Gerard.?

I believe that statement refers to the crystal filter module, without L7 and C212.
There are such modules that are 500 ohms resistive, though they also have
a significant capacitive reactance in the impedance they present.

As Allison says, it is the input to the complete network (including L7 and C212) that should be 50 ohms resistive.

Jerry, KE7ER

开云体育

What I find interesting is uBitx V3 though 6 the terminations for the filter that is
the crystal filter plus matching network is between a Mixer (DBM) and TIA amp.
Both of which are NOT 500 ohms and are optimized for 50 ohms.? So the matching
network has that as its?input and output? terminations.

The filter is characterized as 1500ohms 2pf That is not what it presents,
its what it expects as a load to produce the desired and expected filter shape.
However... I have parts tha texpect 470ohms to as high as 2000ohms resistive
for the termination. So you do need the data sheet for the filter in use.

The above is critical to the filter used as when done tight the loss though the
filter will match the data sheet and also the passband frequency and shape will
be as specified on the datasheet.? Crystal filters are not an exact match to LCR
filters, most LCR fitlers are designed for restive termination where most crystal
filters are designed to expect some reactance as part of the termination.? Altering
that alters the filters behavior (pass band and shape).

Also I have the experience with V3 and a lot of testing.? I found with the supplied
filter and the matching network it performed only marginally well.? Testing using VNA
(Agilent) the matching network values used both skewed the filter response and
created a large amount of ripple in the pass band. causing RX degradation and
also degrading RX and TX audio (for mine).? The values I used were close to
calculated once I had filter data.? I did have to adjust for the trace to ground
capacitance.? The results when properly matched improved.? The filter
pass band was properly centered at 45.000mhz and the loss was lower.?
FYI the filter used wanted a termination of 1500ohms and parallel of 3pf
as a match.

And like Gerry said its not a pure resistive match.

--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.

Hello,

Thank you, Farhan, for pointing me in the right place.
I compiled all info to the single document, corrected typo errors and saved as .pdf into Files/SBITX_V3 folder.
Maybe it will help to someone...

73 - Petr

The calculated solution for 650 to 500 is 969 nH and 3 pF.

It seems to me that the only way to actually find the correct solution is to lift the filter out of the circuit.? Terminate it using a 500 ohm resistor, and put a NanoVNA on it to measure the R +/-j.? Even then, the NanoVNA is itself a 50 ohm device so an approximation may be to use a 9:1 transformer on the VNA to filter to at least match it to 450 ohms.? Even better would be to use the 9:1 and a series 50 ohm resistor.??

Y1 is known to be extremely reactive over frequency.? The model of Y1 is a complex collection of L/C/R configuration that is highly reactive.? The 650 to 500 solution is an over-simplified description of the actual problem to be solved, which is to match at 45 MHz.

Ford-N0FP

I use on-line calculators to figure out matching.? One of my favorites is here:



Using 45 MHz

In the configuration on the schematic, 50 j0 ohms into a series C211 of 0.1uF (X= 0.04 ohms @ 45 MHz) into series L7 of 720 nH (X = 204) into shunt C212 of 22 pF (X = -161) does not appear to work at 45 MHz.? At 38.3 MHz the solutioin comes out almost exactly a series 720 nH to shunt 22 pF.

At 45 MHz, the solve for 50 j0 to 650 j0 is a series 613 nH to shunt 19 pF.

The practical issue of knowing the characteristic impedance of Y1 is clearly a factor.??

Ford N0FP

I have downloaded both prints and can spend the time to figure out which components need changing to goto the 510 finals but was hoping there was a .txt file on github. I looked and didnt see anything, where do I find those notes?

Hi folks,

is somewhere the manual for latest sBITX V3 please? I found just an old V2 document...
Looking for user manual as same as latest schematics and mods done on V3.

Thanks,
Petr

A common problem, especially with petg filament and perhaps a glass bed. Base level is not sticking fully. Happened to me once but I forgot how fixed it. Maybe more squish on first layer? Or more hairspray…

73
Mark
--
AJ6CU/7
KD8CEC 2.0, Nextion Screens,? and open source uBITX Raduino boards for Arduino IOT, BLE, RP2040, Teensy 4.0, and Raspberry Pi Pico
https://github.com/aj6cu

Great idea that shade should be pretty easy to add to the 3d model. Let me think about it when I get back to my desk.?

mark
--
AJ6CU/7
KD8CEC 2.0, Nextion Screens,? and open source uBITX Raduino boards for Arduino IOT, BLE, RP2040, Teensy 4.0, and Raspberry Pi Pico
https://github.com/aj6cu

Gordon,

many thanks for detailed info.
I asked for it as few owners reported cutting first CW element because of long T/R switching time in V3.
I did not find any video where someone would be able to show standard CW keying at normal speed of 25-30wpm.
If the CW delay is possible to set, then it would be interesting to know how low it could be set...?
Anyway, many thanks for help Gordon.

Petr

The webpage Gerard pointed to keeps things simple by ignoring phase.
The L network example on that webpage assumes purely resistive input and output impedances.
Gerard's filter module of 1500 Ohms resistance in parallel with 2pF of capacitance is not purely resistive.

In the general case

The calculation is simple, but the concept of impedance is not always easy.
Few radio amateurs without an engineering background ever figure this stuff out completely.

Impedance is like resistance, but adds the complication of phase.
To keep the formulas simple we use "complex numbers".


Normal "real" numbers only have one dimension, along the number line.
Complex numbers have a second dimension perpendicular to the real axis.
Weirdly enough, the math all works out beautifully if we assume the perpendicular axis
has units of the square root of -1, often expressed with the symbol j.

Once we express impedances as complex numbers, we can use the same formulas
to combine impedances as we do for resistors.

Ohm's law still works too, divide an AC RMS Voltage by and impedance and?
you get a current in Amps, where all three values are complex numbers.
The phase of the complex number for the current will be shifted with respect to
the phase of the voltage, exactly as?you would see with an oscilloscope.



Lets assume the 45mhz filter does have an input impedance of 650 ohms?
in parallel with 3pF as Gerard shows in his diagram.
The 3pF is in parallel with the 22pF of C212, resulting in 25pF of capacitance.
The formula for the impedance of an inductor is zL = j * 2*pi*f * l)
where f is the frequency in Hz and l is the inductance in Henries.
The formula for the impedance of a capacitor is zC = 1/(j * 2*pi*f * c)
where f is the frequency in Hz, c the capacitance in Farads, and zC the impedance in ohms.

That capacitive impedance is in parallel with the 650 ohms resistance,
which can be calculated as:? ?zP = zC*650/(zC+650)
And that impedance in series with the 720nH inductor L7 is zIN = zL + zP

Python is a simple programming language that knows about complex numbers,
We use exponential notation for the big and small numbers, so 720e-9 is 0.000000720
We can write this up as a python script:
On Mon, Mar 25, 2024 at 01:02 PM, ajparent1/kb1gmx wrote:

The IF amps and associated DBMs were 50 ohm not 500 ohms.?
The TIA amp is 50 ohms and the DBM is also 50 ohms.

The actual network is L network and the calculation is simple.

Note C250 and 216 are not there (zero value means not installed.).

Also c211 and C251 are large enough to be near zero reactance at 45mhz
and serve as DC blocking.

So the active network is L7 and C212 and the mirror components on the other side.


--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.

Re,