pcb layout for bitx version 3 linear amplifier  - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: August 28, 2018, 02:38:59 pm
content: I'm planning to homebrew the BiTX20 version 3 transceiver. Got all the necessary components for that. I've even got the pcb of the main board. But I lack the ready to print layout of the linear amplifier pcb. Looked for it on google but got no result. I don't have time to sit and design it on my own. Does anyone know where to get it in pdf format? Also for the sake of keeping the whole thing as cheap as possible , I don't wanna order the pcbs online. The price of those online retailers are ridiculously high. Paying 15 $ for pcb doesn't make sense to me, given that I can make it on my own for less than a dollar

author: CJay
author_rank: Super Contributor
timestamp: August 28, 2018, 03:19:41 pm
content: Given that you can't design your own for lack of time and/or skill $15 doesn't seem like a huge amount of money to me.

author: Dom13c
author_rank: Regular Contributor
timestamp: August 29, 2018, 03:02:45 am
content: I agree but still. With that 15 dollar I can build the BiTX20 version 3 twice given that I each my own pcb. If I just can't get the pcbs , I think building the project in dead bug style would still be cheaper for me.By the way, thanks a lot CJay for mentioning the BiTX20 transceiver to me.

author: CJay
author_rank: Super Contributor
timestamp: August 29, 2018, 06:48:21 am
content: It's not difficult to build the linear, in fact it's probably simpler to build that than the rest of the radio, it's symmetrical and really easy to lay out.I've built amplifiers using dead bug techniques, it works at HF and is easy to do, there are plenty of examples on the web.

author: bd139
author_rank: Super Contributor
timestamp: August 29, 2018, 09:23:15 am
content: Agree.There's a layout for the PA here if you must:https://i1.wp.com/amateurradiokits.in/wp-content/uploads/2017/02/Linear-New-PCB-20mt.jpgIt's not all that critical though. Just keep the leads short and well decoupled. Even an off the shelf layout can oscillate like hell if you're unlucky. You can probably do a better job yourself trivially. I'd use a bit of plain single sided FR4 PCB and gouge out some strips in it and solder the PA transistor over the edge of it so it can be mounted flat on the chassis. Don't forget to add either a small sense resistor in series with the PA or a connector you can short or stuff an ammeter over to set the PA bias current. I tend to build my stuff like this these days in prototype form. Cheap, easy, very rugged and ground is pretty low impedance:I nicked the crystal off it for another project for ref.

author: Dom13c
author_rank: Regular Contributor
timestamp: August 29, 2018, 10:53:08 am
content: That's great. Is there a chance of parasitic oscillation at the linear if I don't use a relay. Farhan, the designer , says it won't be a problem if I replace the relay with a spst switch. . But I just wanna make sure from you guys😸.Had another query . Not sure but is the 50 ohm coax more pricey than the 75 ohm type or vice versa Thanks bd139 for posting the picture of your build. looks far better than mine ones😸

author: CJay
author_rank: Super Contributor
timestamp: August 29, 2018, 11:41:18 am
content: Quote from: Dom13c on August 29, 2018, 10:53:08 amPlenty of PAs don't use a relay but have a look at the Relay switched filter kit fromhttps://www.qrp-labs.com/Those relays will handle 10 watts and are so common, they're similar to the Omron G5V which is about 75p in packs of 5 or you can find 'copies' for even less.This will be sneered at by people I'm sure but there's not a lot of extra loss if you use 75R cable compared to 50R, indeed I know hams who are happily using RG6 75R satellite dish cable because it's half the price of lower quality 50R RG58, it probably won't make a noticeable difference unless you're trying to dredge signals out of the noise floor.If that's what you've got then I'd give it a try, replace it later if you manage to get some 50R coax (I bet you won't be able to notice the difference though).

author: Unknown
author_rank: Unknown
timestamp: August 29, 2018, 01:39:32 pm
content: 14MHz can pretty much be treated like DC (at low power levels) so standard switches / relays are fine. 75ohm coax is also ok, the loss is about 4 - 5 % so there will probably be a similar performance between good 75ohm and bad 50ohm cable especially if you have a long run. If you are building it yourself and understand the circuit, you could tweak the output filter to match to 75ohm. Tv lead matches nicely to a dipole btw.

author: Dom13c
author_rank: Regular Contributor
timestamp: August 30, 2018, 11:54:37 am
content: Is it possible to use SMA connector instead of the BNC type. Currently I have a plenty of them, but I lack the BNC type connector

author: bd139
author_rank: Super Contributor
timestamp: August 30, 2018, 03:44:01 pm
content: SMA is fine. Watch out though, they only last around 50-100 connect/disconnect cycles at average before they wear out. BNC (quality ones anyway) seem to be 2000+.I keep playing with the idea of using SMA and edge mount connectors but I’d kill them in a few weeks.