Homebrewing Antuino : Specs for the two 25MHz 3 pole crystal filters. - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: November 28, 2023, 10:33:04 am
content: This handy RF tool has been known to me for 2 years but it's only recently that I've given it a thought of being homebrewed. From the HF signals page,https://www.hfsignals.com/index.php/antuino/, I've got the firmware and the schematic. I've even found some folks who have modified the design to fit their needs ; the most exotic one being that of EB7ME,https://youtu.be/VTyHUfMKoz8?si=q1CKzr0czXAtyalY.There even was a mod by YO6PIR who did some major redesign work on the analog section, replacing the bulk standard 2N3904 with some proper LNAs.https://qsl.net/yo6pir/sniper.htmlBut none seems to specify the parameters for the 25MHz crystal filter sections. It's obvious, correct me if I'm wrong, that both the filters should have similar characteristics so that they let a narrow slice of the spectrum through and any mismatch might make the slice that the first filter ( viewed from the IF port to the AD8307 path) lets through invisible to the other. If anyone is aware of the intricacies, please allow me to have an insight into it. And also, if any of y'all has any access to the firmware and schematic of the EB7ME version, or have done anything similar and have no problem sharing it, please feel free to pass it over. Help from y'all will be much appreciated.

author: CJay
author_rank: Super Contributor
timestamp: November 28, 2023, 08:00:25 pm
content: There's probably not much of an official spec for it.Given the way Farhan designed the filters in the bITx I would suspect it's a set of cheap 25MHz crystals selected for match and then the measured (observed?) centre frequency is compensated for in software.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 28, 2023, 08:56:30 pm
content: Quote from: Dom13c on November 28, 2023, 10:33:04 amBy the looks of it, its a Cohn min max, the 3 xtals set the steepness of the skirts, the 47pf caps set the filter width (about 3K at a guess) and the series caps on each end help with ripple.You can simulate this very easily in LTspice, use any old xtal model you can find, you do not care about the filter frequency here, only the response and shape set by the other parameters.

author: RFDx
author_rank: Regular Contributor
timestamp: November 29, 2023, 05:22:14 pm
content: Quote from: Dom13c on November 28, 2023, 10:33:04 amThe bandwidth of the crystal filter is incidental to the fixed 50 Ohm port impedance and the motional inductance of the crystals you intend to use. To synthesize the crystal filter(s) you can use the excellent "Crystal ladder filter calculator (DISHAL)" software by DJ6EV.

author: Dom13c
author_rank: Regular Contributor
timestamp: November 30, 2023, 09:17:39 am
content: Quote from: CJay on November 28, 2023, 08:00:25 pmTook a close look at the device firmware, and this seems to be the exact case. Now I'm wondering why he chose 25MHz as the IF. Is it because the Si5351 requires a 25Mhz crystal and he wanted to keep the BOM short? If that be the case and there is no chance of degrading the device performance upon selecting 10MHz as the IF, I can save myself some expenditure and use the load of 10MHz crystals I bought for this exact purpose of building filters.

author: CJay
author_rank: Super Contributor
timestamp: November 30, 2023, 08:16:28 pm
content: Quote from: Dom13c on November 30, 2023, 09:17:39 amPossibly to reduce the number of items on the BOM but more likely that they are dirt cheap and available everywhere.In the Bitx and other transceivers Farhan designed the SSB filters are using commodity 12MHz and 11.0592MHz crystals purely because they're dirt cheap and incredibly easy to obtain in large quantities, similar to the 12MHz and 11.0592MHz crystal he uses in his transceivers.