed to this topic.

Check our help page for help using Forum, or send questions, comments, or suggestions to the Forum Manager.