I am constantly on the lookout for Crystal and Mechanical filters that can be had cheap and will slip right into a project. Winter hamfests are in bloom and this is a good time to look under the tables.
While you are checking out the tables or cruising eBay here are some of the ones that are easy to use in a homebrew radio. All of these are relatively low in frequency and if a single conversion design is in the mill, I would not use them above 40M. This is especially true of the 455 kHz Collins filters. But they work super in a dual conversion project. I have used all of them in homebrew rigs.
Heathkit 3.395 MHz (2.1 kHz) I bought a couple for less than $10 each Z in/out 2000 Ohms.
Yaesu 3.180 MHz I got for $15. 2.1 kHz and a Z=500 Ohms.
Collins 455 kHz, 2.1 kHz Mechanical Filter cost $45 including the SBE-33 XCVR. The bandwidth is a sharp 2.1 kHz and the Z = 50K.
The use of a Mechanical filter required some research to work in a Solid State rig. Many of these filters were used in "toob"radios. There is a photo below that explains the capacitance matching used with the Collins filter.
The $10 Heathkit Filter in a Dual Conversion Three Band Radio https://jessystems.com The link has the detail on how this radio was built with schematics!
Of note in this radio, the small relay in the 1st J310's DGM is to have a different bias on G2 in transmit and receive. (A 3N209 is actually installed.) Thus, AGC is applied on Receive and a Fixed Bias on Transmit -- Another N6QW Trick. The 7:11 Turns match is from 820 to 2000 Ohms or 2.44 to 1. Thus 121/49 = 2.469:1 or as we used to say when I worked for McDonnell Douglas now Boeing -- close enough for government work.
The KWM-4 used a Mechanical Filter out of a KWM-2 and this is a dual conversion SSB/CW radio. I think I paid $25 for this one.
The Circuitry Above the Filter is the AGC, S Meter & Product Detector.
BTW, The Heathkit and the Collins IF Amp stages had small LED's in the Source circuitry. When you had the AGC set right -- these blink on voice peaks just like a Christmas tree. A TKT from N6QW.
So while many hams are perhaps forced to build homebrew filters as justification for their purchases of the Nano VNA and Tiny SA, there are some bargain Commercial filters that are easily integrated into the homebrew topology. Having a CNC helps too.
BTW the first radio shown with the Yaesu Filter now has a homebrew filter installed. This radio has become my homebrew filter tester. A small change in matching transformers and a bit of time with the Arduino IDE and you have a tester.
Boom! Got The Seeed Xiao RP2040 MCU working with a Si5351. The effort required was close to that needed to build the KWM-4. IT IS NOT A SIMPLE DROP IN for the ARDUINO!
You must use a Special Rotary Library from Bill Buxton and the Si5351 Library is from Etherkit. Pin Numbers had to be identified not as D2 or D3 (typical Encoder pins) but as 27u and 28u, the actual Pin number of the device with a u following it. (From K7TFC)
The Etherkit Library numbering system for frequencies is down to 0.01 Hz. So like 7000000 (7 MHz) is entered as 700000000ULL. I never could get that to work so just added one more Zero and when sending "rx" to the Si5351 simply said rx*10.
I never could get frequencies above the LO to work, so for the video used the 5 + 9 = 14 ( for a 9MHz IF) versus the 23 - 9 = 14 MHz which just reversed the BFO frequencies for USB/LSB. Inexplicable -- but with the 23 LO the scope hardly shows anything but the Frequency Reads 230 MHz and not 23.
Now that I have hacked my way through the process there are some advantages to what I have coded so far. I have found a few tricks to move the input frequency to the Si5351 using the code.
The rx*10 mentioned earlier was a 5 MHz frequency. So if you do this in the set the frequency statement (rx*10) puts you on 5 MHz a Another TKT (Tribal Knowledge Tip) from N6QW. A Kludge at best but less struggle trying to get higher order frequencies in the Sketch.
How I discovered this was that I could easily get the 9 MHz BFO frequency to always come to the party but not the 23 MHz frequency. So when I put in the (rx*10) which equaled 5 MHz code for CLK0 that worked.
73's
Pete N6QW